Mixer feedback - like Moog Sub 37

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From Sub 37 manual:
"the FDBK / EXT IN knob takes the output of the mixer and feeds it back into this mixer channel, resulting in a variety of distorted, sometimes chaotic, sometimes mellow qualities"

So it's mixer-mixer feedback, without filter in the loop. Could someone explain what is going on here?

Assuming there are some OTA's and HPF in the loop, does this act as a saturator?
giq

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Would be curious to hear the explanation. Usually if you just connect the output to the input of a mixing stage with ac coupled positive feedback, all you get is sustained oscillation at ultrasonic frequency, about as high as the slew rate will sustain, typically 1 MHz or higher. Unless the circuit incorporates internal bandwidth limitation which would lower the oscillation feedback frequency but otherwise make it no more useful if it is just uncontrolled parasitic oscillation. As far as I could guess. When hardware hacking, making homemade circuits, its the kind of thing one usually avoids like the plague, because it is easy to do accidentally if one gets sloppy.

With negative feedback, would only tend to reduce the mixer's gain. The higher the neg feedback, the lower the gain.

The mixer would seem to need to be doing something in addition to mixing audio for this to have a musically useful application?

Unless they are referring to some kind of control voltage mixer?

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There is a diagram in the manual that confirms this, I've heard that it sounds great.
So I wonder what is it, couldn't find reasonable audio demos.

btw. my first thought was like tanh(x+feedback*y) = y.. that is solvable for sane feedbacks
giq

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I've done this with a friend's Moog Voyager, running one of the outputs to the Ext In. Also NI's Monark has 2 feedback settings (1 mild, 1 wild, based on this principle) -- it's a Reaktor ensemble so you might be able look at the ensemble to see what's happening.

Not sure if this is directly applicable to DSP, but look into analog-domain "Zero Input" or "No input" mixing. People are doing some pretty great things with it.

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Monark has feedback around filter. (voyager as well - afaik).

Nevertheless I modelled a naive approach tanh(x+feedback*y) = y - for positive feedback it acts like a variable steepness saturation
giq

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