I had some great results in experiments with an SE amp design. In that amp, I took feedback from the output tube plate and applied it to a high-gain pentode/p-channel FET input stage. This gave fantastic results. The amp had very low distortion and Zout. (0.027% THD @ 1W and 0.7 Ohms)
This got me thinking about applying this approach to push-pull amps and here is what I came up with for a Unity-Coupled amp. A couple of things stand out:
- Feedback from the output stage plate can be taken from the output tube cathode, since cathode and anode swings are equal in magnitude. The great thing about this is that the cathode sits near GND potential at idle so you don't have a bunch of power dissipated in the feedback resistor at idle.
- Creating a feedback network that works to restore balance in the amp is easy since the feedback network idles at a low voltage. The feedback network can be direct-coupled without needing to be pulled down to something near the input stage grid or cathode potential. It's already there. This makes the input stage work similar to the input stage of an instrumentation amp.
I drive the cathode of the input tube with a p-channel FET here and use the grid as the feedback node, and I configure the feedback network like the input stage of an instrumentation amplifier to correct imbalance in the amplifier.
The source follower drivers for the output tubes aren't really necessary. They simply reflect the way the output stage in my Unity-Coupled amplifier is currently configured. I like the immunity to blocking distortion that they offer so I will keep them there.
The Plitron transformers have pretty low DCRs in the windings so I expect that this amplifier would deliver very low distortion and a Zout of less than 0.4 Ohms, even without feedback around the output transformer.
Some day I'll work something like this into my Unity-Coupled amplifier.
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