• • May 15 at 9:00am until May 17 at 4:00pm

in cooperation with KKV Monumental in malmö, i am teaching a course in making electronic sounds aimed at artists who wish to add sound into their toolkit. it is 3 days of lecture, demos and a workshop to help those in the course with a project of their own. it will be taught in english, although nordic speaking people are welcome to pose questions in swedish, danish or norwegian. KKV has an enourmous workshop with metalworking, woodworking and ceramics equipment at our disposal, as well as a decent modelmaking workshop. the cost for the course is 1900 Kr, i believe. this includes MOMs (VAT). contact KKV for more info…

all the relevant info in swedish, is below. Google translate if you are in the area and are interested in jumping in. or email me.

http://www.facebook.com/events/441912859216874/

Tre dagars utbildning för dig som behöver praktiska och teoretiska kunskaper inom ljudfältet för att applicera i ditt konstnärliga arbete. Denna utbildning vänder sig specifikt till bild- och formkonstnärer som behöver kunskaper i tillämpad ljudkonst. Du får grundläggande kunskaper i att använda ljud, ljuddesign och ljudstyrning i konstnärliga projekt.

Dag 1. Grundläggande begrepp och verktyg för att kunna tillverka utrustning och lösa problem.
Praktiskt orienterad genomgång av ljud som fenomen, vokabulär och verktyg. Mikrofon- och högtalarteknik, förstärkning av ljud, ljudsignaler och oscillatorer. Dessutom arbetar vi med brytare och reläer samt modulation, reverb och distortion. Tekniker som lödning, mätning av ljud och elektroniska grundkomponenter ingår.

Dag 2. En närmare titt på funktioner och komponenter som ingår i komplexa ljudsystem.
Hur man praktiskt kan använda ljud och bygga upp funktioner för att styra dessa med önskat resultat. Tillämpade praktiska exempel och användbara lösningar. I slutet av dagen summerar vi arbetsmetoderna, för att få en överblick över möjligheterna. Efter summeringen specificerar varje deltagare ett projekt eller behov som blir individuellt arbetsfokus under den fortsatta handledningen.

Dag 3. Problemlösning och praktiska möjligheter riktat till de enskilda deltagarnas behov.
Indelning i mindre arbetsgrupper för praktiskt arbete. Målet med dag tre är att hjälpa varje deltagare att definiera och arbeta med ett eget projekt, redan påbörjat projekt, eller mindre utmaningar som går att lösa under dagen. Handledning samt möjlighet att nyttja KKV Monumentals andra verkstäder vid behov. Gemensam redovisning och utvärdering vid slutet av dagen.

Om handledaren: JC Morrison har studerat konst på Parson’s School of Design, NYC. Under 2009 undervisade han i
analog ljuddesign på samma skola. Mellan 1997 och 2008 var han senior design engineer på Electro Harmonix och
utvecklade elektronisk utrustning inom ljudskapande. Han har även arbetat som konsult för ett antal företag inom
musikbranschen med utveckling av instrument och utrustning och assisterat ett flertal konstnärer, bl a Billy Klyver och
Robert Rauschenberg. JC har också skrivit ett antal artiklar inom ljuddesign.

För mer information kontakta Gunn Hasvén, KKV-Monumental, Tel: 040-966009, info@kkvmm.se

Anmälan görs till Gunn Hasvén, KKV-Monumental, Tel: 040-966009, info@kkvmm.se. Anmäl dig snart, få platser!!!

why does that make me so happy?

i have been periodically buffer obsessed. it is always an issue. i need a good buffer always, and tubes have trouble driving low impedances. when i say low, i mean 50 or 100 ohms to 5 volts or so. okay, yes one can parallel devices and use power tubes…. but it gets so big and bulky. and there are jobs, such as a mixing desk, where you need lots of buffers and space is at a premium. so i have tried out a lot of different things.

this one works. very nicely.

okay, some caveats: the key to making this work is degeneration. yes. the resistors in the cathode/source part of the circuit need to be adjustable so you can balance the two currents. then, they become one! yes, the tube slowly drifts as it ages… that is why a servo is a worthwhile addition. other pjfets will work well, but the resistances will have to be adjusted. basically, when the 2 null (and therefore draw the same current) you have the right values… you could use a high quality pot… a cermet trimmer with 3 to 10 turns is a good choice. it can be hard to find exactly the value you want. a 200 ohm pot would work well in this case. but you can always parallel a bigger pot with two fixed resistors… there are many ways to accomplish this job. the transconductances of the two devices can still be somewhat different so the spectrum analysis will show that some pfets are better matches for certain tubes and operating points, than others. degeneration is a way to match the gm as well as the current. clever choice of operating point is the key. using a sim, can help.

a note about servos. i am always a little creeped out by servos. but if you are into direct coupling, they are inevitably waiting for you. but where to apply the arm-bending? the input/s is the place most engineers head directly for. that creeps me out the most. i think i can hear it. at least i could in the past. i have tried lots of different methods and sometimes am forced by design to also do it like that. but often, there is a better sounding way to go. in this case, it has proven to be the tube cathode. that is what drifts… yes, the cathode is way more nitpicky than the grid, which is easy. and the opamp can’t exert nearly as much control of the low impedance cathode… but this application doesn’t require much. and the diodes and the 20k resistor limit the range of influence. a good trick is to use an ic socket for the opamp, turn on the thing without the opamp installed. adjust it for null… put the op amp in. you are done.

of course some provision for DC at the output also becomes a question, and i will take that up another time. today i just want to put this out there. i have several silbatone products that use this idea out there already, and more new stuff on the way. i think it’s time to share it.

give it a try!

been some discussion lately about both current feedback and current amplification in the circles i run in… here is an approach that works. i know because i have built several now and am listening to one as we speak. the one above is simplified from what i prefer, so as to have just one power source (the 300 volts can come from the 450 volt supply). i changed this to make it more likely someone would try it. actually, you can direct couple and still use one 600 volt supply, thanks to a large 1.6K 50 watt power resistor (or many smaller ones), along the lines of loftin-white. (see below). both will be stable and safe, although don’t let this be your first amp. 450 volts and up is nothing to play with if you haven’t already… etc.

some observations: this arrangement with high gm 9 pin tubes, a depletion mosfet, and a russian 6550 or KT-90 wired in triode, is a down and dirty way to get high performance current amp in single ended form and cheaply… the D3a can be replaced with a 6E5P or 6688, although some components would require different values (i’m not going to do all of the work for you…). the opt trans can be anything you can get between 1.2K and 2K. i used a lundahl 1627 1.6k 120 mA SE opt for the last one.

the open loop gain for this version of the driver stage is between 90 and 100dB (!). add to that the gain of the opt tube… and there is 60 dB returned to the first stage in current feedback. there is no nfb around the transformer, and no voltage feedback. the transformer quality will really determine the “sound”.

the zobel network around the opt trans is absolutely necessary. i like the russian “KGB” silver mica caps for this purpose. the big ones. current amps have a high output impedance and will track the load impeadance… speakers are not flat, so some kind of equalization is important for both the transformer and the speaker (it should also have a zobel).

and here is a pic of my own… this is meant to be a bit of a fuck you to the vintage crowd. shitloads of current feedback, current drive, and unpopular tubes. but it is lovely, of course! and there is still plenty of fuck you to the traditional EE scene too. actually more than usual. enough for now.

ahh, and just so you know that it can be done with the vintage stuff too, i tack on some images of mr. han’s we 275 current amps which are running WE-16As in seoul. i designed them last fall.

here is a schemat for that…

i am a subversive contrary bastard. and i make things with this in mind. it would be a lie against nature to deny it.

