Global Negitive Feedback with Top Cut?

[DuaneOh]

I am designing a new amp and have a question about feedback and how it interacts with a Vox style top cut control. I am planning on making the feedback switchable with 3 positions, high and low feedback and off. I am wondering how much a top cut control would screw with the negative feedback loop?

I'm thinking that maybe when the top cut is turned up the feedback loop will try to boost the highs so it will have little effect?

Can a top cut control cause a phase lag in the feedback loop resulting in oscillation?

[thetragichero]

i have nothing but empirical results, but it seems to work fine in a build with switchable negative feedback and the cut control as the only tone control

[2deaf]

I'm thinking that maybe when the top cut is turned up the feedback loop will try to boost the highs so it will have little effect?

Naw.  It'll still work.  Even if you feed the entire output signal back it would still work, just not as well.

[2deaf]

I just looked at your profile.  It says you're from San Luis.  I grew up in the hills above Pismo.  We used to go to San Luis all the time to cruise on Friday nights in the sixties and seventies.

[DuaneOh]

Naw.  It'll still work.  Even if you feed the entire output signal back it would still work, just not as well.

Thanks for your help, still trying to understand negative feedback and will probably have more questions.

I just looked at your profile.  It says you're from San Luis.  I grew up in the hills above Pismo.  We used to go to San Luis all the time to cruise on Friday nights in the sixties and seventies.

I moved to SLO about 3 years ago when I retired to escape the rat race of Orange county. I have a good friend who went to Cal Poly in the '70's and came to visit, I've wanted to live here ever since.

[2deaf]

Thanks for your help, still trying to understand negative feedback and will probably have more questions.

For this discussion it's all about the gain without NFB (open loop gain) and the gain with NFB (closed loop gain).  Here is a cheat sheet for NFB with an amp with a LTPI and an example with the Fender AB763 Pro-Amp.  Feel free to ask me any questions about this.

 

[wittyjeff]

I've just finished up an AC30 build with switchable NFB.  Mine works great. It didn't seem to bother the Top Cut at all, although NFB is always going to affect the tone and may require adjustments of the tone controls and/or Top Cut to get the exact tone with NFB engaged.
I only have on or off on this build.  I've used a three-position switch with two settings of NFB in past and definitely will again).  In this build I ended up using a 33K resistor (with an "ice pick" bypass cap) to give it more NFB, 45K was more subtle so I decided to go with a bit more (it would be nice to have both options though). In my particular build, I wouldn't go any higher than 33K, it's a significant NFB.  I like your idea of giving the option of a heavy, light, and no NFB.  You'll have to try some different resistor values to get it just right.  I've included my schematic in case it's helpful.
Best of luck in your build!