To misquote Morrissey, some knobs are better than others. The Manley Neo-Classic 300B amplifiers that I’ve been listening to, for example, have a knob marked “feedback” that goes from 0 to 10. I’ve learned so much from using it that I’ve come to believe that if your amp doesn’t have such a knob, it should. You see, the higher you set this control, the better the amp will measure. Applying more global negative feedback to these amps lowers their nonlinear distortion and noisefloor, increases their bandwidth, renders them less sensitive to the speaker’s impedance variations, and otherwise makes them more linear, stable, and efficient. In fact, by applying lots of feedback to an amplifier, it’s possible to reduce distortion to barely measurable levels.
So what’s the problem? Well, a few turns of the knob suggest that negative feedback isn’t as useful as it appears on paper. The Manley website urges the listener to dial in a “tasteful amount of feedback.” For me, that’s about 3dB, which tightens the bass without affecting the listening experience adversely. But turning the knob past 3dB progressively reduces my ability to enjoy the music: It robs it of color, texture, presence, and drama until these ravishing tube monoblocks begin to remind me of a receiver from the early years of solid state. Well, that might be an exaggeration, but you get the idea.
As it reduces the total amount of distortion, negative feedback adds higher order harmonics that sound nothing like the simple harmonics we associate with musical instruments, and after a while the brain begins to call bullshit on the idea that dialing in more of it is bringing us closer to the recording. The knob on the Manley amps makes these relationships—which usually reach audiophiles mainly in the
