This project is a follow-up to previous experiments with the cheap MCM 555-7125 speaker line matching transformer. The goals I had in mind were to prove that it's possible to make a decent single-ended power amp with this transformer and to produce a listening experience dominated by the characteristics of a single power triode -- free of any possible benefit or drawback from the driver section. My power output goal was 5W, mainly because that's about all I expected from the "30W" OPT at very low frequencies.
I feel that I've succeeded, to the point that I now plan to transplant the breadboard circuit into a two-channel amplifier chassis with it's own PSU. Midrange power output approaches 10W at clipping with regulated lab power supplies. Grid current commences at 5W. There is no visible change in the THD analyzer residual waveform as power output increases beyond that point. The 3.3nF feedback cap was needed only to insure no-load stability. It has no obvious effect on 10KHz squarewave response. The 100Hz squarewave shows a small linear tilt. At 1KHz, there is no visible deviation from perfect.
Regarding the voltage amplifier section, I owe a nod to Frank Blöhbaum for his articles on Multiplied Transconductance Amplifiers in Linear Audio.
The secondary flux cancellation feature produced a surprise: LF distortion at higher power output levels can be reduced significantly by flux overcompensation. I haven't decided to pursue this finding any further at present, but the ability to control core flux independently of tube bias current is a unique feature of this architecture that could facilitate research.
I've attached the amplifier breadboard schematic and a proposed PSU schematic for two channels. Comments are invited.
I feel that I've succeeded, to the point that I now plan to transplant the breadboard circuit into a two-channel amplifier chassis with it's own PSU. Midrange power output approaches 10W at clipping with regulated lab power supplies. Grid current commences at 5W. There is no visible change in the THD analyzer residual waveform as power output increases beyond that point. The 3.3nF feedback cap was needed only to insure no-load stability. It has no obvious effect on 10KHz squarewave response. The 100Hz squarewave shows a small linear tilt. At 1KHz, there is no visible deviation from perfect.
Regarding the voltage amplifier section, I owe a nod to Frank Blöhbaum for his articles on Multiplied Transconductance Amplifiers in Linear Audio.
The secondary flux cancellation feature produced a surprise: LF distortion at higher power output levels can be reduced significantly by flux overcompensation. I haven't decided to pursue this finding any further at present, but the ability to control core flux independently of tube bias current is a unique feature of this architecture that could facilitate research.
I've attached the amplifier breadboard schematic and a proposed PSU schematic for two channels. Comments are invited.