These days, most modern guitar pedals offer true bypass. This means that the signal bypasses the effect circuit when the pedal is switched off, passing from input to output. Pedals that don’t have true bypass merely have a switch that turns the effect circuitry on or off with the signal still running through the circuit.
The advantage of this is that it introduces a buffer against signal interference and won’t colour your guitar tone with the circuitry of the effects pedal in use. This colouration is referred to as ‘tone sucking’, and can easily be tested by comparing the tone of your guitar when plugged directly into an amplifier to that when first run through a pedal. If you are running multiple pedals then the affect on your overall tone and power will be cumulative, which is obviously not desirable. The impedance of the guitar pickups vs. that of the pedal will also have a bearing over which factor dominates the signal more.
TYPES OF BYPASS
There are three main forms of bypass and the description above describes the most basic end eldest, which uses a single switch to route the input to the effect or to the output. The downside of this is that the effect circuit is always connected to the output, which can result in losses of both high frequencies and dynamic range. Another noticeable drawback is that these types of switches are known to create audible clicks when switched on or off, which is obviously not desirable.
Consequently ‘logic bypass’ was developed in the late 1970’s and featured in designs by many major manufacturers, such as Roland and Ibanez, among others. They utilised a transistor-based switch to cut the effect out of the circuit, however this meant that the signal still has to pass through the switching logic even in bypass mode and consequently ended up colouring the tone itself.
True bypass takes from both of these designs, using two switches that operate in parallel, with one at the input stage and the other at the output. When bypass is selected the signal literally bypasses the effect circuit, avoiding any possible impedance to your tone.
To achieve true bypass in a pedal with an LED on/off indicator requires more complicated switch, called a Three-Pole Double-Throw (3PDT). While these switches have just two positions for on and off, both of these make three circuit connections, moving the guitar signal from the effect circuit to the output as well as switching the LED on/off.
THE ALTERNATIVE: BUFFERS
One of the problems with true bypass pedals are that while they do not impede your signal per se, they also do not aid it in any way other than not getting in its way. One problem with this is that the signal will lose power the further it moves from the original sound source to its final destination.
To combat this, most mass produced pedals these days come with a standard buffer – a small unity-gain preamp - built into them specifically to tackle this problem. Assuming it is well made, the buffer helps to maintain the tone and volume of your signal as it makes its way across the pedal board and through multiple guitar cables.
However, true bypass pedals do not have a buffer built into them, often resulting by default in a weaker signal. Because of this some guitarists choose to use true bypass pedals and also incorporate a dedicated buffer/preamp into their signal chain.