Feedback loop pedals open up a ton of sonic possibilities and can bring new life to boring old pedals, but are rarely seen in use and never stocked in your average guitar store due to their highly unconventional and misunderstood functionality. Perhaps it’s because they feedback too much? I’m not talking about feedback caused by strings vibrating from an amp that was turned up too loud, but rather man’s desire to harness the power of sonic infinity–for a sound that neither ends nor begins because it’s too busy looping back upon itself over and over. For those of us who seek this uncontrollable power but have yet to witness one of these noisy stompboxes in action today is your lucky day. The premise is simple: you have an Input, Output, Send, Return, and a single knob for adjusting the amount of feedback. The Send and Return work like your standard f/x loop except on crack, and in between the two you can patch in any combination of effects pedals that you would like to feedback upon itself repeatedly, usually resulting in some form of controllable self-oscillation or depending on the pedal often unique and unpredictable results. Ever turned the feedback up on your delay pedal until you could feel the repeats growing exponentially and cascading out of control into a noisy wall of oscillations? That’s pretty much it except the feedback loop works in a unique way on pretty much any type of effect to bring out the most extreme properties of each effect. There is absolutely no signal processing done inside the pedal itself, as the goal of the circuit is to just route the f/x chain to properly loop back on itself. There are some pretty nifty looking feedback loop pedals out there like the Total Sonic Annihilation by Death by Audio ($150) or the Eye of God by Devi Ever ($100). But the truth is these items are terribly overpriced considering you can build your own feedback looper for less than $20 worth of parts.
And there you have it! Behold the wonderful True Bypass Looper with Feedback Loop schematic care of Beavis Audio Research! Consisting of a mere 4 ¼" jacks, 1 resistor, 1 LED, 1 3PDT switch, 1 SPDT switch, just 1 potentiometer, and lastly 1 cool pedal housing (not shown). You can even strip down this design even further, by removing the resistor, LED, and SPDT (and switch out the 3PDT for a DPDT) in exchange for sacrificing the true bypass functionality and the LED which you don’t really need. You can generally tell if this effect is on or off without a little light telling you. The total cost of components for this project will run you around $20-25 at most if you purchase your components from Mouser or Small Bear.
For the complete beginner there are a few points to note about the above schematic. First, you will notice that all the sleeve lugs are connected together. This is called common ground and is typical of all circuit design. Second, the single potentiometer (or pot) is a variable resistor that controls the amount of feedback in the circuit–notice that it is nestled in between the switch and the Send regulating the flow of signal into the feedback loop. Lastly there are two switches: A 3PDT (3 pole double throw) which controls the true bypass loop and LED switching and a SPDT (single pole double throw) which turns on/off the feedback functionality of the loop. You may wonder if a DPDT (double pole double throw) exists, and it does but we don’t require one for this project. The stomp switches are named in the following convention: the first two letters indicate number of contact sets, and the last two letters indicate the number of contact positions. This means a 3PDT switch will have 3 sets, with 2 contact positions per set, which actually equals 9 total contacts and visually resembles a tic tac toe board. In the case of the 3PDT the center column of contacts is connected either to the top or the bottom row of contacts, alternating between the two every time the switch is pressed. Logical, right? A 3PDT switch provides the minimum number of contacts required for true bypass with an LED and usually runs between $5-10 each accounting for the majority of the cost of this project. Resistors/LEDs generally cost pennies each and the jacks will run a dollar or two each. You can also get slightly blemished factory seconds BBE-sized pedal enclosures from Small Bear for $5-10 each or you can just cannibalize one of your older more unappreciated pedals for its pristine non-biodegradable case.
Once you have all of the tools it’s just a simple soldering job and you’re done. I’m going to assume that you have basic soldering knowledge but if you don’t you can search Youtube for some quick visual learning. Double check your wires and make sure they are going to the correct leads as labeled on the schematic. The hardest part will be getting the solder to stick on the switch/jack lugs. To do this hold the soldering iron to the lugs for a minute or so to heat them up before you apply solder or you will have a cold solder joint that is an incredibly weak bond and is guaranteed to come apart with little pressure. Luckily you don’t even have to mount a PCB for this project, although your enclosure will need the proper amount of holes to cover 4 ¼" jacks, 1 pot, 1 dc in and LED (if using an LED, otherwise you don’t need any power source for this pedal to work), and 1-2 switches depending on whether or not you decided to include the true bypass feature into this pedal.
One modification that you can make to this project is removing the true bypass entirely and just having a permanent f/x loop while retaining the ability to turn on or off the feedback circuit. To do this you can completely drop the 3PDT in the schematic and use either a DPDT (if you want an LED) or even just the SPDT if you don’t need an LED. For SPDT, wire the Input to Send, the Output to Return, and the center lug of the SPDT to Send, and one of the side lugs to the center lug of the 500k pot. Lastly wire one of the pot’s side lugs to Return. Now the effects chain will always be on, but the SPDT will toggle whether the feedback loop is on or off.
Conversely, you can eliminate the SPDT that controls whether or not the feedback is on and just have the feedback permanently engaged whenever the f/x loop is selected. This is my preferred version, because I have no intention of using the pedal simply as a true bypass looper for normal effects purposes–I only seek extreme volume and oscillating feedback functionality for cooking up delicious noise soup. To achieve this goal you simply imagine the SPDT in a permanent ON position, and with that mindset look again at the schematic and you will know to wire the center lug of the 3PDT (red output wire) to the blue lug on the pot and voila, feedback is always on when the loop is engaged. Done. This guy seems to have the right idea:
Thanks to Experimentalists Anonymous for providing us freaks with the schematic and a wealth of information on this project, not to mention for fostering a thriving online community that is very friendly and welcoming to novice pedal builders. In addition to this project they host an enormous catalog of free schematics for any conceivable and inconceivable diy project and I highly recommend diving into their community if you are the least bit interested or curious about building your own pedals. Until next time.
