passfan wrote:swt wrote:can someone explain to me, please, the use of those feedback networks...sonically speaking...
Feedback is somewhat an error correction device. In this case I believe we are correcting for distortion in the signal and possibly linearity. By giving up output we inject that back into the circuit, out of phase (someone correct this if not so) with the original signal thereby cancelling out the offending artifacts and allowing the preamp to "boost" the signal even more. It's a balance between how much output and how much correction...like a see saw trading off one for the other. Dumbles use of local feedback (section specific) instead of global feedback may be part of the key to the SSS being so loud and clean. Hifi guys use feedback to correct for linearity and distortion all the time ..... sometimes to correct for poor circuit design due to lazyness.
EE here. Yes and no... negative feedback (NFB) effectively trades gain for reduced distortion and lower output impedance.
NFB does indeed make a stage more linear and "clean". The tradeoff is that the distortion that remains is the higher-order harmonics that people find more musically objectionable, so there's a balance between too little and too much.
NFB reduces the output impedance of a stage. This is useful for a couple of things. When used in the power amp, it increases the damping of the stage, which means that the power amp can more tightly control the motion of the speaker. (This makes the amp feel not as "tube-y" and many guitarists don't like the feel.) When used in the preamp, the lower output impedance means that the stage does a better job of driving capacitive loads like a tonestack or filters without the high frequencies getting rolled off (see feedback to cathode of V1a/V1b). When used in the reverb send a la ODR/SSS (see feedback to grid of V5), it increases the damping so the stage can do a better job of driving the reverb tank.
Finally, you can adjust the overall frequency response of a stage by tuning the NFB. When you adjust the cathode cap, you're actually tuning the NFB of that triode stage. You can see that Dumble did this by adjusting the value of the capacitor in the NFB loops to V1a and V1b (.1u in one and .22u in the other).
So when is it a good idea to use NFB? If a stage is never going to be driven into clipping, and the stage has more gain than you need (e.g. you're going to be dumping some/most of the signal with a voltage divider anyway), and lower distortion / reduced output impedance is something you care about, then it might be a good idea.
When is it not a good idea? I don't usually like too much of it in a stage that's going to be overdriven. The NFB results in a more sudden onset of distortion -- as long as there's gain/headroom to maintain the NFB, it stays clean, but when it gets pushed to the point that the negative feedback signal doesn't have much effect any more, then suddenly you've got clipping. This is why people love Vox AC30's for jangly dirty rhythm -- no global NFB means a seamless transition from OD to clean as the signal decreases. I used to have a Traynor Bass Mate with no global NFB that did that sound better than anything else I've played.