Global Negative Feedback in Pedals: Applying Signal to the Emitter

[YouAre]

Hello all,

I've been looking at a few different circuits that provide some sort of emulation of tube amp power amps within an overdrive pedal. The best example of what I'm looking at is the ROG Professor Tweed.

http://www.runoffgroove.com/professor.html


It looks like the output of Q3 (which is a stand in for the amp's power tube and transformer) is tapped and fed back to the emitter of Q2. I would assume that this would act as (global) negative feedback, but is it? The output of Q2 is inverted, then inverted again at Q3. This signal being fed back to the emitter of the Q2 is not inverted.

As I understand negative feedback (ignoring the local negative feedback from each of these gain stages for a moment), you're feeding a portion of the circuits output to the negative input of the gain stage that begins our feedback circuit block. I'm defining the feedback circuit block as the stage(s) to which feedback is applied. 

Does that mean that if you feedback a portion of the inverted output signal to the same node as the input signal, you're applying negative feedback also? Or is that a bad assumption?



I do understand that the Professor Tweed and the Princeton have a different "phase inverter" circuit compared to most push/pull amps, which utilize long tail pair inputs and their negative inputs. I'd like your help in understanding if a single JFET stage is a suitable input stage to a feedback circuit block.

Thank you all for the help.

[antonis]

http://www.runoffgroove.com/professor.html
It looks like the output of Q3 (which is a stand in for the amp's power tube and transformer) is tapped and fed back to the emitter of Q2. I would assume that this would act as (global) negative feedback, but is it? The output of Q2 is inverted, then inverted again at Q3. This signal being fed back to the emitter of the Q2 is not inverted.

I've to admit that you've managed to confuse me enough..

Q3's out is in phase with Q2's Emitter so we have a positive feedback on Q2's Emitter..
This acts as a negative feedback on Q2 - it lowers it's gain for the shake of other purposes..
(in the way that any Emitter resistor acts - more current on it --> higher voltage on Emitter --> lower current on Collector..)

As I understand negative feedback (ignoring the local negative feedback from each of these gain stages for a moment), you're feeding a portion of the circuits output to the negative input of the gain stage that begins our feedback circuit block. I'm defining the feedback circuit block as the stage(s) to which feedback is applied.

"Ignoring" is the (wrong) key word..
ALL fedbacks should be negative (except in oscillation cases) so you have to face them as "individually cascoded"..

I'd like your help in understanding if a single JFET stage is a suitable input stage to a feedback circuit block.

As long as JFET inverted out is added to block's input is more than suitable..
(A trillion of inverting inputs of JFET Op-Amps can't be wrong..)