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previous: Ken Gilbert I don't mean to rain on anybody's p... -- 11/4/1999 2:13 PM

Re: Advanced feedback loop questions..

11/5/1999 1:33 AM
SteveR	Re: Advanced feedback loop questions.. >>I suggest you all start playing around with LOCAL NFB, around ONE stage. It's very easy--just take your average gain stage and add a single small resistor from output (after the coupling cap) back to the grid. Voila, NFB loop.<< Ken, Can you recommend a value to start with? When you say small, are we talking tens of ohms, kilos, or megs? The Halfer Triple Giant has an NFB loop around one of the stages of the Clean channel. It's got a 22pf in series with a 4.7M resistor going back to the grid. The loop is taken right from the plate and there is a separate coupling cap to the next stage. There's a 2.2M resistor paralleled with another 22pf cap coming from the previous stage which the joins the NFB at the grid. (Clear as mud? I hate describing circuits with words) Anyway, 4.7 seems like a pretty BIG value resistor which means LESS feedback. What would be a good range of values? Ooh. Ooh! Duh. I could put in a pot (maybe a small trim pot) to make this a variable control, and it also just occured to me that tone controls here, particularly a baxandall stack, would make a tone section that didn't only cut signal. Instead, the "cut" of the tone control would be removing feedback signal ultimately boosting the net output. Hey it's a presence control!! Would this still have the 6dB per octave falloff that most passive controls provide? Any thoughts on this? -s

11/5/1999 1:33 AM

SteveR

Re: Advanced feedback loop questions..

>>I suggest you all start playing around with LOCAL NFB, around ONE stage. It's very easy--just take your
average gain stage and add a single small resistor from output (after the coupling cap) back to the grid. Voila,
NFB loop.<<

Ken,

Can you recommend a value to start with? When you say small, are we talking tens of ohms, kilos, or megs?

The Halfer Triple Giant has an NFB loop around one of the stages of the Clean channel. It's got a 22pf in series with a 4.7M resistor going back to the grid. The loop is taken right from the plate and there is a separate coupling cap to the next stage. There's a 2.2M resistor paralleled with another 22pf cap coming from the previous stage which the joins the NFB at the grid. (Clear as mud? I hate describing circuits with words)

Anyway, 4.7 seems like a pretty BIG value resistor which means LESS feedback. What would be a good range of values?

Ooh. Ooh! Duh. I could put in a pot (maybe a small trim pot) to make this a variable control, and it also just occured to me that tone controls here, particularly a baxandall stack, would make a tone section that didn't only cut signal. Instead, the "cut" of the tone control would be removing feedback signal ultimately boosting the net output. Hey it's a presence control!!

Would this still have the 6dB per octave falloff that most passive controls provide?

Any thoughts on this?

-s

Replies:

Ken Gilbert The value you can use depends on wh... -- 11/5/1999 9:02 PM