Mechanical Switching

Audio is switched inside the actual footswitch.

A survey of the possibilities.

Final section includes troubleshooting tips.

Switch-the-Output Jack or Half Bypass

Input jack is connected to the fx input and one side of a SPDT.

Output jack is common of a SPDT.

fx output is on the other side of the SPDT.

Commonly used in vintage pedals. This scheme would work fine if the fx input was perfect and had near infinite input impedance while maintaining a stable operating point and low noise. Practical inputs have a less-than-ideal input impedance to achieve stability and low noise, so this bypass scheme suffers from "input loading," also known as "tone sucking." These pedals are the typical candidate for a "true bypass mod," but there are alternatives such as adding a buffer at the front of either the chain or before the offending pedal, or placing the offending pedal in a "true bypass" effects loop. If practical, the pedal could also have its input circuitry modded to be higher impedance, or (space allowing) a buffer circuit could be added inside the box.

Adding a pole with a DPDT allows for a LED on the extra pole.

Buffered Switch-the-Output Jack or Buffered Half Bypass

Input jack is connected to the fx input, buffered, then split between the actual fx circuit and one side of a SPDT.

Output jack is common on a SPDT.

fx output is on the other side of the SPDT.

Adding a pole with a DPDT allows for a LED on the extra pole.

Also common in vintage pedals. Some designers recognized the "loading/sucking" issue and did something about it. Buffering the signal before trying to drive both the bypass and effect inputs simultaneously avoids the tone suck. Unfortunately, the input buffer might have its own noticeable effect on tone (brighter/darker) or amplitude (perhaps causing an undesired boost or cut) which may be resolved by either modifying the buffer or converting to a full bypass scheme.

Reverse Half Bypass or Switch-the-Input Jack (Impractical)

Input jack is connected to the common of a SPDT.

Output jack is connected to one side of a SPDT and the fx output.

fx input is on the other side of the SPDT

This arrangement is impractical. The low impedance of the fx output would load the input signal when in bypass. Just included here for completeness.

If the input was a known low impedance, and a resistor was added in series with the fx output, this could work, but as a general purpose bypass scheme it is impractical.

Buffered Reverse Half Bypass or Buffered Switch-the-Input Jack (Unusual)

Input jack is buffered, then sent to the common of a SPDT.

Output jack is connected to one side of a SPDT and the fx output.

fx input is on the other side of the SPDT.

This arrangement is unusual, but would work. Note the resistor between the fx output and the output jack. This resistor isolates the input buffer's output from the fx output (presumably another active output). Suffers from the same issue as the former buffered scheme in that any tonal or amplitude variations from the buffer may be objectionable.

Douglas Self's "Virtual Switch Contact"

The virtual switch concept is taken from Douglas Self's Small Signal Audio Design. This is the next logical step in our progression from the impractical "switch-the-input jack" and the buffered "switch-the-input jack."

In many cases, the last stage of the effect will be some kind of op amp output anyways, so the "FromFX" op amp could be part of the effect and not something added for the bypass circuitry. The only requirement is that it has a very low output impedance.

(A practical design would include another resistor in series with the output jack, but after the node with the switch and resistor shown to protect the effect output amp from short circuits.)

The value of the resistor R is a design choice with a tradeoff between the "offness" (Self's word for rejection of the "other" signal) of the dry signal when the effect is engaged and the effected signal when the switch is set for "bypass." Self gives measurements for "offness" are given using 2k2, 4k7, and 10k resistors: good starting points for your own experimentation.

The benefit to this design is the mechanical simplicity. As the mechanical parts are the most likely to fail, minimizing mechanical elements increases the durability of an effect pedal. Although the simplest application would be a latching SPST footswitch, this could also be a SPST relay, or 1/2 of a DPDT relay with the other section controlling a LED (footswitch would drive an electronic circuit to toggle the state of the relay).

Switch Both Jacks, Full Bypass, Hard Bypass or "True Bypass"

Classic wiring:

Input jack is connected to the common of pole 1 of a DPDT.

fx input is connected to pole 1 side A.

Jumper wire connects pole 1 side B to pole 2 side B.

fx output is connected to pole 2 side A.

Output jack is connected to the common of pole 2 of a DPDT.

The "classic" wiring is so named because many vintage pedals that include a full bypass used this arrangement. This allows for a neat straight piece of solid wire to serve as the jumper connection.

Compact wiring switch-the-output jack:

fx input is connected to the common of pole 1 of a DPDT.

