Re: Tone Controls
Turns out that incorporating the Baxandall circuit into a power amp feedback path turned into fairly common use. There is even a short section of Doug Self's Audio Power Amplifier Design book, page 101 of 6th ed. "Power Amplifiers combined with Tone Controls". He says it was/is fairly common in Japanese amplifiers, and picks the Yamaha AX500 from 1987 as just one example of "many many more".
He says that audio power amplifier design is challenging enough without putting gain variation in the feedback loop of 20dB or so, that he finds the whole business puzzling.
So although the Armstrong might seem uniquely quirky, the Japanese took the idea up with a gusto.
Craig
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