simmconn

I was wondering... why the first and second stages are designed at such low operating points. 12AU7 is at 1.2mA and 12AX7 at 0.5mA. Wouldn't higher operating points give better distortion and lower output impedance and hence better frequency response at the higher end?

Oh I'll definitely put these in a proper enclosure with volume control and input selection, etc. This is just temporary. By the way, do you have any suggestions to reduce the gain? Increasing the NFB would certainly do it. I'm more looking for a way to reduce the open loop gain I guess.

I built the amp on a heat sink taken from an old Denon home theater receiver, and decided to try it out before investing more time and $$ on a 'real' enclosure. It just so happens that the heat sink has the exact length for two boards mounted side-by-side, and width to mount the transistors on both the PSU and amp boards. The big electrolytic caps are mounted on the opposite side of the PSU board to double as temporary mechanical support. You can see the green Soviet military style HV transformer. I got it a number of years ago on ebay when the international shipping wasn't that crazy. It's better suited for circuit requiring 400V or 350V B+, though.

I just finished building the KGSSHV Carbon and Golden reference PSUs, well, using the PCBs from AliExpress. The performance is superb. It registers 0.0006x% THD+N up to 583Vrms and the distortion profile is very well behaved (2nd order slightly higher than 3rd order). The noise floor is at 1.1mVrms wide-band without power-line frequency harmonics. I found that my comfortable listening level is around 10Vrms and would probably never need that kind of dynamic headroom. Is there a lower gain option or mod that can bring the noise down?