mic pre power (part 4)

this next installment of the EH 12AY7 mod series is going to deal with the power situation. the original has a very clever (if i say so myself) and very economical power supply completely organized around the idea of compact size and market/regulatory acceptability… (much of the pedal world functions far far away from the high voltage world of vacuum tubes). it makes all of the various voltages needed from one 12 volt AC wall wart, with few parts and without any switchmode anything (still too noisy for cheap analog). while fulfilling the design requirements, it is not ideal in terms of the best performance. mainly, because of the limits of the wall wart and supply. also, the method for distributing all of the various currents interconnects them… a drag on any one of them, really affects them all. this is mainly a problem because of the phantom supply. certain modern mics draw a fair amount of current from the phantom. some of them, more than is available for the preamp…

the upgraded circuit improves the isolation and the drive capability of the buffer, but also requires even more current than the original. the solution for all this is a more optimized and dedicated set of supplies for the various jobs at hand. this way we can optimize the various functions. let’s start with the big one: the high voltage B+.

double regulated 220 VDC supply

ok, here you see a pretty straightforward solid state regulator. it has three basic sub-circuits. it is based in it’s entirety on National Semi’s© venerable applications propaganda for the LM317. i am a big fan (although they didn’t write this stuff, bob pease and jim williams are such heroes for me. because of guys like them, the quality of the writing and depth is routinely excellent. the national applications notes are deeply useful and well done. the guy who did the work for the LM317 was very matter of fact and sometimes really funny). an added mosfet current limiter ahead of the chip, and a transconductance regulator after, make it much more sophisticated and safer than originally proposed. some small refinements to reduce the likelihood of high frequency oscillation, and better short circuit protection have also been added. a few words about all of this are warranted.

the high voltage rail calls for 220 Volts DC. this was provided for with a “pi filter” following the rectifiers in the original. in order to reduce hum and variability of the rail, we can arrange for a separate supply and then regulate it. mic pres often deal with tiny signals and the balanced differential circuit used in the design of the mic pre only has so much power supply rejection, and it works less well the higher in frequency you go. a tightly regulated supply takes care of that! however, an interesting problem does arise. high feedback regulators, such as the LM317, or even any of it’s improved versions (such as the wonderful LT1084, 85, 85…), also make high frequency noise that isn’t so easy to compensate for. all amplifiers add noise, but especially those with gain, so it isn’t that remarkable. it’s analog! deal with it. those chips have a LOT of gain. the unregulated supply did have some hum and sag, but no high frequency crap at all. now you know why the DIY world is so dependent on passive solutions… it takes work to deal with this crap! the remedy is not that complex. following the chip with a “no gain” regulator, the stiff regulation is mostly retained, but the noise is reduced by a huge amount. we can also decouple the final regulator with a small film cap, which will respond to transients with aplomb.

let’s have a closer look at the sub-circuits involved. the first mosfet is your basic source follower, with the reference voltage derived from the regulated output of the chip regulator downstream. the 100k resistor and the 15 volt zener make sure the gate is never more than 15 volts away from the output of the chip regulator. the 20 ohm resistor is sized so that if there is a short circuit at the output of the reg., the increased voltage drop across it will begin to cut off the mosfet and protect the LM317. this could be bigger in this case because the total current draw is so small, but since it will be used for stereo, and i am lazy…  the mosfet will heat up a lot under shorted conditions (a decent heatsink is needed on all the chips), but normally doesn’t get hot. the LM317 has a maximum voltage rating of only 35 volts. by combining it with the mosfet in this way, it can handle 100s of volts.

how is that? the LM317 and it’s family of adjustable regulator chips are “floating” regulators. the built in error amp works to match the drop across the reference resistor (in this case, R6), which is connected to the output, with it’s internal voltage reference. in this arrangement, a better more accurate reference is used instead (the LM329). the point is, this drop can be set without a direct ground connection, hence the “floating” moniker. the LM317 can source up to 1.5 amps provided it has a good heatsink and is protected from shorts. the Linear Technology© versions have better performance all around, but do cost more too. i use them all the time for low and high voltage regulation.

the final sub-circuit is very important. sometimes called a “capacitance multiplier” or a “transconductance regulator”, it is essentially a source follower (cathode follower for the vacuum tube folk) with a high impedance voltage reference on the gate that is bypassed heavily with a large electrolytic. this is important. the time constant needs to be LONG. that will multiply the effective “capacitance” of the output by the Gm of the device. a small film cap at the output can be made to behave as if it is orders of magnitude larger. handy. and it has no gain! if arranged in a stable configuration, the output noise can be in the nanovolt range. in this case, the uVolt range is good enough! we are talking about a 200 volt supply! 220V/1uV is 10^9 noise reduction! this is an improvement over the 2mV hum level of the original.

in the case above, i have deliberately injected 100mV of 60Hz hum on the 250 volt “unregulated” input. the 0.8 mV PP output ripple across a 10K load (22mA) speaks for itself (you will need to download LT spice to sim this). mosfets have the Gm and low rON necessary to do this right, but tubes can do a fair job of it too! as long as the load is constant, the supply won’t sag. additionally, while the impedance of a 6V6 or 6L6 cathode follower can’t match the mosfet, it would  be fine for line level stuff, or a screen reg in a push pull amp.

some will complain that it isn’t a tube regulator. i don’t care. tube regulators can sound great! but they are not capable of this level of performance. at least not without a large increase in complexity and cost. and you will never reach the really low noise level. however, if you are willing to go hybrid, there are some interesting possibilities… even just following a tube reg with a mosfet “no gain” reg will do quite a job. that’s for another time.

the heater supply on the original pre amp PC board is regulated and can be kept as is. the prudent upgrade is to feed it from it’s own transformer and filter. perhaps, just removing the chip and tiny heatsink from the board and mounting it on the chassis or larger heatsink will be enough improvement there… we’ll see.

the phantom supply is next.

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