ALL ABOUT BYPASS
� 2004 H. Davis
BYPASS, DIRECT BYPASS, TRUE BYPASS, etc. have been ambiguously applied terms. There are several methods of bypassing the effect circuitry in guitar effects pedals to get dry (unprocessed) signal at the output, and each has certain advantages and disadvantages. I'd like to propose more specific terminology here to clear up the confusion:
BYPASS of all types makes the dry (unprocessed) guitar signal available at the output through the footswitch or other switching circuitry. Without the preceding terms DIRECT, TRUE, or TOTAL, bypass often means that the dry signal is taken from the output of a gain or buffer stage following the input, which is usually an op-amp or transistor. This is also known as virtual or buffered bypass. The signal is subject to the limitations of that first stage and its loading on the guitar pickups. With this form of bypass, in the event of a power failure or dead battery, no signal is available at the output in either the bypass or effect-on mode. The only advantage of such a bypass arrangement is the buffering of the signal, which makes it insensitive to noise pickup and loading effects from the pedals and cables following it in the signal chain.
DIRECT BYPASS or TRUE BYPASS means the output jack is directly connected (hard-wired) to the input jack in the bypass mode. The dry signal is thus available even in the event of a power failure, but it still may be tonally degraded through the loading of the guitar pickups by the first (input) stage of the pedal and any pedals and cables connected after it.
TOTAL BYPASS (my term, to distinguish it from the above methods) uses a 2PDT switch to disconnect the input stage from the input jack in the bypass mode, as well as to connect the output jack directly to the input jack. Thus there is no loading effect on the pickups due to the input stage, and direct signal is still available in the event of power failure. The terms DIRECT BYPASS and TRUE BYPASS have been used for this in the past, as well as for the above method where the input stage remains connected. TOTAL BYPASS is the best descriptive term for the 2PDT switch arrangement that disconnects the input stage, and this is superior to the other bypass methods. The presence of a 2PDT switch does not necessarily prove that a pedal has total bypass, as one pole may be used to operate an effect-on indicating LED. A 3PDT switch is required when such an LED is used along with total bypass. Circuits have been devised to operate an LED with only a 2PDT switch wired for total bypass. These are not always reliable, can introduce switching noise, and to my knowledge are not used in any commercially manufactured pedals.
Some guitar effects pedals such as the Electro-Harmonix Deluxe Memory Man have an adjustable gain stage (a drive or level control) at the input that introduces complications with bypassing methods other than the buffered bypass it was designed with. This is because with other bypass arrangements the variable gain can result in a volume mismatch between the dry and effect-on signals when the switch is thrown. The level control can be set so the dry and effect-on volumes are the same, but the problem is that this level setting may not be optimum for signal to noise ratio, which requires as high a drive level as possible without causing objectionable distortion. It is also possible with very hot pickups that the effect out signal will become overdriven before its level can equal that of the dry signal. My Deluxe Memory Man Hot Pickup mod can correct this.
So you can have a total bypass mod done on such pedals, but you then lose the ability to adjust the drive level exactly as you might want it for noise and distortion, and also keep the proper balance between bypass and effect-on volume levels. In such cases where total bypass may not be desired, a mod that increases the input impedance of the first stage can be almost as effective as total bypassing in reducing tonal degradation due to pickup loading. If you use other pedals or long cables following this pedal, increasing the input impedance is the better choice, as the benefits of buffered bypass are retained, which is not the case with total bypassing.
The reissue Deluxe Memory Man now comes from Electro-Harmonix with direct bypass, but with the input level control stage as it was when wired for buffered bypass. This is workable with many pickups, but bypass level matching problems can occur with hot pickups. For Memory Man mods done by the engineer that designed the pedal, contact me:
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