Guitar Feedback Module Ideas?

I’d really like to have a module to create guitar/amplifier type feedback, but not quite sure where to start with a design. Would this even be possible entirely in silicon? Could I do something with a PT2399 in a feedback loop? Maybe I could have a tiny little speaker and a contact microphone in a feedback loop?

Just a pie in the sky idea, but curious how the wizards here would approach this problem. There are a few guitar pedals which emulate feedback, but I think they’re really pitch-tracking oscillators under the hood, which is not quite what I want.

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I suppose gluing 2 piezo mics together could work… Could be a bit noisy though.

Google brings up this:

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Ooooh. Yeah, something like that, I wonder how they’re doing it. Looks like quite a complicated PCB. I’m mostly interested in the kind of sounds that happen around ~1:00 in that video.

The resonance control on a filter varies the amount of output signal that’s fed back to the input. When it’s raised high enough it “self oscillates”, which is a polite way of saying you’re getting feedback sound.

So yeah, you can get something like guitar/amp feedback in silicon. Of course a typical VCF does not give you anything very much like it, but it’s maybe a start. A VCF type circuit with resonance varying between large and ridiculous — voltage control over both cutoff frequency and resonance — maybe also an all pass filter phase shifter in the loop? Something like that… maybe…

I’m not sure you need time delay, but it might be worth experimenting with a PT2399 anyway. Or at least it’d be fun.

The speaker and contact mic idea would be sort of an acoustic version of an optoisolator, with the output being an approximation to the input. That would not be necessary for feedback, nor sufficient unless the speaker and mic had suitable resonance characteristics. It’s connecting the output of a resonant system back to the input that makes feedback, and that can be done just as well and more controllably with a filter and a piece of wire.

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I think that’s maybe a good start, but acoustic feedback is more complex than filter self-oscillator, which basically just makes a sin wave at a single frequency.

bilde

I can make a circuit that somewhat approximates this in VCV Rack:

Screen Shot 2021-07-24 at 10.51.55 AM

Which actually gives pretty good audio feedback sounds, but it doesn’t quite behave like what I want - the feedback is always present, I want to compress level of the feedback down when there is new audio signal coming in. Maybe I can figure out something with an envelope follower and an inverter? Not quite sure yet.

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I still think a resonant filter, or maybe several of them in parallel, ought to be useful here in addition to what you’ve got. The resonant oscillation doesn’t have to be a sine wave if there’s some clipping/distortion in the feedback. With the resonance right on the edge of oscillation it might respond pretty well to the presence/absence of input signal. But the idea of controlling the feedback level with an inverted envelope seems good. It doesn’t need to be an envelope follower unless you’re processing an external signal like a guitar; if the input comes from the synth then you can just use whatever envelope generator is creating the input amplitude.

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This sort of works!

Demo: Vocaroo | Online voice recorder

I did this in Rack using a +10V constant voltage and a dedicated “Subtract” module to create the inverted envelope. I’m not sure how I’d do that with what we’ve got in the Kosmo stack right now though, since inverters usually take a 0-10V and make it a 0 to -10V, where as I want to take a 0-10V envelope and turn it into 10-0 volts. Maybe use two OBAs, invert the envelope through one and boost it with the other?

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Seems to me you should be able to do it with one OBA: take say a 0–10 V envelope, offset it by -10 V to make it -10–0 V, then use the attenuverter to invert it to +10–0 V.

Sounds good!

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Very cool, definitely in the right ballpark. I would slow down the attack of the feedback coming in though

I found this cool open source module builder: Monotropa v3 – Reverselandfill

Sounds a bit more “digital feedback” than audio feedback, but still a fun and super noisy DIY’er. He’s got a few other cool open source modules too.

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thx, very interesting sound ! a good distortion here, it will make me want to do a stripboard layout
(downloaded :wink: )

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That’d be amazing!

A few things I don’t understand with the schematic: a) What’s the point of having two rather than one EQIN/MIXOUTs? Seems like they’re the same. b) What are the pot values for FB and Sum? c) What’s with the weird EQIN flags on loop in/loop out? Is that just for labeling, or does that require a switched jack?

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a) I guess some people just like to provide for multiple simultaneous inputs and outputs. (LOOPOUT is the same as MIXOUT and MIXOUT2, too, except see c).)

b) Doesn’t really matter, since they’re voltage dividers and there’s a resistor to a virtual ground in one case and a only direct connection to a high impedance input in the other. But for proper input impedance the FB one should be about 100k. Log or linear, I don’t know.

c) The flags are global labels and they indicate those two points are connected. So if nothing is plugged in at LOOPOUT or LOOPIN the circuit output is connected back to the input… I guess… I don’t understand the logic. But yes, it’s using switched jacks.

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c) The flags are global labels and they indicate those two points are connected. So if nothing is plugged in at LOOPOUT or LOOPIN the circuit output is connected back to the input… I guess… I don’t understand the logic. But yes, it’s using switched jacks.

Ohhh, okay, that actually makes sense to me. It’s a feedback circuit with send/returns, so this way if there’s nothing plugged in, then the output is fed back to the input to drive the thing. If something is plugged in to only one it’ll break the feedback, and if the send/return is used then it’ll add a modulation effect to the feedback. Thanks for explaining that.

I think I’ll skip the multiple ins/outs and just use external unity mixers/buffmults instead.

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Here :slight_smile:
I put only one imput, and also 1 output, and the Feedback pot without “switch jack” (but easy to add if you want build with)

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WOW! That was fast! Nice wire style.

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Too complex to put “real wire”, i made this to not made spaggetti , we must follow the color code like a children :laughing:

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I’ve just edited my layout , because i’ve just look at the BOM ( when all is finished :roll_eyes: ) and it’s all 10K for the pot :slight_smile:

I think that’s too low for the FB pot. Again, as far as this circuit on its own goes, any value should work, but the input impedance really should be about 100k. In fact I’d say R3, R4, R12, and R16 would better be 100k too.

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Like this ?