(rant alert: sometimes i just have to get it out… it won’t be pretty, but you don’t have to read it, either)

in addition, the “adult” world is generally a great disappointment to me. and, whatever it is, or purports to do, i don’t believe it… until i’ve seen it’s underpants and broom-closet.

but what good could nature possibly get out of me? does nature need contrary, or even give a damn? or is it civilization i should be worried about? am i just one more irritably discontent jerk in a sea of uncivilized assholes, raining on the parade or just messing with shit? it takes millions of well-meaning, orderly and faithful souls to put this world together… just so a few sour grapes can paint it black?

and if there is some good in contrary, what qualifies as quality, when it comes to going against the grain? are there better or worse kinds…  what does it mean to make subversive things? what good could they do?

perhaps, it’s good for stirring up the pot when things get totally stuck or boring? increase the opportunities for natural selection? fluff the herd for an entertaining ritual sacrifice? start a revolution?

or is it just random pain in the ass on the evolutionary road, social friction, doomed to inevitable flattening (by a fat suburban douchebag in a hummer with a gun rack, rushing nowhere, with a supersized mountain dew and a mouthful o chick fil-A…)? well, it sure isn’t enough just to cross-dress and walk backwards (although, it certainly doesn’t hurt either…).

i have a suspicion there has to be an element of collective or general advantage to any successful subversive/contrarian effort, even if no one likes the way the medicine goes down, and in spite of the powers that be. real contrary is not simply opposite, it is an opportune break with the way things are. but what the hell do i know?

one thing is clear. i don’t feel like i have a choice in the matter. and it’s probably a bit of all of the above, which means it might be part natural. so there’s hope for me…

the present moment has never been particularly kind to contrarian jerks, but history has been downright generous. most of the heroes of the written record are contrary rat bastards. and despite the requisite hard knocks, even bottom trawling subversives such as myself do often get a fair amount of positive contemporary direct/indirect marketing… always have. not necessarily for their benefit, of course, but some do cash in. i can’t wait! (still waiting…)

in america, the mainstream leaves a little space for people like me, up to a point. this includes academia and industry. the marketing business often channels a little contrary mojo, just enough to carry a whiff of difference – scratch and sniff contrary – to imply a generic “new” relevance, dark edge, hipsterness or sometimes actually just in total desperation. a subversive patina appealing to some deprived sense of humanity, something missing, and therefore convince itself that it is capable of change (which it actually doesn’t want or approve of, and has no intent of ever allowing). apparently, civilization likes a little contrary from time to time. as long as it’s in the proper measure (no actual change at all).

yes, and there are quite a few brightly illuminated “contrary” motherfuckers around as role models… sadly not myself. pundits. mainly of the sort that smoke in the subway, or preach weird gospel for money, hawk conspiracy theories, or just sell things that will soon be thrown away. but all of them without actually contradicting the state of things. they hock the flipside. neo-nazis (and fox news) are often working this angle, for example. QUESTION: need to get a rise out of ordinary folk who never pay attention to anything outside of their own tiny expectations? what is the most exploitative, disreputable and unpopular political position a person could take these days… especially if one has no connection to what it originally meant, can barely read or write, and is completely dis-enfranchised by the mainstream? give a rebel yell… seig heil! this is actually idiot contrary, and not worth spending much time on. it does pay some, and there is plenty of it to go around, but it’s just more of the same, only upside down. libertarians fall into this category. american “survivalists” too. troglodyte rebels. the teenage hang behind the seven-eleven (smoking pot and drinking night train or colt 45) is similar… but much more innocent. that isn’t contrary, it’s desperately ordinary. self absorbed. mainstream. sniffing glue, chugging cough meds and smoking embalming fluid… now that is fairly contrary (and really skanky). but all of that still changes nothing, which is of course the point.

that’s not really what i mean…

i guess i am thinking of a more visionary contrarianism. something along the lines of babette’s feast (netflix it). something that de-constructs the dominant discourse, and posits substantial and timely difference, and simply by it’s very existence, flips the adult world the stink finger. and things are better afterwards. it’s been said before that whispering is often a better way to be heard, if everyone is yelling and screaming. whisper something really resonantly subversive, make them a nice sandwich, kaboom!

it is smaller, perhaps more durable transformations, that interest me. things that directly affect the withering reduction in quality of life that the industrialization of humanity has wreaked on the planet, on culture, on human relations. mass produced crap, that ends up in the garbage. mountains of garbage. disposable work and manufactured obsolescence. oceans of shit, carbon dioxide and plastic. a view of humanity as disposable. a view of nature as disposable. people who can’t tell the difference between fake and authentic because it actually doesn’t matter. fake is way more important than real, regardless. our civilization is totally invested in it, no matter what country you come from. and here you get a sense of my paranoid take on things: “they” are ready to prevent big changes… with media, guns and the law. big changes are scary and affect profits and planning for the future. but small contrary steps add up. governments and corporations can’t keep track of them so easily and when bigger things emerge, it is too late to stop it. this encourages me. i need some hope, myself. because i want many things to change and it seems unlikely. like i said, i am a contrary bastard. there has to be some alternatives to the right-wing religious pyscho fuckers… whether it’s al qaeda or the christian right. also the institutional systems designed to prevent all real innovation: state, church and industrial workplace. against wall street: the enemy of everything innocent.

the slow food movement, for example! it woke me up. very nice subversive work there. no one needs fast food, and it doesn’t help anyone but the 1%. okay, the occasional pommes frites is nice, but what goes for the most of it, is terrifying. not all food has to be healthy, sophisticated or good for the planet. but, most food? in fact, you can count me in for permaculture and rooftop jungle. farmers markets (hell no, monsanto!) bring it on. in seoul, a big concrete jungle if there ever was one, greens and spices are grown right in the city year round… why not in new york? there are so many wonderful small contrary ideas that could practically work. elephants in nebraska… roaming the plains? if the puritan english could move to the states, why not elephants? and black rhinos… let them in, i say. better them, than any more white europeans. it will be hard to keep the yokels from shooting them though. simply because they’re different. but i’d bet they’d do better in nebraska than in namibia… they used to live in the midwest anyway. my point is that contrary can and could be fun, and great for the place.

so, subversives can perform some service, to civilization and nature. it’s a dirty job, but someone has to do it. technology? what about craft? design? art is already steeped in the marketing of contrary, so i will leave that alone. but what about science and engineering? let’s get out our ipads!

todays engineers are generally cowards. most of them (high voltage folk exempted). and, like priests, almost incapable of self-reflection, remorse, or sense of larger context for the job at hand… (again, high voltage people exempted). yes, that is what i think. they live in a world of conventional and predictable details, expertly modeled, defensively distanced by the size and “objectivity” of the job description, and stubbornly convinced of the wisdom in it. it’s all figured out. and they are so NOT concerned with the potential consequences of the interactions of their work with the work of others. or with culture. not that i am totally against that, just that it is never deliberate. for these people, it is “not worth” considering…

it’s not completely their fault. engineering is taught that way, and the value of the work is proved in the marketplace. it’s not their job to think in wholes. you don’t get paid to think big. “context”, especially in emotional terms, is a tiny word in their vocabulary. yes, it is mean and probably foolish to make such sweeping generalizations, and sure, there are many exceptional people and visionary threads of development (if it is actual development, then it is already contrary)… but overall, the nature of this work is baby steps with avarice. suspicious, reluctant “non-progress”. and yet, when then rules do change, ultimately eager to throw everything into the garbage and forget about it. because profit, the uber value, stands over all of it. that hasn’t changed, not for some time. the measure of all things engineered.

by the way, there is nothing ever contrary about profit, except doing things for free. nothing more deterministic or mechanical. if there ever was a place that needed some subversive thinking, it is here. it is, but tucked away in the holes and corners of industry and society, hoping to avoid detection by middle management.

to be fair, there is so much specialization now it is not really possible for one person to have a comprehensive sense of the whole, much less do much with it. there are too many disciplines. those who handle the macro vision have little comprehension of the micro, and the other way around. there are too many layers of interaction. too many managers (“please don’t kill me, i just do eyes”…). our understanding of the physical universe has long passed human scale. so has the sense of interconnectedness. still, i’ll stick by my diss… they’re supposed to be the clever ones. they could try for some integration… they generally avoid it like the plague. it does make it easier for all of them to be exploited, by both the “scrupulous” and the not. that’s a good thing for some.

