Adding a Feedback Loop - Input/Output out of Phase

[SeaWitch]

I'm trying to add a feedback loop to this Modded Rat build (on a momentary switch).

The usual approach of Effect Output -> Effect Input feedback loop does not seem to work on this circuit.  the signal is killed when set up this way...which may mean the input is out of phase with the output...??

So, how can I determine, on this circuit where an “output” on the board is in phase with the input?"

Does anyone have any ideas? I'll do some more experimenting, but if there are any suggestions or knowledge about how to incorporate a feedback loop into this circuit, I'm all ears!

[induction]

The only active subcircuit in a Rat is the non-inverting op-amp stage, unless there's an input or output buffer, which will also be non-inverting.

In general, the way you predict phase inversion is by familiarity with the building blocks: non-inverting op-amps and common collector amplifiers (transistor buffers) and their FET analogs will be non-inverting. Inverting op-amps and common emitter amplifiers (COT-50s, etc.) will be inverting. Etc. An even number of inverting stages in series get you back to non-inverting again.

The output signal of a Rat is qualitatively different enough from it's input signal, that you wouldn't get complete cancellation anyway. Just a change in tone and probably a loss of volume.

It's hard to say for sure what went wrong here, because it depends on the details of your implementation (a schematic would help a lot), but it's possible your output signal has DC voltage on it and is ruining the bias of the op-amp input. That would often give you silence. It's also possible that you shorted the input or output to ground, or something else went wrong.

[induction]

I noticed your post in the other thread and then looked at the Blower Box schematic and the layout you posted above. Instead of an output buffer, this circuit has a common source amplifier after the tone stack. This is the jfet equivalent of a common emitter bjt circuit, and is indeed inverting.

I still don't think that should cause you to get complete silence in a feedback arrangement.

(Edit: Having thought this over,  I believe I was wrong about this. It's not a pair of similar but reversed signals like out-of-phase pickups, but an infinite negative feedback loop, with each iteration cancelling a little more of the remaining signal, ultimately driving the output down to zero. Nonetheless, if you get it to work, at best it will sound heavily filtered and quiet rather than the crushing sonic mayhem I assume you're looking for.)

Bottom line, if you want to add a feedback channel to this you should probably tack an inverting buffer onto the input or output, or convert the op-amp to an inverting configuration. (Edit: Better yet, if you're using an external feedback box, put a switchable inverter into that. Then you can also use it with any pedal, inverting or not, without any more mods.)

[SeaWitch]

Thanks for the troubleshooting!

I'm going to stick a LBB-1 in there and see if reversing the phase on the output will do the trick.

Thanks for the ideas and help - I'll report back with results when I test this out.

Thanks again!!

[ibanez48]

http://experimentalistsanonymous.com/board/index.php?topic=134.0.    Don’t know if this might help you. You can check it out

[SeaWitch]

Yes, perfect.  I think I'll go this route of building a stand alone feedback looper.  Thanks!