Electronic Switching
Part 2: JFETs, MOSFETs/Analog Gates
Transistors can pass audio or block audio. How they do this, and how well they do this, is a much more complicated topic than how simple metal contacts pass or block audio. Despite the complications, a well designed transistor audio switch is perfectly acceptable to our ears.
BJTs are rarely used for gating audio. They can do it, and they do pop up in some tremolo pedals (see the Schaller Tremolo), but FETs are the preferred device. FETs come as JFETs (usually discrete), or as MOSFETs (usually in IC packages, called "analog switches").
FETs can be arranged in either series or parallel to block or short the audio signal respectively. A combination of both types results in the best attenuation of the signal, but this is overkill for guitar pedal applications.
The audio signal path is comprised of the source and drain of the FET. The control voltage is applied to the gate terminal. FET switching requires a buffered signal to drive through the drain-source circuit. The clean signal is applied to one FET, and the effect signal is applied to another FET. The control voltages will be logically opposing, i.e. the clean gate will be HI while the effect gate will be LO and vice versa.
An alternative logic scheme can be made by combining series FETs with parallel FETs. For instance, the clean signal could be passed or interrupted by a series FET, while the effect signal could be passed or shorted by a parallel FET. This allows for a logically agreeing signal, i.e. if both are HI the effect is bypassed, and if both are LO the effect is active.
Latching switch control of FETs
The Maestro PS-1 Phase Shifter uses a latching SPST switch to toggle the effect. The original version uses two JFETs to toggle between dry and mixed paths. The second version eliminated the dry FET, so the toggle either mutes or un-mutes the phase shifted path.
Discrete JFETs are commonly found in Boss and Ibanez pedals. Most JFETs will work so long as your power supply can provide the proper cutoff voltage. Since 9V is a relatively low power supply, this does put a limitation on what JFET types will actually work consistently. Some great information on this can be found at geofex.com The common MOSFET IC of choice is the CD4066. Less common is the older version, the CD4016. The 4066 and 4016 can be referred to as "analog transmission gates" which is a fancy way of saying they can pass or block analog signals. An advantage to using the CD4066 is that they are very consistent from one IC to the next. A disadvantage is that the MOSFETs are much more prone to death from static electricity.
Back to Bypass Schemes