every month scads of new details about how things fit together, or don’t, are published, peer reviewed and disseminated. this is not to say there aren’t people or groups of people who have a perspective of what they are doing or what is changing… darpa for example (“creating and preventing strategic surprise”) still has graduate engineering and science students working on grant funded elements or components of enormously complex systems that are secret and sometimes very contrary. those working on the parts have no idea (?) what they are for. one nuance of computational linguistics, added to another algorithm for pattern recognition, networked to a specialized search function of a database (kept by the DOD), which now can be cross referenced with facial recognition software, and a fast frequency-hopping microwave uplink to a remotely piloted drone over yemen, with the results crunched by a new program for analyzing crowd behavior… and que theory, that relies on another 3D mapping technique based on stereoscopic, or multiple-camera overlay software recently adapted from work done in the 1890′s and in the 1940′s on stereoscopy. what the hell could they want that for? oh right, to find and kill people by remote control…

the point is that few of those working engineers and scientists ask why. its not their job. they solve the riddle of the immediate problem. what emerges from it is “someone else’s” problem. physics is too “flaky and impractical”. politics and psychology “have no place” in design work. yeah sure. engineers aren’t supposed to do “wet work”. this leaves the bigger picture to business managers and to marketing departments, and politicians… the “experts” on what is good for humanity. in systems that organize above a certain level of complexity, there is no question that this method of working and networking the work of many together, is powerful. think of apple, or mercedes benz, and their impressive list of accomplishments. but it also creates both blindness and sometimes disaster, which you can’t “see” until it happens. this fact, combined with the small mindedness and the tiny cajones, insure a constant supply of highly engineered gadgets that leave out crucial features, serve the interests of the wrong target group, or glitch tragically when the ride turns left. both the car industry and the covert drone warfare industry can attest to this (brakes that accelerate the car… and remote recon systems that can’t tell the difference between friend, non-combatant and foe), although they won’t do it loudly. and it can be a very subtle set of small interactions between disparate functions, from which emerge large and extremely undesirable behaviors. it can also be very hard to fix because the bug/s can be very hard to find. and the teams very much separated by culture, fear, and distrust. but, most of all, the success of their way of working apart, has an overall effect of diminishing the perspective these gadgets were meant to enhance.

these gaps and blind spots are the life blood of hackers, by the way. hackers are often undeveloped engineers with an acute sense of context. and the best analog engineering is hacking.

for example, some of you are sure to be circuit benders, or know of them. you/they can be asinine, especially if they come off serious… laughter is crucial for that kind of hacking, and in the right balance, a cure for the stuck industrial soul. it is a semi-sensitive manner of getting to some new sounds that might (generally not) expand the context of what can and should be meaningfully done. that is very difficult to come up with on your own these days. “new” is harder and harder to come up with. kind of like gecko stickiness. no human thought of non-chemical stickiness until they started looking at how geckos stick to things… it is becoming a “new” thing now. we didn’t come up with it. geckos did. but we needed to have a deeper sense of the value of sticky, before we could recognize what geckos did that was different and special. the key thing is that chance and the natural world gets a piece. that is circuit bending.

control is a relative idea. to “bend”, you follow wherever it leads. benders use hammers and icepicks. paper clips and handfuls of aluminum glitter. hairspray and coolant are also useful… even fire. breaking, shorting, glitching, and or otherwise destroying the normal function of an electronic musical or audio gadget for the purpose of getting new and unusual sounds – all is fair. this is the polar opposite of the modeling business. you can’t know ahead of time. i have heard my share of it… at it’s best, it is beautiful and exploratory. at it’s worst… well, there is plenty of dreck out there anyway. that is why it is especially important that the bender has a sense of humor. but, the interesting thing is the element of chance, and the complete rejection of the industrial arc. “you can’t manufacture this!” it’s a wild card. gives back a little power. a little sexy human power…

this edge is never pushed by the mainstream. but the mainstream relies on the undertow to give it relevance and even a sense of hope, even as it resents and stifles any and all significant change. at least until it really needs it and can’t wait anymore.

the design of analog electronics is rife with opportunities to do it contrary, break old habits and get some benefit. for music making, sound design and for research. slow food for the ears.

filter skelter

okay, today’s project is a variable filter arrangement that is particularly well suited for sound generation or processing… and it is based on one of the oldest and most durable circuits known: the twin T filter.

i think the tubed active filter is one of the most delicious applications… and one of the least explored. op amps have made it easy to make complex filters with flexible tuning and tweaking. and it would take lots of space and power supply to replicate that approach with tubes. it is not the only way. sometimes, it is worth looking closer at old tricks for clues and hints of new direction. cleverly simplified direction.

the twin T filter is mainly used in it’s most “tuned” form, shown below. in fact, it is unusual to see it any other way, these days. one could call this a special case of a bridge circuit, because the two legs are made to work in balance with one another. like most other RC circuits, the tuning frequency, f = 1/2piRC.

below you see a sweep of the filter, with typical values for 1 KHz. note the deep notch and phase shift characteristic. the phase shift in the two legs reaches 180 degrees together, and nulls out everything. also note the insertion loss is very small. there is very little attenuation outside of the attenuation band.

in order for this filter to perform as well as you see in these pix, very close tolerance parts are required. in practice, you will have somewhat less of everything. the loading of the circuit is also very important. loading it down reduces the attenuation and Q of the filter. because this is an RC filter, the attenuation characteristic is gentle and slow, and spreads across several octaves, either side of center. but you still get a lot of loss at the null.

okay, nice one trick pony with great features. or is it? notch filters can be turned into peak filters and oscillators as well. first thing to do is make it a part of a feedback loop in an amplifier. why? because now the gain of the amp will be maximum at the frequency with the most attenuation (least feedback), and will have much less gain at all other frequencies.

here are two practical arrangements: the first one uses a cascode amplifier and the second a pentode. multigrid arrangements have many ways to get signals into them… which is what you need. grids are generally high impedance inputs (the screen grid of of pentode is not) which makes things easier. a buffer will be a big help in order to get the low source impedance necessary to drive the filter and decouple the plate load of the cascode from the lower grid (the feedback input). there are many ways to do this, but this way is venerable, having been used in the 40′s and 50′s for radar and special applications (see valley and wallman). i used this approach to design the TUBeQ for electro harmonix.

and here is the sweep…

here below, is the pentode arrangement. note in this case that an additional cathode follower is needed to get the input signal into the low impedance screen grid. also note the pentode is loaded with a constant current source and a resistor to ground (a transconductance amplifier), which gives enormous gain. a dual triode/pentode is perfect for this job. i have used the 6BL8, which i like very much.

here is the sweep…

note the effect of having more gain in the amp… the Q is higher and the amp more selective. but the transitions are still “soft”.

okay, now lets look at a very useful modification that extends the usefulness of the peaking amp, even further. below, we see the cascode feedback filter with the “control” signal returned to the first stage through a series voltage divider: 50k/50k. this could simply be a 100k log pot. not only does this reduce the feedback level, but it increases the driving impedance ahead of the filter. it also increases that part of the feedback signal which is not frequency dependent.

here is the sweep… please note that not only has the voltage divider been added, but also the loading of the filter has been changed (to 2K from 1 meg). again, the legs of the filter have slightly different requirements for balance. you will find that for a range of input Z, there will be a widely varying Zload (R12) for perfect balance. for most practical uses, a 1 meg dual pot with 100K in parallel with one gang will work ok. for more precision, a 2 pole rotary switch with the right values for every setting could be used. the important thing here is that it is a relatively simple thing to vary the Q of the peak…

now, if one wanted to vary frequency, it would seemingly take a relatively complex switching arrangement, or expensive custom pots to vary all the resistances over a useful range… for this reason, and because of the finicky balance, very few people have used a twin T to make a variable frequency filter. op amps have made cheaper and simpler “brute force” filters practical. but you don’t see so many parametric filters using vacuum tubes. that’s too bad! i can’t accept that. there is, as always, another way to approach this.

photo cells, or “LDR”s (light dependent resistor) have been used as variable filter elements for a long time. but not much for twin T filters! (anthony barmentloo, a fabulously clever effect designer and analog hacker, and myself, have done a bunch of gadgets for electro harmonix that use this idea). R is simply replaced by one photo cell. R/2 is made by putting two photo cells in parallel. i have used silonex and perkin elmer photocells in this application without problems. buy them in batches and usually the characteristics are close enough for sound effect use. buy a bunch and measure if you are making a precision filter.

for those unfamiliar with the idea, an LDR in close proximity to a light source (lamp, LED, electroluminescent panel… etc.) in a light proof container creates a third component known as an opto-coupler. optos use light to control resistance. four matched optos can be combined to make the R, R, R/2 needed for a twin T. the C is determined based on the range of off to on resistance of the particular LDR. values from 1 to 40 nF are typical. depending on how precise you want the filter to be, and where the center frequency is meant to sit.

in any case, frequency can be varied now with a pot, or a control voltage or current, to the light sources in the optos. simple sweep controls and active level sensitive sweeps are simple to arrange, once you’ve gone this far. below is a solid state combo frequency control (pot) that can also be used with a boss type expression pedal… this will work well with the tube active filters shown above. the expression pedal would be plugged in with a 1/4″ TRS jack. R6 and R7 represent a 5K linear pot (hard to draw in LTspice). D3,4,5,6 are part of the 4 optos in the twin T filter.

