feedback pedal ... does it exist?

[Dr Ron]

I'm wondering if anyone has designed a pedal specifically for feedback for one or several pedals.

I've seen a few posts about adding a feedback loop to a distortion pedal.  Feedback is common in delay and memory pedals. Negative feedback is used with a filter to remove unwanted feedback onstage.

I suppose a feedback pedal would be useful for low volume levels, since most amps will feedback with enough volume.
I also heard that RHCP used feedback on vintage pedals to get new sounds.

Is this a cool idea, or something of little use?

[Mark Hammer]

Is it circuit feedback you want, or the sound of feedback from amp to guitar only at lower volumes?

[Dr Ron]

>> Is it circuit feedback you want, or the sound of feedback from amp to guitar only at lower volumes?

I'm not sure, although I think the latter is what is desired by guitarists.
I'm also not sure what exactly RHCP was doing with the feedback.

In general, isn't circuit feedback used to clean up the signal and lower overall gain?

[syndromet]

Well., the idea is cool. The only problem is that feedback is generated when the audio-waves from the speakers push the strings to vibrate. this would be verry hard to simulate. I guess the closest you will come is something like this.

[petemoore]

  Negative feedback...used in Tube Amps...just taps a bit of output and sends it back to the input to 'dampen' [the reversed phase mixed in cancels]
 Positive Feedback...tapping a small division from 'later' or at output of a circuit and sending that back to 're-inforce' the input...this can make the circuit self-oscillate and do other wierd stuff, to the point where the guitar is 'half controlling' it or it's 'out of hand'...[the in phase fed to input re-inforces].
 It would seem that one could take a pot, and wiring lug 3 to a source of gained up [nearer output] in phase signal source, and lug 1 to an out of phase signal source and have a negative or positive feedback knob, the wiper feeding some amount of damping or re-inforcing signal back to the input.

[Paul Marossy]

I remember a Death By Audio circuit that created feedback by adding the output into the input or something like that. I know a few people at this forum have built it.

EDIT: Check out this thread http://www.diystompboxes.com/smfforum/index.php?topic=10408.0
Just in case you want to explore the possibility. 

[Peter Snowberg]

Several years ago (late 90s?), one of the DIY electronics magazines (Popular Electronics?) had a project for a feedbacker with selectable phase. I think they had it labeled "resonance". I wish I picked that issue up.

[Mark Hammer]

The old Boss DF-2 Distortion Feedbacker(http://filters.muziq.be/model/boss/compact/df2) pedal attempted to mimic a guitar feeding back by syncing an onboard oscillator to the fundamental of whatever single note you were playing.  If you stepped on the pedal and held it, the "feedback" would gradually fade in with just a hint of vibrato, and remain on for as long as you held the pedal down.  The user could adjust the balance between same pitch and octave up to vary the quality of the feedback sound.  I have one. Works okay, and sounds sort of like amp feedback.  Loses its excitement value quickly though.

The Line 6 Tone Core "Dr. Distorto" (http://www.line6.com/tonecore/drdistorto.html) picks up where the DF-2 leaves off and introduces a little more control.  Here you can adjust the level of the overtone, plus the manner in which it fades in and out.  Again, works okay (a little better actually, given the accuracy of digital technology), but I haven't played mine in quite a while. 

Generally, what people want from "feedback" is a result that teeters on loss of control, and both these boxes are about having more precise control.  So, some of the tone is there, but the experience is not accurately captured.

Another part of what people use "feedback" for is to sustain a note in an almost oscillator-like fashion, rather than for the screech.  Here, the different boxes work fine, as do things like the Fernandes Sustainer units that are linked to.  Unless you can be more specific about what it is you *want*, though, it's hard to make any strong recommendations.

[Harry]

 Negative feedback...used in Tube Amps...just taps a bit of output and sends it back to the input to 'dampen' [the reversed phase mixed in cancels]
 Positive Feedback...tapping a small division from 'later' or at output of a circuit and sending that back to 're-inforce' the input...this can make the circuit self-oscillate and do other wierd stuff, to the point where the guitar is 'half controlling' it or it's 'out of hand'...[the in phase fed to input re-inforces].
 It would seem that one could take a pot, and wiring lug 3 to a source of gained up [nearer output] in phase signal source, and lug 1 to an out of phase signal source and have a negative or positive feedback knob, the wiper feeding some amount of damping or re-inforcing signal back to the input.

Could you tell me more about what you mean by 'dampen'?

[Johnny Guitar]

Off the top of my (rather empty) head, I'd try experimenting with some VC synth circuits: namely a VCA and a VCF (with the Q turnned up pretty high), and preferable having some sort of FtoV converter to control the frequency of the VCF. You might need to try different slope VCFs to get a convincing feedback sound. I don't have a FtoV converter so I don't think I can do any useful experiments in this area.

I would think that even a rather primitive guitar synth like circuit (often used to control VCOs in a guitar synth) would work very well in this regard since you can fade into the feedback sound anyway.

Isn't this kind of how Harry B. uses his guitar synth in performance?

[petemoore]

  Dampen...reduce.
 A small amount of *output of the amp is fed back to the *input [*they are reversed in polarity, so this has the effect reducing whatever input level is just past the feedback loop] so the 'negative feedback' reduces the input, ITCase using the output to dis-re-inforce [reduce] the input [letting a little of the output sections 'mojo' influence the input of course] , the amount of output let back to the input via a large resistance makes it a small amount in comparison to the input, these are often labeled 'presence' knobs on guitar amps, and can be viewed in schematic form on say a Marshall Plexi schematic.

[billings]

The boss device sounds like an individual oscillator that tries to track the appropriate feedback pitch, right?

Would it be possible to setup an oscillator that is instead tuned to a particular note, producing output when the input is resonant with the oscillator?  Not related to what the original poster was asking about, but...  curious.

Also:  if it were possible to sweep phase shift from 0 to pi instead of panning between 0 and pi (as per pete's suggestion) that would be...  well, different I would guess.

[JisforJustin]

Here is the Death by Audio pedal:

http://www.killerrockandroll.com/deathbyaudio/totalsonicannihilation.html

[Ge_Whiz]

$150 hah hah haha.