Negative feedback tone control idea

[midwayfair]

I was looking at negative feedback and presence controls, and I thought there might be something of use in there. It looked to me as though in amps, the presence control works out to, essentially, a variable bypass cap sort of doing double duty ... highs are boosted because the AC gain at a frequency goes up, and the negative feedback loop loses those frequencies, so they aren't cancelled by the negative feedback. Maybe someone here can explain if that has any benefits over a more traditional tone control, or if maybe it's just overly complicated, but I also thought to myself that I hadn't really seen a negative feedback control (in pedals, or amps, though I'm not as familiar with the latter) that did anything with the BASS frequencies. So I drew up some ideas (the two on the left are old familiar arrangements obviously, the two on the right are things I was wondering if they'd work):



Unfortunately, the only pedals I can think of that use negative feedback are Fuzz Face and Tone Bender type things, where the bypass cap is large enough that it's going to eat most of the frequencies and the negative feedback would have less and less of an effect as the gain is turned up (am I correct about that?), but I THINK op amps use a similar connection with a resistor and capacitor, right? Is there any way to apply this idea to that? I do know I've seen small caps used to bleed off high frequencies in the middle of the feedback loop in a fuzz face type circuit, but I can't say where I saw that. (Though I'm willing to bet Gus has done it!)

#3 might be unworkable in some ways ... I would think that the resistance across C3 would have to be large enough that it might futz with the negative feedback loop. The values for #4, if we go by AMZ's stupidly wonderful tone control, are much more in the realm of possibility, since they're 57K, which is close to the Bandmaster's value, and half of a Fuzz Face's 100K (could be reworked to hit that).

[tca]

Take a look at Garnet amplifiers preamps. They work really well with mosfets (with a few changes).

Cheers.

[midwayfair]

Take a look at Garnet amplifiers preamps. They work really well with mosfets (with a few changes).

Cheers.

Yeah, you showed me the Garnet preamp tone control. I'm having trouble finding legible schematics for those amps. Do any of them use a negative feedback control like the presence control in Fender amps? Or does that control count as negative feedback? The wiper goes to ground, so I would think there's no negative feedback on that control.

[tca]

> I'm having trouble finding legible schematics for those amps.
... that and good trustworthy schematics of those.

The following are good!

See here:
- http://www.lynx.bc.ca/~jc/garnetAmps.html
- http://www.lynx.bc.ca/~jc/garnet.html

Cheers.

[Pojo]

but I also thought to myself that I hadn't really seen a negative feedback control (in pedals, or amps, though I'm not as familiar with the latter) that did anything with the BASS frequencies.

I think the resonance control some amps have (ei. peavey 5150) is basically a power amp bass control that functions via negative feedback. I like your ideas though and would like to dig into them for my 'never finished' little amp I have going on. It has switchable neg. feedback using an on/off/on to be able to select 2 different resistances but rarely use it in anything other than off completely.

[midwayfair]

Okay, I can confirm that putting a cap to cut the bass in the negative feedback loop works. Simply adding it in series with the NFB resistor in #2 in the above schematic seems to put a bass cut on the NFB and then a treble cut; this makes the NFB loop a bandpass, with the final result being a little dip in the midrange and a boost of both the bass and treble! Very handy, and it definitely has a different flavor than put negative feedback. I used the next lowest value for that cap than the "C2" value. And it's far gainier than leaving in a blackface tone stack, but not as gainy as lifting the negative feedback altogether.

This is really cool ... I might have to make a Fuzz Face style circuit like that.

Updated the schematic in the first post for scoop and boost NFB controls.