you will find that some adjustment to the filter component values will be necessary for the most useful range with the most consistent Q. if you start with the Q maxed out at one setting, it will not practically be that way anywhere else. if you deliberately detune the filter a little bit, it stays more consistent over the range you sweep it. below i have detuned the C about 10%. with R = 10k, the sweep shows a peak at 2K, but at a much more modest Q than the highly tuned version above.

now, deliberately mismatching the value of one R to 100k and the other to 110k has no meaningful effect. the tuning frequency has shifted to 200Hz, a factor of 1/10. but the Q is not so different…  it is hard to keep the component tolerance consistent over a sweep. this is important for practical musical instrument use, and even for post production effects… it’s good for tone generation too, but the input level will have to be higher.

the high Q peak filter is really useful for tone generation and resonator design, but not always the only way to do it. often, there is a practical narrowness of the peak that sounds good, beyond which there is small improvement. simply going for maximum Q does not ensure a useful audio filter… that is a big subject and worth going into more.

one or more of these filters, and perhaps two shelving filters (low bass and high treble) in parallel and with a common input, with the outputs summed, with some variable gain, would make a very useful tubed active filter good for music production. i have a tubed wah wah pedal that is almost exactly this.

this is enough for now. i will come back to this again.

it’s been awhile. a tough bunch a months.

at the time of writing this there’s an election looming, in the usa, that will set the course of the next generation and it’s economic future. there’s “frankenstorm” on the american east coast and an earthquake and tsunami on the west: all matter of unnatural and natural catastrophe. in the usa, very few seem to realize there are just a few small mistakes left between generally crappy and a major meltdown. that this was caused by the former republican president and his advisors has been both completely forgotten and successfully made vague. “it’s the black guy’s fault”. the de-distribution of american wealth that began under bush’s watch has continued helter skelter. never in the history of personal wealth have the wealthy been so rich and in my opinion, so undeserving of it. many of them make nothing. they speculate, gamble, con or just plain steal. like goldman sachs. like morgan stanley. like citigroup. like bain. on the flipside, poverty is now almost mainstream. it is crazy, to me, no one seems to care how this happened. most just want a slice off the flank. it’s business as usual. those responsible for informing the public have been downsized, both teachers and journalists. propagandists have replaced them, and are actually much more profitable. conspiracy theories, the price of gas and who gets to boss the vagina around gets more attention than why it’s that way in the first place. the mainstream media’s open racism and radical revisionism of all history, and even of science, has tainted the process to the point where it is actually hard to find out what the fuck IS going on. you have to watch the fake news to make any sense of it (thank goodness for john stewart and stephen colbert). comedians, like chris rock, make fake public service announcements that are way more honest and useful than any political organization. it should be funny…

hanging over the american election: corporate money has utterly corrupted the democratic process. if romney wins, he will have proved that elections can be bought in the usa.

and things are hard everywhere! the storm that trashed the east coast just now, trashed the caribbean before that. there’s no electricity or safe place to sleep, or sanitary way to take a shit, or a shower… for lots of people there. the eu’s money problems persist and inequality there threatens to split the union. the arab spring has collapsed. syria, jordan and lebanon are primed to blow together! beyond the false pride, economics and human costs of the wars we just can’t seem to “win”, corporate greed and avarice have become status values. the answer. things to take comfort in. “at least our billionaires are richer than yours”, “we” must be doing something right? this is how we do business. god bless.

it feels a little odd and even a bit shameful to be thinking about circuit hacking under the circumstances… about art and craft. about making weird machines. although it is a nice place to escape to. i have friends who have no lights, heat or water at the moment. it’s getting cold and there’s a nor-easter on it’s way to nyc. i guess i want to say that i feel compassion, sadness, and anxiety for the people over there. i am grateful i have a job and a roof over my family’s head.

i want to go back to a discussion of current amplifiers and also of current feedback. i’m a little rusty at the blog thing…. this will be a good way to ease back into it.

the ee universe is a man’s world, sadly. yes, and a man worthy of the name, pays attention to the volts. you think i’m joking?! i am simply stating a fact. ours is a voltage-centric discipline. a “thevenized” kind of bag. thevenin is the ralph cramden of engineering attitudes, and there is nothing more conventional and everyman than that. CAD relies heavily upon it too, as nearly all equivalent circuit simplification starts there. if you are not sure of what i am talking about, please go look it up. it is worth your time and will help you in your work. i am simply going to the punch line because i need to get some where else, but i want you to know why. because of thevenin, we tend to analyze active devices in his terms. an ideal voltage amplifier is a voltage source with an output impedance of 0 ohms. a short circuit. and it likes to work UP, into it’s load, which has some finite value.

as with everything else under the sun, there is another side of the tracks… and it is called norton. the bald headed goofy well-meaning idiot-savant. emphasis on savant. well educated EEs will of course point out, and rightfully so, that in the end, thevenin and norton are simply flipsides of the same insight: that no matter what kind of electrical circuit you have, it can be simplified into one relationship between volts, current and impedance right where the source of power meets the circuit. they come to the same conclusion. one pushes a bus and the other works in the sewer. yes yes… well said.

i say, there is a practical limit there which tends to be pushed aside in our haste to get to the end of the job well done. there isn’t a practical amplifying device made today that is a short circuit or reliably drives one.

on the other hand, norton has a different perspective of these things. the current-centric view. and a current source can be viewed, ala` norton, as an infinite impedance looking DOWN into a finite load. so what? as noted, you wind up with the same result. the big difference is that all of the practical devices we have to work with today are much more reliable, or how about “happier”, as open circuits than they are as shorts. vacuum tubes in particular, and especially tetrodes/pentodes, could be said to fit this view very nicely.

voltage feedback and current feedback have some related twists relating to thevenin and norton practically, but as my perspective is controversial, i am going to save most of that for another day. it is uncontroversial to say that voltage feedback and current feedback transform practical amplifiers in slightly different ways. there can be advantages to current feedback in the way it handles impedance transformation… even though in the end you have the “same” result.

below is a simple example of one of the most basic ways to return negative current feedback to the first stage. most modern voltage feedback op amps are actually based on this exact arrangement. the main difference is that this is half of the circuit. the op amp would have a differential input. but in this case, the “input” for the feedback circuit is low impedance… the cathode. the input for the signal is very high impedance, the grid. i think this is accurately describes a current feedback amplifier. i also believe modern current feedback op amps are different in that they respond to current at the inverting input. but that shouldn’t change this discussion. we’ll see who yells at me for all this…

here, you see two grounded cathode inverting gain stages, plus a cathode follower buffer, which returns it’s current variations (the swing through it’s Rk) back to the cathode resistor of the first stage. these current variations are in opposite phase to those in the first stage. consider what happens when the cathode follower has to dump extra current into a load…. loading it down. there is less current variation in Rk. less CFB. more voltage into the buffer. this tends to stabilize the output. this is classic negative feedback error correction, done with current. one fabulous feature is that the error signal is DC coupled.

above is a very fancy practical hybrid version with metal pentodes and extraordinary performance. the 6SJ7 (triode wired) is set for a gain of 24 dB. and the C3g is a Gm amp, with a gain of 65 dB. that is roughly 90 dB of gain for the two. the bandwidth is DC to -3 dB at 18KHz open loop. the buffer is a depletion mode mosfet with 10mA going though it. that is probably not enough but it is what i have in the supply. there are better parts for this job… yes, this combination is sure to piss off a few! of course that’s why i do it. i will wire up a version with submini tubes (solder leads) under the hood for comparison. here, i have dialed in 58dB of current feedback for a closed loop gain of 32 dB. the 20pF compensation cap is for stability. below is a view of the prototype. the heatsinks are for the current source and buffer mosfets.

here are some measurements for you. first is a 250 Vpp sine wave at 20KHz. it looks like a sine wave but nice and big. 50 volts per centimeter. the next image is much more impressive. it is an X/Y measurement of the same signal. note the simple straight line. it is completely closed. without a distortion analyzer or phase meter i can still say that this 250 Vpp is provided with well under 1% distortion, and well under 1 degree of phase shift. probably under 0.1% and 0.5 degrees.

that this was accomplished with 5 active devices is pretty cool. the two gain stages are tubed. it could be all tube, but that would put a bunch more holes on the top panel, and there isn’t any real advantage. the current source and the buffer don’t have to be tubes. also, to do this in only solid state is not that easy. most of the practical devices available have too much input capacitance (and the C is too voltage variable) and the high voltage would cause problems. you could go to more devices and get to the same place where this design is… cascode high voltage transistors, or mosfets. but there would be a great deal more phase shift at 20KHz with any 2 high voltage transistors and the same mosfets. the tubes i use here have very low input C and and are voltage controlled.

