Feedback in Depth - Gain

[aziltz]

I was wondering if someone might to explain a few specific examples of gain circuit feedback to me?

3 Circuits I'd like to wrap my head around are the Klon, Honey Bee & something FET-Based like the ROG Prof. Tweed.
Schematics curteousy of Madbean and ROG:
http://www.madbeanpedals.com/cd553ghi/Sunking.pdf
http://www.madbeanpedals.com/cd553ghi/YellowShark_ver1.5.pdf
http://www.runoffgroove.com/professor.html


Let me try to throw down my understanding quick.

I pretty much comprehend how negative feedback works immediately around op-amps.  It seems like the Honey Bee has 2 extra sources for Negative Feedback around the CA3130, but I do not understand what effect the feedback running through the "Nature" Knob has, or if that's even feedback?

In the Klon, it seems like all the extra filtered paths run back to the input of a non-inverting amplifier.  Is this positive feedback or rather feedforward of some kind? 
These are the kind of circuits that seem interesting to me.


I also understand that Common-Source Amplifiers like the Fetzer Valve and its derivative stages in the Prof Tweed invert the output compared to the input.  So I can see where a Negative signal is derived from.  What I don't get specifically on the Prof. Tweed is the effect of running that signal back to the Drain of a FET stage.  What is the effect on the output?


Thanks in advance to anyone who can take a stab at this.

[Thomeeque]

http://www.madbeanpedals.com/cd553ghi/Sunking.pdf

Here's not any other feedback then the "immediately around op-amps" one Even it's bit hidden there, IC1B stage is normal non-inverting amp (with a little bit complicated gain adjusting network ) and IC2A stage is normal inverting amp used as a mixer (where two input branches are somehow equalized and adjusted clear signal from IC1A output and third branch is distorted signal from IC1B stage).. What you have to note is that:

• GAIN1 pot's wiper is "grounded" from AC point-of-view, so no signal goes thru it "back" (or "forward" ) and that it makes (again from AC point-of-view) two electrically independent parts:
  a) input divider (with R6||C5) for signal heading into IC1B's non-inverting input
  b) part of gain adjusting network of non-inverting amp made by IC1B
  
• No signal goes "back" from inverting input of IC2A

T.

[earthtonesaudio]

In the Yellow Shark, there is "extra" negative feedback coming from R7 and R9, but that's it.  The middle lug of the Nature control is ground, so no signal gets past there.  It combines a tone and gain control into one pot.  Gain and treble increase in one direction, both decrease in the other.

In the Prof. Tweed, the 22k/1uF feedback to the source of Q2 is just negative feedback.  It also affects the loading on Q3 (even more than "normal" negative feedback because the source is a low-impedance node) so that will have some consequences for the dynamics and compression of the pedal.

[aziltz]

thanks a lot, that helps my understanding a ton.



In the klon, the fact that R7 and R19 meet at C16 still confuses me. Feedback or Input to IC2A?

[earthtonesaudio]

See for yourself:
http://www.muzique.com/schem/filter.htm

There is some trivial amount of positive feedback (around IC1B) from pin 2 of IC2A below 10Hz, but more notably there is feed-forward (from IC1A to IC2A) below 100Hz.
Note however that before the signal from IC1A gets to this point, it goes through a high pass filter formed by C3, R6, and the total resistance between lugs 1 and 2 of Gain1.  So effectively this is a low frequency bandpass filter, which feeds signal from IC1A to IC2A (fed forward past IC1B).  When the gain control is at minimum, the high pass cutoff is higher than the low pass cutoff, so essentially no signal passes.  As Gain1 is turned past 5k, you get more and more bass (below 100Hz) fed forward to the input of IC2A.

The purpose of this might be to allow the bass frequencies to bypass the clipping diodes, resulting in less mud.  That's just speculation though, I've never heard one of these.

[aziltz]

thanks for that explanation earthtones.  I'll be building one from madbean's board in the coming, hopefully my ears will sort of detect what's going on.