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previous: Ken Gilbert Yes Steve I've got that book. -- 11/3/1999 9:19 PM View Thread

Re: U.L. Fenders (calling Ken Gilbert)

11/4/1999 3:51 PM
Dave Stork
Re: U.L. Fenders (calling Ken Gilbert)
My own take on NFB in guitar amps...  
Some like the sound and "feel" of an amp that puts out a clean signal up the point of clipping, at which point distortion products increase exponentially due to the now nonlinear action of the loop. Some players find such an amp more useful than an open-loop arrangement where the distortion products increase gradually as you turn up the gain. An amp with at least some measure of NFB is useful when you want to go from crystal clean to balls out distorted with a flick of your guitar's volume pot. I've found it difficult to attain such a characteristic from open-loop output sections.  
I agree with Ken on a couple of points. NFB is a powerful tool that should be treated with respect. I get nervous when experimenters with rudimentary electronics knowledge and inadequate measuring equipment start diddling around with the NFB in their amplifiers. Fuck around with your phase-gain margin and your amp might turn into a dandy marginably stable amplifier/ultrasonic oscillator, although you'll never know it without a scope and the knowledge to test the stability of the finished product. I don't like the idea of playing around with NFB voltage dividers until it sounds rockin', dude :) Having said that, in a tube amp, it's generally fairly safe to experiment with reducing NFB slightly.  
I also don't really care for the classic presence control topology. Frequency selective elements within a NFB loop should be minimized. There's already the nonlinear Bode response of the transformer and speaker to deal with :)  
Lastly, I think the reason for the uniformity of NFB ratios in guitar amps is the fact that most amps are "designed" by copying what someone else did, no questions asked.