most tube circuits aren’t expected to work this well. especially in the DIY world. but also in the “high end” world… of course, there will be many who immediately ask, “but, how does it sound?”. “feedback! what the fuck!” “who needs it to be that good!” and, “it ain’t natural” (for some reason i hear jeffrey jackson saying this to me…). look, electronics is the antithesis of natural. i say, it all depends on what you use it for… i plan to drive some big globe 50 tubes with it. i’m pretty happy about it. i’ll have more to say about this… later.

the square wave pic simply shows that the X/Y plot is very real. it is boring in it’s simplicity. no ringing, no rounded edges… it’s just a plain old 1 KHz square wave. 22 Vpp with half a volt in.

now for an interesting variation, imagine making the 3rd stage a transformer coupled grounded cathode stage… but what about the phase? it is an inverting stage at the plate but the current variations through the tube are the same as for the cathode follower. it is still negative cfb returned to the first stage. but in this case, the output impedance at the plate will be increased. the unbypassed resistance of the first stage’s Rk (which is in series with the output stage’s Rk), times the gain against the error signal there. variations in load impedance will have the opposite behavior of the typical voltage amp. with the source impedance (the amp) higher than the load’s, the output current will tend to be regulated by the feedback, and the voltage will track the impedance.

now why would that be cool?

well, imagine you are interested in driving a “full range” speaker (a misnomer) or a compression driver connected to a horn. both of these speakers have difficulties at the bottom (near resonance) and at the top (roll off from the mass of the piston). a traditional voltage feedback power amp (with an “infinitely” low source impedance) would have a good “damping factor” and would dump more current into the load as it approached 0 ohms, and less as it increased. it would have to do this in order to keep the output voltage “regulated” as compared to the input signal. but, this would result in a loss of bass and treble into both “full range” speakers and compression drivers, as the impedance is rising at both of these frequencies… and the efficiency is fading at the low frequency end, at the same time. a current amplifier, with it’s higher source impedance, would look at these particular loads quite differently. the output voltage would track the impedance of the speaker. and you would have extension of both the bass and the treble.

this is interesting for me as i use manger drivers in my system. mangers are complicated drivers. they are bending wave drivers above 400 Hz, really inefficient piston drivers below, and beamy laser beams above 1 KHz. i have a love/hate thing with them, for sure. but they do one thing better than all standard moving coil dynamic drivers: their impulse response is almost perfectly in time with the drive signal. if you can cross them over to a bass driver low enough, and keep the woofer output out of the midrange as much as possible, and then deal with the beaminess… (i have gone the way of the diffusor), you have something pretty special. at least for a small to mid sized room. they do make excellent monitors for recording as there is no appreciable group delay or phase shift over the range of 150 to 30KHz (!). they can reproduce a square wave. the number of speakers ever made that can, can be counted on one hand.

if i could get a bit more extension at the low end of the manger, it would help me with the crossover to the woofer… i want as little woofer output as possible mixed in with the manger. so i put the woofers on the sides of the baffle, with an electronic crossover (18dB/octave). this will be steeper in practice with the mechanical crossover of the baffle added in… also, i can use the back wall as a boundary reinforcement, to get a boost in the very low frequencies. this allows use of smaller woofers (2 of them). but the fact is the manger starts to crap out at 400 Hz. and X max is 2mm. they are easy to blow up, and crazy expensive. we have had a great deal of experience with them at silbatone.

so, back to the current amplifier. i have made one using an 807 beam tetrode. it drives the manger safely to 100 Hz, and drops off. at the top, i get a 3dB rise in output up around 9KHz with the amp, which flattens out when the diffusor is installed. it’s a touch dull with the regular amps. i’ve been listening to it full range for a few months now and it is wonderful. a good mix of compromises. the diffusor was designed by MJ Chung. it works very nicely. no more laser beam.

but this isn’t the only thing i am working on… i think the most useful place to use a current amp is on compression drivers. the big advantage now shifts to the high frequency end of things… because of mass roll off, many compression drivers, especially older ones, are limited to a fundamental range of about 6 to 12 KHz. think of a WE555 or an altec 288. modern drivers have wacky tricks to extend the range by use of resonance and peaking. it has never impressed me as a sound approach. the classic approach was equalizing the driver and using a super tweeter. a current amp offers a middle way. the output rises with impedance. the amp equalizes the output. it is very natural and not anywhere as harsh as what you get with the modern beryllium peaking types. i have only heard this on a an altec 288A. but plan to test out some things soon on the WE555 and WE594. more on that, later. the real interest for me is with full range horns. and drivers like the WE555. the vitavox low resonance freq. compression drivers are also very interesting to me.

obviously, if you are into “full range” drivers, current amps are something to check out. by the way, nelson pass has been into this for some time. way longer than me. he has a bunch of really interesting stuff on his website along these lines. i do have to control my gag reflex when it comes to lowthers, but many people like them. a current amp could (nothing can save those pieces of shit) help…?

here is the 807 current amp design adapted for 2A3s. i have substituted more common tubes than i used. i can recommend the 2A3 EH (made in russia), but 6B4Gs or any of the extended family would do… or even a 6AV5 or 6550 could be crowbarred in there. i used one electron output trannies for this test and they didn’t disappoint. you can use whatever you can get… fun, and interesting.

addendum!

after a great deal of trouble driving the output stage with the current feedback driver stage shown above, i had to back up and get some help… 2 quick conversations with morgan jones and frank blöhbaum (at etf) helped enormously. i blew up a bunch of fancy depletion mode mos, and normal run of the mill hexfet enhancement mode mosfets too. it was hard to see what the failure mechanism was (morgan was extolling the virtues of inexpensive digital oscilloscopes for the ability to record events… i guess now i have one on my christmas wish list). it eventually turned out to be the gate charge and the extremely high impedance of the gM amp (the C3g stage). you learn something new every day. i had diodes and up to 5k gate stopper. boom. especially on big transients. i ended up using an IRF730 with a 10k gate stopper (!), a 12 volt zener from gate to source, and a 100 ohm source resistor in order to make it reliable. but the performance is affected. the much lower C in of the IXCP45MS allowed a large amount of current feedback (58 dB) with relatively modest compensation. the much larger value of input C on a hexfet changed everything. it was much harder to manage it without oscillation. i ended up increasing the compensation cap to 420pF! it had been 20p. and it is still just stable at clipping… i will work on making the fancy mos more reliable. while this measures and sounds excellent, it can be better. it makes me appreciate tubes all the more.

i switched to a submini tube as a cathode follower (because a solder lead tube fit under the chassis and i didn’t need to drill any more holes) and everything was perfectly stable with much less compensation. it is so easy to compensate tubes, at least if you know them… but the insertion loss was 50 volts! the mos dropped 5. my 250 volts of swing became 200 just like that. tubes just don’t have the transconductance to make really good simple buffers. they need help. they sound good, don’t get me wrong, but solid state does buffers without losing so much level. at line level, who cares. but for lots of volts, they eat up swing.

so back to the mosfets. i have been compensating the driver stage by using a shunt cap to ground from the C3g plate. this is supposed to help define the pole associated with this stage and prevent it from screwing with the other poles… particularly the buffer and opt tube. i have switched to using a “miller” compensation scheme (basically the cap goes from plate to grid) instead and everything works much better… why throw bandwidth away for stability without getting anything for it? miller effect is negative feedback. you get the compensation needed AND distortion is reduced in the attenuation band. now, even the fancy depletion mos is stable and i don’t need more than 22p of C.

i meant to drive a 50 tube and test something i have wanted to for years. 50′s are tough to drive because they have a largely variable input admittance. for this reason, they expressly do not recommend fixed bias and most of the amps you will ever find that used them were transformer coupled to the grid (low impedance) and self-biased. the grid current varies from negative to positive quite a bit from cutoff to saturation. with fixed bias, there is nothing to prevent runaway if the bias shifts due to clipping or asymmetrical waveforms of large amplitude (drums and synth pulses from dance music…). i want to push the envelope and see what i can get away with. the problem is the resistor connecting the 50 grid to the source of bias: for cap coupling, this is usually on the order of 100k to 500k. this allows a cap of reasonable size to be used, typically .047uF to .47uF, for low frequency performance. this maximum requirement is always reduced for fixed bias… an EL34 in fixed bias shouldn’t have it’s Rg exceed 50K. most amp manufacturers have no clue. this is a major source of failure in the musical instrument amp business. a 50k grid resistor would need a minimum of 1uF and probably more like 2uF for anything quality in an RC coupled fixed bias amp, if you wanted a reasonable low frequency performance. that is a big and expensive part.

