Negative feedback tone control ideas

[midwayfair]

I've been busy!

Okay, so there aren't a ton of pedals to use these in, but if you have an amp with negative feedback, they're easy additions. But we do use negative feedback in things like Fuzz Faces (that's the 100K resistor), so maybe some of these will be of use!

Anyway, if you're not familiar with how negative feedback works, Uncle Doug has a good vid:

Here's a little schematic I drew with some ideas:



Note that a lot of these look familiar -- I'm just proposing putting them on the emitter or cathode of a circuit in a negative feedback loop, which makes it sort of "semi active", and won't load the transistor or valve it's affecting the sound of. (It will load the place you take the feedback from, but that's usually a low-impendance place in the circuit!), which could be extremely useful in some circumstances. They do remove some gain overall, but sometimes that's a good thing, and it's not as bad as some circuits. The mid scoop circuit I show, for instance, is a very mild notch, less severe than a blackface, but far GAINIER as well.

[GrindCustoms]

That's definitly a «printable» to keep handy!

Thanks for sharing this Jon.

[jtn191]

I like #1. You could do a blend between two different values of bypass capacitor something effectively like a blend between 22uF and 220nF. The control becomes a Bass boost control.

[midwayfair]

You could do a blend between two different values of bypass capacitor something effectively like a blend between 22uF and 220nF.

Yup, AMZ suggested that as an alternate gain control for the MOSFET booster:

[chromesphere]

Really interesting Jon, thanks for sharing.  Subscribed to Uncle Doug.  Idk if it would work so simply, but I would be interested to see how NFB pot and presence control would work with the fuzz face.  Its already got the resistor right there.  Could make for a more interesting tone control then SWTC or a simple input cap tone control.  Fuzz face + treble boost has always been a nice combination imo. Might have to hit the breadboard!  Thanks for the inspiration!
Paul

[stecykmi]

I like #1. You could do a blend between two different values of bypass capacitor something effectively like a blend between 22uF and 220nF. The control becomes a Bass boost control.

cool chart, but it should be only considered illustrative, not universal. the cut-off varies with the value of the emitter resistor / gain. there is a formula to calculate the capacitor or cut off that shouldn't be too hard to google if anyone is interested.

[jtn191]

Thanks for the info. There's an online calculator on ampbooks.com. Guess that picture references the most common scenario (100k plate resistor etc)

[midwayfair]

I like #1. You could do a blend between two different values of bypass capacitor something effectively like a blend between 22uF and 220nF. The control becomes a Bass boost control.

cool chart, but it should be only considered illustrative, not universal. the cut-off varies with the value of the emitter resistor / gain. there is a formula to calculate the capacitor or cut off that shouldn't be too hard to google if anyone is interested.

Yeah, I should have brought this up.

It's 1/(2pi * R1 in ohms * C1 in Farrad)

The series resistance coming from the the negative feedback source is in parallel with R1 I think, and C6's high-pass filter is calculated against the cathode resistor.

Some people may recognize this as the generic tone cutoff frequency formula. I didn't find the calculator on Ampbooks useful, because it doesn't give you a number, just a chart. I spent a day futzing around with a calculator before I realized that you can just use this: http://sim.okawa-denshi.jp/en/CRhikeisan.htm

It gives the proper cutoffs. Merlin's chart is, I believe, done with a 1k8 cathode resistor.

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Chromosphere, for the fuzz face, you can do it a couple ways. You can split the resistor in the middle and put a cap to ground. You could ditch the "fuzz" part and shrink the emitter cap on the second transistor to brighten the sound. You could put a capacitor in series with the fuzz face's negative feedback like the bottom left (should be #5) to potentially get a bass boost, or do that whole setup and see if it gives you a midrange boost. No idea how big of an effect it will have on the fuzz face!

[chromesphere]

Thanks Jon!  Gunna have to try this could make for a really interesting / different tone control.

[midwayfair]

Check out the schematic at this time stamp in the newest Uncle Doug video for a really complex negative feedback tone control.

[ChrisM]

Check out the Garnett tone control. It's a cool (simple) tone control circuit that interfaces between the plate and the cathode.

[davent]

Check out the schematic at this time stamp in the newest Uncle Doug video for a really complex negative feedback tone control.