okay, just a quick update.
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.