i have always got around it by direct coupling. no cap is the best cap. but i wanted to know just what the range was… and my contrary nature began to tug at me: who says it can’t be done! so i want to make a reliable cap coupled fixed biased 50 amp. driven with metal pentodes and very high performance. fixed bias gives a bit more power. when you are talking about a 4 watt amp, an extra watt doesn’t hurt.

so here it is. after a bunch of head scratching, it works fine. not only is it fixed bias, but cap coupled. it sounds fabulous as it should… it makes just under 6 watts at 2% thd. almost 1 and a half more watts. i even tested an electrolytic coupling cap (4.7uF 400V, bypassed with a silver mica .001) and it still sounds and tests excellent. look, i just can’t help it… tell me i can’t or shouldn’t and i just have to try. panasonic won again, by the way, over sprague and illinois. but now i am using a 2uF film and foil. i worry about such big AC swings across an electrolytic… if it weren’t such a rare output tube, i would let it rip. the grid resistor is 12K. i started out with 5k (and the 4.7uF electrolytic). the drop across the resistor at clipping is 0.4 volts… not enough to worry about. at 50k, this would be 3 volts less bias. 100K would be 5 or 6. you see the problem. the grid current and the voltage drop across the grid resistor would destroy the tube.

and finally, here is a look at the top. now i have to make the other channel… bleh! i need an intern. more soon…

lately, the discussion has been gently poking around in the sonic muck, for the purpose of a closer look at distortion as an electronic sound effect. electric guitar is an obvious and simple context to begin with, but i want to make it clear that the threads lead in and out in ALL directions. there are other places to start in terms of sound design. film (especially animation) is a good one and so different from popular music, for example. still, the electric guitar emerges as an instrument of change. in the 1960s, particularly in connection with psychedelia and the british invasion, new sounds and mass production transform music forever. but in terms of distortion, there are guitarists whose uncanny intuition have transformed the perspective of what it means, both as a vocabulary and a source of symbols.

the blues guitarists definitely relied on distortion to do what they did. but it wasn’t intentional design: little amps turned up loud, aggressive and cutting though. nothing wrong with that! when one thinks of distorted guitar, while it starts with the blues, it rapidly began to shape shift. link wray, jimmy hendrix (and in some crazy parallel way, sam andrew and james gurley at the same time or even a little earlier…), lou reed, robert fripp, and jody harris and bob quine (as a Gemini pair) are seminal figures. that short list could be expanded a bit, but it’s in the ball park. they transformed the electric guitar into both a new voice and into an expanded environment. what isn’t so obvious is the boxes they stomped on to do what they did… distortion effects by rodger mayer, bob myer, and others, are crucial to this evolution. my bloody valentine might seem a long way away from muddy waters, but not if you follow the sound design.

there is no doubt, to me, popular culture preceded academic exploration of sound design in the 20th century. electronics has everything to do with this. primarily because most of the advances in the technology appeared so fast, once they appeared, the uses lead the understanding by a progressive margin. it did make for a lot of bad music! still does! but it also changed music forever. sound design is now a permanent feature of most music making. this process did eventually slow down, in the analog context, and reversed itself almost entirely. now, people search for ways to use analog techniques to make new sounds… as they do with digital stuff. even “classical” acoustic music has taken on this idea in an utterly analog sense. lou reed’s “metal machine music” was performed by an all acoustic orchestra in germany not long ago, for example. but it didn’t start out that way. the sounds came first.

the digital era has kickstarted a new cycle which is ongoing… but the entire arc shouldn’t surprise anyone because it is a feature of the post modern. it’s intertwined with the economy, politics, and disaster. many will point to musique concrete, stockhausen, or even colombia university in the 50′s and 60′s as the root of the tree… i want to say out loud, this is bullshit. raymond scott, in one perspective, yes. more than anyone else, he created a system to compose with sound design (for scott, words are also sounds) in a popular, widely understood manner, before anyone had imagined such a thing. the others: important, yet only partially formed and mostly referential experiments (stockhausen’s “hymnen”). still, the bad apples led the way, and the rest of the power structure didn’t catch up for some time. they have done their best to see it doesn’t happen again. but it most likely will. maybe not in the USA? we americans could use an “arab spring” ourselves.

john cage fits into this, but not for the simplistic reasons most people imagine. i think he took for granted the similarity between words and the “language” of images and sound (it’s the reverse of raymond scott: sounds are words)… but it isn’t a perfect fit. it is uncontroversial to say academia has always had a problem with the human body. the physical senses, other than sight, aren’t far behind. the gestural powers of the body are different from words. it is a blind spot that still messes with the academic perspective. there is a long pattern of alternately completely denying or hysterically wallowing in “physicality” (that’s a word, not the meat). does it have to be so stark? as far as the current discussion is concerned, i will suggest there is an inexorable connection between distortion and the human ass. words cannot explain what these two know without them. and, there are things that cannot be explained well with words. a wall of sound needs no words. and the hands of the musician, together with the technologies that extend the reach, aren’t separable. for me, it’s another form of dance.

in the previous posts, i separated the most well known distortion effects for guitar into “fuzz” and “overdrive”, and linked them to two approaches: clippers and intentional “maladjustment” of amplifier operating points and headroom… i want to add a subtle but very interesting feature of older designs. most modern distortion effects rely on op amps to do these jobs. because the gain is so high and feedback is necessary to define the transfer characteristics, the slide from linear to non is very quick. when discrete devices are employed, the range of operation near cut off and saturation can be used much more sensitively than is possible with op amps. there is much more of a range of nonlinear distortion generated by operating a stage in this way, even though clipping may never occur. strange uncorrelated artifacts can be generated that have a wider and often less palatable sound, but still contain enough of the original signal to make out the source. this idea, in combination with other techniques, can be used to make truly original sounds, that cannot be made any other way. that is a problem for a manufacturer trying to make something repeatable… but there are ways to do it. let’s have a closer look…

a word about triodes: triodes were the first practical voltage amplifier, and also the simplest. the gain is low to modest… they are relatively linear without tricks, and the cutoff and saturation transitions are not very complex, especially at signal currents. compared with bipolar transistors, they aren’t as rich in terms of distortion generation because of their electrical simplicity. modern VHF triodes have more to offer here, than the older audio ones, because they have sharper transitions, are more sensitive to grid current, and can make a lot more higher order harmonics before clipping. pentodes offer even more interesting possibilities… especially in the transition from the “resistive” part of the characteristics (at low voltages) to the “constant current” part of the characteristics. in order to keep things simple and get the most from the discussion, i am going to use a hybrid circuit to demonstrate the ideas. it’s easy to build and listen to… but it is absolutely plausible to do this with tubes alone.

here is the idea… you can clearly see the connection to the previous circuits.

the first stage is simply a jig to make the various Q3 operating points adjustable. i have shown this stage with a typical low noise NPN bipolar transistor. but, what you need to get here is a germanium transistor. use a socket, try anything… the transition areas are much stranger than modern discrete silicon, and more interesting. the buffer is absolutely necessary so don’t waste your time trying to wiggle out of it. this is not hifi. it could also be a FET, and you could current source it for upscale. i made all three transistors the same… this is for demonstration purposes. use what you have.

the point of being able to adjust the bias and B+ (and the load of the tube stage) is that you now have control of the operating points. you can dial things in the middle for symmetrical clipping, or skew off towards cutoff or saturation (the diode line). because there are two stages, the flexibility is outrageous. in the example above i have leaned out the amplifiers and lowered B+ so as to plumb the areas at the very corners of conduction. this makes for a very asymmetrical waveform that creates an odd mix of harmonics… by varying the bias alone, you can get an effect similar to a ring modulator. the added overtones are related to the difference in the tops and bottoms… it can be awful! it can be genius. have a look:

here you see output compared to input. note both the asymmetry in the X axis as well as the Y. this setting bottoms out at -20 mV… but doesn’t top off until around 140 mV! note also the rising level… this would be whackier in reality. in practice, this is not as predictable from one transistor or tube to the next. especially at low B+. i have not talked at all about the coupling issues between the stages and why they are done as they are… suffice it to say for now, that slowing things down a bit and choking things off a bit (with grid or base current) is also a technique. one thing at a time… the range of adjustability of the “jig” is more than enough to account for differences from one gadget to the next. this is but one example of many possible. 1000 sounds and no presets!  next, i think it will be time to look at EQ for distortion… that is the main ingredient to the “special sauce”. more soon.

