feedback looper questions

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FAP
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Re: feedback looper questions

Post by FAP »

Shunting the unused fx loop is a great idea; I plan on building a modified version of this circuit I found:
http://www.generalguitargadgets.com/pdf ... p2c_sc.pdf

To me, it makes more sense to shunt the unused return than it does to shunt the unused send, since a high-gain or self-oscillating effect could still be audible without an input to draw from.

If I were more ambitious I’d try to figure out how to bridge A & B together in a chain as well, but I’ve already spent more trying to remedy this $30-ish pedal than I would’ve hoped; I also just realized I have a Boss line selector to fall back on anyways, which is a far more flexible piece of equipment.
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Re: feedback looper questions

Post by NoiseWiki »

I could see a reason to put a switch on both the send and return so that you can stutter the input going into a reverb and not cutoff the effect or perhaps you do want to cut off the effect
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Re: feedback looper questions

Post by FAP »

No, it just cuts off the effect that isn’t currently in use. Basically doing everything possible to make sure there’s no signal bleed from the unused fx loop.
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Re: feedback looper questions

Post by NoiseWiki »

FAP wrote: Mon Aug 29, 2022 9:53 am No, it just cuts off the effect that isn’t currently in use. Basically doing everything possible to make sure there’s no signal bleed from the unused fx loop.
That makes sense .. I'm just making a case for both if you were to use an external effect instead of just a feedback loop
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Re: feedback looper questions

Post by FAP »

Alright gents, I've come across another odd quirk with this build that I'd like some second opinions on, but first, some photos for reference:

Exterior of completed build:
face.jpg
Gut shots:
guts.jpg
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As you can [hopefully] see, I used shielded cable for the input & output leads, and twisted pair for everything else, with the exception of the LED circuit (far right column of both switches) and the two white leads jumping between the switches (middle row, first and second from left columns on the 4PDT switch).

Schematic I used, modified from one I believe I posted previously:
schem.jpg
Note the top and bottom rows of the 3PDT switch have changed places in the final build i.e. the bridge between top left & top middle lugs in the schematic = bottom left & bottom middle lugs in the final built: this was done so that bypass is initiated when the switch is flipped "up." Other than that, it's pretty close to the final build.

It's a bit messy but basically what's happening here is the input runs into the 3PDT first. When in "bypass" mode, the input signal goes directly to the output. When not in bypass mode, the input signal then goes to the 4PDT. The 4PDT switches between fx loops "A" and "B." When fx loop "A" is selected, the positive lead of Return "B" is shunted to ground & vice-versa: this is to prevent signal bleed from whichever fx loop is currently inactive. Meanwhile, the active loop passes on through to the output.

Here's the problem I'm running into: if I use certain pedals in either loop, there's a slight added feedback to the output signal, which doesn't otherwise appear. To put it another way, if I stick a contact mic into x-pedal, then run the out of x-pedal into my mixer/amp/whatever, it works normally; if I instead run the contact mic into the input of this fx looper, then run x-pedal through one of the fx loops (send/return), then run the output of this fx looper ino my mixer/amp/whatever, there's some feedback added.

Now this only appears to happen with high-gain pedals; specifically, I was using the Mantic Hivemind and DOD Grunge on A & B respectively. Swapping either of them out for a Boss DD-3 (digital delay) doesn't yield the same added feedback effect i.e. the sound of the DD-3 is identical whether I'm using it in an fx loop or not. It's like this fx looper "amplifies" the high-gain pedals just enough to feed back into themselves, but not completely so.

I've ruled-out erroneous connections or poor solder joints as the culprit; the same issue happens in both A and B loops, and signal-tracing confirms I wired everything up exactly as the schematic dictates (minus the alterations previously mentioned). With that in mind, I'm starting to think my problem may not lie in the execution so much as in its conception: TL;DR does the schematic look okay? Is it just a quirk of the high-gain pedals? Or something else entirely?
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Re: feedback looper questions

Post by NoiseWiki »

Well my theory is thst the 4 pole isn't sufficiently shielded internally allowing signal bleed.. it seems like the only place where it could happen. To prove this I would manually make the connections with the wires for the send and return to bypass the switch.

Now another idea would be instead of having the send and returns connected next to one another on the 4 pole try putting the sends next to one another and the returns next to one another so that the matching send and return are not immediately next to each other .. this may help with the crosstalk if that's what's going on
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Re: feedback looper questions

Post by crochambeau »

Are the pedals sharing power, or powered separate? Rule out bleed on the power rail.

I've never been inside a switch that was shielded between throws (I take almost every failed component apart, so my experience may just be limited to cheap shit that breaks). If you pinpoint bleed at the switch, is it possible to reconfigure the paths so they are not running on neighbouring throws? Kind of like the old "allocate the drum track to an outside position on the multitrack tape so it only bleeds over into one track" trick.

Your wiring layout/lead dress is pretty rats-nest ball of worms-like. I intend no offense, I've done worse. That said, if bleed is an issue it may behoove you to rip out all the wiring and route different branches/legs as far from one another as possible. Kind of a Tetris with open air thing, instead of a meatball.

Or you can get creative weaving in foil connected to common and see if that helps.
When in doubt, add resistance.

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Re: feedback looper questions

Post by NoiseWiki »

Wait so is this only happening when you have a pedal on A and B unless one is a the low gain delay? Does it happen if only one of the Hi gain pedals is connected?

As I've mentioned in o e of your other threads I've encountered strange results with certain pedals and one spot adapters so you'll want to eliminate that as an issue
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Re: feedback looper questions

Post by crochambeau »

I also notice that while you have twisted pairs with ground & signal, your actual ground reference on a few of those jacks looks like it's depending on a mechanical connection to common through the metal enclosure to complete.

My advice: NEVER use the enclosure for signal path. Ground return/termination counts. Any resistance that builds up, be it galvanic reaction over time, simple things like nuts backing off, or invisible films impeding solid connection will steer you right into the sort of troubles you're trying to solve. The enclosure's job is to keep the guts safe and shield stuff from EMF. If you add signal termination to the list useful life of the build will diminish.
gutsstug.jpg
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At the very least, a straight wire, uninsulated bus from IN to OUT that you can run little common drain wires from the send/return jacks to the ground bus.

If you're trying to "play Tetris" with twisted pairs start each pair at the jack, route the twisted pair like a single wire until you split the "live" signal into the switch area and the common wire to the ground bus.

Crossing paths with other wires is better the more perpendicular the crossing is.

You have three dimensions, and a straight wire IN to OUT connection offers some structural framework that can be of use.
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Re: feedback looper questions

Post by crochambeau »

I realize I threw a bunch of shit at you.

Implementing one solution may be all you need.

We're not trying to build bridges, just make some racket.

Good luck!
When in doubt, add resistance.

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