Input jack is connected to pole 1 side A and pole 2 side B.

Output jack is connected to the common of pole 2 of a DPDT.

fx output is connected to pole 2 side A.

(Pole 1 side B has no connection. May be connected to ground to silence some noisy overdrive circuits.)

Compact wiring switch-the-input jack:

Input jack is connected to the common of pole 1 of a DPDT.

fx input is connected to pole 1 side A.

Output jack is connected to pole 1 side B and pole 2 side A.

fx output is connected to the common of pole 2 of a DPDT.

(Pole 2 side B has no connection. May be connected to ground to silence a noisy output or used with additional circuitry to form a RAT or Millenium bypass.)

Adding a pole with a 3PDT allows for a LED on the extra pole.

Using the compact wiring "extra" contact to drive an electronic LED circuit is known as a RAT or Millenium bypass. (Note the mispelling of Millennium. This is how the word was misspelled in R.G. Keen's original article.)

All 3 of these wirings show up in practice. In troubleshooting footswitch loud pop problems, simply switching the wiring from one type to another (including the grounded or NC options) may fix or reduce the problem (this comes from anecdotal reports on stompbox forums).

About 3PDT footswitches

The 3PDT footswitch has become extremely popular in the world of guitar effects pedals. The 3PDT craze appears to have grown from Mike Fuller's Fulltone brand of pedals. He may have even coined the term "True Bypass." He was very vocal on his website about the 3PDT and its benefits. Fuller is credited by Analog Mike in the AMGTVE as being responsible for bringing the reliable 3PDT to the market.

Although the ubiquitous CIC blue 3PDT switch is a fairly reliable thing, they are the most commonly replaced component in my repair shop. Having 3 latching poles may simply be too mechanically complex and the chance of something going wrong appears to be higher in these switches than in simpler designs. They are extremely convenient for hobbyists and small scale builders to avoid an electronic toggling design, but I would personally like to see more small builders step up to the challenge of incorporating electronic switching. "True bypass" can be maintained with relays, and I can't remember the last time I changed a relay in a stomp box.

Mechanical switch troubleshooting

Footswitch failure has to be the #1 problem for guitar pedals. I'll have to check my numbers, but I want to say I replace dozens of them every year. I think I replace more busted footswitches than input jacks, toggle switches, potentiometers, etc.

A bad footswitch is rarely fully broken, although that happens too. Most of the time the customer will have some general or vague complaint about the pedal cutting out. Customers are more likely to identify a faulty input or output jack than a faulty footswitch. The idea that the switch itself could be bad seems to be lost on the average user.

The Slow Push Test

To test a suspect switch, I connect a clean sine test signal to the input, and monitor input and output simultaneously on a dual channel scope. (By observing the input signal, we can verify that nothing is shorting the signal, or note the presence of dc offset.) The output is also sent to a monitor amp. With the sine wave present on the scope, and audible on the monitor, I'll toggle the switch a few times with my hand and note any noticeable cutting out or strangeness. A real bad switch fails quickly. If it passes the first round, we move to the slow push test. The slow push test is exactly that: push as slowly as you can. You can treat it like a game where you see how far down you can get the plunger before it toggles. A good switch will only have two states, and it should snap quickly between the two. During the slow test, there should be no cutting out and a sharp and sudden change between the two states. A bad switch will have some kind of in-between state that you slip into with the slow push. A slightly bad switch will only momentarily go "in-between," sort of creating a slight "delay" between states. I recommend replacing the switch if you can do this. A bad switch will actually latch in the in-between state and require another push to toggle over. The in-between state might be muted, might be attenuated, and it may affect the audio, the LED, or both. Observe all of these things, and if anything appears to malfunction during the slow push test, I would replace the switch.

True Bypass Popping

The fact is that you open yourself up to popping problems if you insist upon a "true bypass" scheme. These come in several forms. A footswitch can be mechanically noisy, although I find this to be rare. The gain from a distortion pedal may amplify a mechanical switching sound, but again I find this to be rare. Most commonly the pop is the result of coupling caps discharging. Most pedals are designed so that the coupling caps will have resistors on both sides to maintain a good voltage reference no matter the switch position. The problem will rarely be within the pedal that "pops." What happens is that some other pedal, often a Boss pedal, will have the capacitor charging/discharging issue because they did not design around hard bypass type schemes. The way to troubleshoot this is to check each pedal independently, then slowly combine them until the offending combination is discovered.

Back to Bypass Schemes