i mentioned the possibility of both “soft” and “hard” clipping and put up some images of what it would look like on a scope. the addition of odd order harmonics (3rd) and frequency emphasis/de-emphasis are key elements to the sound, but limiting (clipping a waveform is unambiguously limiting it in amplitude) is also a crucial feature. why? given enough gain after the input, the dynamic range of the output is brutally reduced. it doesn’t make any difference if the input signal is small or large, one size signal comes out. this translates into an overall smoothing of variations and a smearing of all intent or accident into one contiguous sound.

a brilliant feature of this radical reduction in dynamic range, is another interesting effect: sustain. because small signals are boosted to the amplitude of the clipping threshold, and large signals are reduced to the same, any overdrive or decay is essentially transformed into one long (sometimes insanely so…), one sized event. think of carlos santana playing lead in “oye como va”… that was a big muff, the classic clipper fuzz.

sustain is an interesting transformation of the guitar. electric guitars are plucked instruments whose decay has mainly to do with the lossy qualities of the mechanical parts and the thermal absorption of the materials it is made out of. those instruments made with bolt on necks and light wood and metal parts (stamped steel bridge) have very little natural sustain and a signal envelope that rises fast with plucking the string and dies out soon after. it’s NOT a piano. the fender telecaster would be a good representative example of this kind of thing. the resonant qualities of the physical construction don’t help either in storing much energy in the bandwidth of the instrument (to drag it out longer). twang! by adding a fuzz box to the signal chain, this percussive dynamic sound is transformed to a smooth legato vocal or bowed like sound that pushes the guitar into roles other than comp or texture, which was the first role of the guitar. now, the guitar is a voice. for good or evil? hahhahahahahahahah! jimmy page used a distorted telecaster in “heartbreaker”, so that may hint at the answer to that question?

clippers are a good place to start with this stuff because they are simple. but their simplicity also makes them one trick ponies… all the way fucking out. that’s an important concept for the effects business. every box, a specialist toy. you can understand right away how idiotic hifi is as a business model today: a bunch of old smelly men, looking for status and distraction, in all the wrong places. the sense of exploitation is both closer to the surface and the herd more obviously delusional. for every piece of hifi gear sold, hundreds of musical instrument or pro audio gadgets are sold. it’s a really big business. i wander… if clippers are the logical far end of the distortion business, then what comes at the beginning?

“overdrive” is the industry term for small to considerable amounts of level sensitive progressive distortion. it can go as far as total clipping madness, just as fuzz boxes automatically do, but it can take a while to get there, and do some other things along the way. as i briefly mentioned before, exceeding the input range of a gain stage, deliberately, will also distort the output signal more or less, and depending on the arrangement, can wind up clipping or, even cutting off the amplifier entirely. clippers can’t do that in a progressive manner, because of the generally restricted dynamic range. not to be underestimated in the subtlety or heinous destruction possible, overdrive can be crudely described as having both more dynamic shifts in effect AND a wider range of possible harmonic and enharmonic sounds.

because there is a period of transition from “clean” to distorted, and this range can be related physically to the fingers of the musician, a “touch sensitive” character to the effect is often the important quality. with fuzz, there is no touch… it’s all or nothing (actually, it’s all or noisy). overdrive is ALL about the touch. but how does one build that into a design? good question? i was hoping you might ask… (talking to oneself is a sign of madness).

the transducer responsible for getting the movement of the guitar strings into the amplifier is generically called a “pickup”. it is a magnetic pickup related intimately to an alternator… the magnet(s) and coil are fixed and the steel strings move. by varying the field in proximity to the coil, an alternating current is induced in said coil. there are 2 common arrangements of this design: the single coil and the humbucker. the humbucker is essentially just two out of phase series connected single coils with one positioned to cancel common mode… (by reversing the magnetic field in one) it does this at the expense of some bandwidth, but gains output.

as you can imagine, it is not a particularly sensitive approach… the strings don’t have much mass or flux varying permeability and the magnets have to be powerful and the coil thousands of turns. just in order to get 50 – 300 mV of peak AC signal… all that copper and impedance invites stray pickup… especially of line voltage. shielding helps, but the long cables typically employed mean hum and noise pickup. i can imagine jeffrey jackson or dave slagle making field coil pickups, with pure iron or permendur poles! if they don’t, someone will sooner or later. but it was talked about here, first, you johnny come latelys… i don’t care. it won’t make or break the system. humbuckers can double the output and can have decent common mode rejection… but the sound is different. a bit less dynamic. more rolled off and midrangey. fortunately, musicians are WAY more pragmatic about this sort of thing than audiophiles generally are. the electronics that follow are heavily relied upon to fix, enhance, or modify shortcomings of the transducer/instrument front end.

okay, given that the input range will go from nothing to approximately 50mV (vintage single coil) and up to 1 V peak to peak (“super hot” humbucker), that gives one a place to start. the amplifiers typically used for guitar sound reinforcement are already designed with this input sensitivity in mind. exceeding it will overdrive the amp. that is a discussion for another day… because there is always some kind of input level adjust at the amp, differences in pickup type and output level can be taken to account. in designing an overdrive effect unit, we will want to match the output range to something similar.

in a previous post, i drew a test jig in which the bias, load and power supply range could all be adjusted. the purpose of doing this was to be able to practically experience what shifting the operating point deliberately into the “wrong” could do for effect. if we drive this stage with a good clean signal, and provide some guitar sensitive eq to the result, this will make an excellent test bed for overdrive. let’s have a closer look how this might be arranged…

the original concept has been developed here to include a safely adjustable B+. with the “bias” pot at 2.7K, the “load” pot at 220K, and the B+ pot at minimum setting, this is the operating point you might wind up with… lets look at the waveform.

ahhh, nicely clipped! with 1 VAC pp sine wave in, we have a very slightly asymmetrically clipped 70 VAC pp. with only 630 mV of bias on the 12AX7, even self biased, we have totally exceeded the input range.  lets mess up something else… just by turning the “load” pot down to 20K, and leaving the other pots as they were, we get:

and the waveform is now very asymmetrically clipped and the distortion is much more complex.

finally, i will leave you with the next logical development. here, we have a “clean” gain stage with some house cleaning added… the 68K grid resistor helps with the rfi and emi crap the cable and pickups find. the cap bypass, so often hated in hifi, and so truly beloved in musical electronics, is very important for leading edge “bite” and dynamics. some of the “touch” comes from doing it this way. now, the input range can be pickup sized: it’s shown as 50 mV. danelectro territory. the 1 meg input gain adjust pot is the grid leak for the overdriven stage… with the “bias”, “load”, and B+ pots all set as shown, here is what you get…

here are images of representative waveforms of this approach. soon, we can add some EQ and further housecleaning…

ok, i should stop. but here is the added extra stage arrangement… not as well behaved as the first one. but it has adjustable output level. you may not care. these are some of the techniques one can begin with to make interesting distortion effects.  this is enough.

among the diy hifi 1%ers and the ultrafi diaspora, distortion has mainly received a seriously bum rap. yes, some like to give the 2nd order harmonic distortion artifacts an easier time of things, much to the mainstream tradition’s chagrin. most often, distortion measurements are used as a blunt weapon to humiliate, belittle and cajole the errant peers and maladroits of the electronic “sciences”…  and of course, as meaningless marketing. there is some older research to back up the lack of importance of even order harmonic distortion in terms of listening. but is it really all that bad. for playback, within reason, i say yes. the whole rationale for hifi hangs in the balance…! but is distortion just inherently bad? as it turns out, for music making, there is almost no amount of distortion too much… in certain quarters, it is the very foundation of musical flavor.

one interesting observation after many years of looking at practical linear circuits: most of the really troublesome analog problems come from switching behavior… interestingly, most of the problems facing digital systems are analog effects. turning on or off is not something any analog device does perfectly. all the devices we use to make digital systems are completely analog at some range. in terms of making electronic distortion, this dualism/factoid has much to offer.

ahh, distortion, oh glorious buzz, crystalline harmonics and angelic glitch! a head full of droned out cranked up amps and i see two opalescent visions of myself, and there’s one rising from each of my shiny boots… and if the melvins, swans (from back in the day, “nobody beats you like a cop, when you’re in jail…”), and cop shoot cop (shine on elizabeth!) don’t wake you tha fuck up to the world of right now, nothing will. maybe you are wondering what the hell i am ranting about?

