Attenuators

A transistor can be an active attenuator. Typically the collector-emitter or drain-source terminals will be attached to the signal path and ground. When the base or gate is biased so that the device is off, the signal will pass with no attenuation. As the base or gate is forward biased the signal will attenuate.

JFETs allow for shunt or series operation. These can be used individually or combined, as shown above. If combined, the control voltages would be opposite, i.e. when the shunt CV is high, the series CV is low, and vice versa.

Perhaps the most famous example is the UREI 1176N compressor. A JFET is used as the active attenuator. Whenever someone describes a compressor as a "FET compressor" they are probably referring to this type of topology. The Dan Armstrong Orange Squeezer uses a FET attenuator for its compression.

Most Boss and Ibanez and other commercial pedals use JFETs or MOSFETs as attenuators or audio switches in their bypass schemes. JFETs are used in series with the audio path. They "look like" very large resistances when "off," and very low resistances when "on."

A "gate" type effect fully saturates the transistor when the signal level falls below a threshold, causing the signal to mute.

Discrete VCAs can be build from transistors. Early synthesizers, and some DIY designs from PAiA are great examples of this. VCA usually means "voltage controlled amplifier," but in most synthesizer applications the VCA is really functioning as a "voltage controlled attenuator" where the max positive CV gives unity gain and max negative CV mutes.

Tremolo effects can be realized by muting the signal in a rhythmic pattern created by a LFO.

The Schaller Tremolo is a great example of discrete transistor circuitry in that it uses both a discrete transistor attenuator and a discrete transistor LFO. The Gretsch Playboy/JEN HF Modulator is another example. Kay Tremolo T-1.

Phase shifter effects can be realized by attenuating one side of a split signal using JFET attenuators. The Maestro Phase shifter and the MXR Phase 90 are the classic examples.