True Bypass Wiring

Older vintage pedals can often color the tone of a guitar even when they’re turned off. This is usually referred to as “tone sucking” — due to the loss of high frequencies and overall volume — and is caused by the so-called Hardwire Bypass.

Hardwire Bypass

These layouts permanently connect (“hardwire”) the input signal to the effects circuit as well as the bypass line. With only the output signal switched one can install a cheaper SPDT switch. However, the input impedance of the circuit is loading the signal line even in bypass mode.

True Bypass

A simple modification to the Hardwire Bypass is to also switch the input signal and thus have a direct connection from the input to the output jack without the effects circuit being involved at all. That’s called “True Bypass”.

True Bypass Wiring for D/3PDT Switches

Here’s the True Bypass wiring for D/3PDT switches. If you don’t need a status LED a DPDT switch will do, otherwise use one with three poles. LEDs with a DPDT are possible but slightly more complex.

The red wire is the bypass line. JI and JO are the input and output jacks, BI and BO the input and output of the effects board – also called effect send and return.

Improved True Bypass Wiring

In bypass mode the guitar is lifted from both the input and output of the circuit board. Any voltage accumulated by e.g. DC-blocking capacitors will slowly (or faster, depending on the quality of the caps) leak out and build up as a small charge potential.

Without a path for the current to drain off it can cause “pop” or “thump” noises when the pedal is switched back on and the voltage discharged into the signal path.

Grounding the effect send BI in bypass mode allows any capacitor charges to leak off when bypassing the pedal and get rid of any voltage that could cause pops later on. The same can be achieved via pull-down resistors.

3PDT Wiring Boards

PCBs that simplify wiring a 3PDT foot switch are easily available. Most of them follow the Improved True Bypass wiring from above with the effect send grounded in bypass mode. They make for much nicer builds and their labels make it harder to get things wrong for beginners.