fundamentally speaking, “distortion” is any deviation from the “original” signal that results from passing through a stage of amplification or other electronic function. obviously most functions create distortion under this definition. just think of a log amp, or an equalizer… the very concept of EQ actually puts passive speaker crossovers into a whole new light: distortion boxes! but most hifi people think only about linear amplifiers. enormous effort goes into making a “transfer system” (having a transfer characteristic) which is in all possible ways equivalent to the input multiplied by the gain (a fixed number). one overlooked issue among many engineers and hackers is that just by using an amplifier, we have given up on this idea, because the time it takes to traverse such a practical transfer system made with the devices we actually have to use. the inertia added makes it impossible. time, all by itself, has a say. the deviation can be admittedly small, and can be rationalized by the tradition as a reasonable “group delay” to be factored in. but i am a fan of honesty in electronics. the more stages, the more complex the demands on the supply, the greater the inertia and delay. but i stray…

in this case, we can do the typical western intellectual division of the universe into two opposite forces (one of them is always implied to be “bad”, without saying so. mind/body, mind/nature, male/female, etc.) and describe the types of distortion as “correlated” or “linear”, and “uncorrelated”. correlated distortions are largely going to be a matter of amplitude and phase (time) variations. simple harmonic distortion comes to mind… all the rest falls neatly into the other category as “non-linear” distortions. some of those are sort of “semi-correlated”, such as inter-modulation distortion (sum and difference) which has roots in the linear stuff. but some stuff is just plain whacko, extra evil gnarly shit and who wants to share space with that? (it’s all a matter of perspective…) however, as it turns out, there are subtle interconnections between the two sides that cannot be totally separated. as is nearly always the case!

an interesting analog question to raise is whether or not there are parallels in the material world to these electronic distortions? the answer is yes absolutely, and no. there are some very obvious comparisons. air, for example, is very compressible. but not linearly so; that is to say, non-linear at small and great magnitudes of compression. the sound of lightning, the thunder clap, is a perfect example of a complex acoustic pulse of large magnitude containing all sorts of linear and non-linear components… all created by rarefying and compressing (and ionizing) air, very suddenly. there are more eloquent people to talk about this than me. the amplitude and phase deviations possible, along with the more complex failure mechanisms, one observes with practical transfer systems can also occur in the natural world. not easy to compete with a thunder clap, though… are there particularly electronic distortions that have no equivalent in the material world? yes. and they are all non-linear. more about this later.

most physical musical instrument sounds are ultimately dependent upon distortion of the fundamental event that generates it, even if the spectral content can include mainly non-linear components as well! think of drums, or bells, and other mallet instruments. the sound they make is fundamentally 2nd harmonic: the strike is heard as a high odd order pulse (which could be very complex if it is a hard stick, or simpler if it is a felt ball) but compresses the air in the column or the material of the piston (might be skin, wood or steel, or…) and damps it. the dimensions of the piston and the speed of sound in the material it is made of, determine the root characteristics, but getting it moving requires energy, which in this case comes only from the mallet. the mallet “stops” the piston during the blow. the release can, and usually is, much greater in magnitude than the blow… this is out of phase with the strike and distorted in amplitude over time… etc. the resonant qualities of the “air column” behind the piston (if there is one) filter or store the energy further, enhancing or attenuating some frequencies at the expense of most others. just trying to describe this in such a simplified way gives one the sense of just how “un” linear the creation of physical sounds is. drums are fairly simple! think of a piano… distortion is very much at the heart of our musical taste and imagination.

what makes distortion “musical” or just plain nasty? let’s keep an open mind about this for the time being… this is a long conversation. however, i can give one important and simple starting point as a reference. the sound of rock and roll (and the blues!), is mainly the sound of 3rd harmonic distortion, in higher or lesser amounts. this is a good place to start because it isn’t as complex as a ring modulator or bit crusher… and many people know what rock and roll guitars, keyboards and vocals are supposed to sound like (?). 3rd harmonic distortion is created in abundance in the crappy output stages of push pull beam tetrodes amps (Fender), and in the overdriving preamplifiers of many other well known musical instrument amps and pedals. below is a graphic for 20% 3rd harmonic… a typical value in a blues guitar sound. this is minus the treble boost, mid cut and bass boost you would also typically want… one step at a time.

so, back to the original question, but now lets ask: how do i do that? well, i am going to describe some of the most basic variations of electronic distortion creation, and suggest gently, as is my wont, what the combinations might beckon… the goal here is primarily 3rd harmonic in character. but if we go a little further… cthulu!

one of the most obvious and brute force electronic distortions is clipping distortion. this is what happens when a signal is, by various means, hard limited in its amplitude. as signal amplitude increases beyond a certain threshold, the “top” and/or “bottom” of the waveform is “clipped off”. this can be done with analog means by using a switch, arranging the bias, the power supply, or the gain of the system to deliberately exceed the linear operating range with the available signal. for example, you can make any linear amp into a clipper simply by sending 100 times the input range into it! a further nuance of this approach is whether or not the onset of clipping is “soft” or “hard”: soft clipping is notable in its slower transition to “clipped”… a gradual rounding over that ultimately results in the limiting value. hard clipping is fast and unambiguous. soft clipping is much more complex sonically, even though hard clipping produces far more odd order harmonics and is brighter. the gooey slewing of soft clipping is an area ripe with variations and subtle differences…

in the image below you see a basic diode clipper… two shunt connected diodes, in reverse, will turn “on” at 600mV (the forward voltage of a 1N4148) or so, “shorting” the signal to the clipping threshold (the forward voltage). with 1 volt pp AC in, there is still some sense of the sinewave visible in the slope of the waveform between the clipped tops and bottoms. in the image below that, that is no longer obvious because the signal level has been increased to 20V pp, and the result is almost a square wave. almost because the top is not perfectly flat! diodes are not apparently perfect switches after all?

one of the simplest ways to improve upon the quality of the switch is to put it in a feedback loop (ahh, the dreaded nfb has already appeared). below, you can see how this would be implemented.

here it is… and below that an image of the waveform. here we don’t have the need to drive the diodes with a large signal (the 12AX7 does that) and the top is flatter, although still not perfectly flat! the small bypass cap and the input “impedance” are also necessary for symmetry. the 12AX7 does need to be loaded well, over the frequency range of interest. additional steps can be taken to “improve” the “fidelity” of the clipping (steeper sides and flatter tops).

this is one simple technique, and kind of a blunt tool at that. with this kind of technique, no matter what the size of the original signal, as long as it exceeds the threshold of the clipper, the output is limited and distorted in a predictable way. it is easy to see the similarity between the ideal 3rd harmonic distorted sine wave and what a clipper can do at modest and overdriven levels. this is classically known in the rock and roll business as “fuzz”

traditional fuzz boxes/diode clippers like the “big muff” or the “octavia” or even the “tube screamer” are all based on this approach. but what about more sensitive techniques? perhaps something that can straddle the linear into the hard clipped? for this, we can look first at deliberately choosing the “wrong” operating point for an active device, so as to create a range where the signal level can determine how clean or distorted the output might be… the bias, DC operating points can be chosen to make this possible. another technique could be to cascade specially designed stages so as to optimize just how “wrong” this can get.

next is a test jig you could make that uses pots to make the bias, loading and B+ all adjustable, simply so you could vary everything and really see what “wrong” can do. immediately you should notice that in the middle, the variations don’t make a huge difference… at the more extreme settings, things get more screwed up.

this can be tried with vocals, guitar or keyboard sources for some interesting effects. remember that more signal conditioning will be necessary to make these sounds useful and able to couple to your other equipment. i am trying to talk fundamentals here.

LT spice doesn’t do pots, so i have drawn them as series connected resistors with the wiper connection between…

a 12AX7 is a good choice for this job because it is high impedance and will only draw 5 mA or so if you turn low the bias, load and B+ pots… it will still cook if you leave it that way, but not much danger to anyone if it burns up. cheap or used 12AX7s are ideal for this treatment. russian 12AX7WAs are perfect. because the 12AX7 is a dual, it is a great opportunity to cascade two identical stages like this for extra sick weirdness. the work isn’t done yet because some housecleaning and eq will be necessary to really get the most out of what different kinds of distortion you can pull out of it. but this could get you started… you can also have another look at the completed distorting preamp i did earlier in the tremolo article, for tips with this.

cascading two FB clippers will also do wonders… here you have the core of a tubed “big Muff” pi, minus the tone control and buffer… i’ll save that for later… the waveform follows…

this was fun. more to come…