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Pro and Cons of NFB Circuit

Discussion in 'Shock Brother's DIY Amps' started by andrewRneumann, Aug 24, 2020.

  1. andrewRneumann

    andrewRneumann Tele-Holic

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    NFB-Circuit (1).png

    I'm sure someone has tried this before. It's a variable feedback pot with a high-pass filter on the wiper. I'm curious what everyone thinks and if there are any drawbacks/pitfalls to this circuit. The idea is to feedback more high frequencies and thus cut the high-end more than the low end when the NFB pot (R2) is turned down (wiper to the right in the schematic).
     
  2. jsnwhite619

    jsnwhite619 Friend of Leo's

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    I've used a NFB bright cap for that reason. I think it's called an ice pick cap. 100pf or smaller cap bypassing the NFB resitor to cut some extreme treble.
     
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  3. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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    So you want resonance/thump control?
     
  4. andrewRneumann

    andrewRneumann Tele-Holic

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    I’m don’t know what to call it. I want to control the high frequencies generated in the power section in overdrive. And I want to do it with the NFB instead of adding yet another control.
     
  5. andrewRneumann

    andrewRneumann Tele-Holic

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    That’s what I want to accomplish, but I have this pot and I want to make the ice pick filter adjustable so the player can put the high freqs back in if into that sort of thing.

    Now that I think about it, your version might accomplish the exact same thing but is simpler. I will experiment.
     
  6. jsnwhite619

    jsnwhite619 Friend of Leo's

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    I'd start with 100pf or smaller. It doesn't take much to turn a filter into a blanket.
     
  7. XTRXTR

    XTRXTR TDPRI Member

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    Why not use an ON OFF ON with or ON ON ON with different caps. You could even rig up two switches, one on each speaker 16,8,4 and then the other bypassing the NFB resistor. I was thinking of that on my amp as one of the mods to experiment with. With the ON OFF ON the center would always be in circuit so you could start really small and add on either side for instance 47pf in center and 47pf on one side and 100pf on the other so you end up with basically 100/47/150pf. When I get to it i want to see what it does when its really cranked for stage volume.
     
  8. andrewRneumann

    andrewRneumann Tele-Holic

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    Learning on this: I tried both methods described above. Although the NFB bypass cap is really simple, it relies on the value of the cathode resistor to create it's potential divider. On an amp where all these values are set and unchanging, that is probably the way to go. Unfortunately, this is a homebrew amp and ALL the values are subject to change, which means if I go back and change the cathode resistor, I changed the frequency response of the NFB. So the pro of creating the filter early is that it is independent of what you hook the circuit up to.

    I tried various values of C1 with a substitution box and 100n was indeed the most effective with the NFB pumping hard. It's hard to know if it's "better" but it works.

    @XTRXTR I appreciate the ideas! But what I'm looking for is a little more treble cut with the controls I already have. Right now the "presence" knob can really take the edge off the final product. Or it can be turned up and things get a bit more bright (or blaring if overdriving). Or it can be put somewhere in between. I've been spending some time with the amp right side up... and just playing and listening. Sort of an "engineering freeze" on all work.
     
  9. Guitarteach

    Guitarteach Doctor of Teleocity

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    I put a variable NFB pot in my champ alike... i like it.

    On my ‘79 JMP 50w, I added a similar NFB mod to add some thump/resonance. Its essentially like a presence control in some configurations, so its nice to have it in a pot to dial it in.

    def, worth a play.
     
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