I have some reservations on the microphone mute circuit published in the Circuit Notebook pages of the April 2007 issue. First, there is no provision for powering the FET in either version. Phantom power provides 48V, with positive on both pins 2 and 3, negative on pin 1. The battery-powered version does not connect the battery negative to the circuit.

Second, it is very poor audio design to short out a signal path, especially a low level one such as a microphone or to impose anything prior to the preamp. Normal muting is done by interrupting the signal by either a hardware switch on the microphone body or in the mixer, or more usually by a VCA such as a noise gate.

I used several methods of control of leakage on front of stage mikes as far back as 1972, when I designed the Grateful Dead’s Wall of Sound system. We used two instrument-grade differential mikes which cancelled in a preamp and inverse summing circuit at the mike mounting. Later, a VCA circuit was built with an industrial-grade switch pad (such as is used to control heavy machinery) and connected via an insert jack on the mixer.

The switch pad must be sourced from an industrial machinery dealer; I had to go to the US for mine. All of the commonly available switch pads are lightly constructed of flimsy foil, easily damaged and very unreliable. This is very important to allow a quick bypass in case of failure of the pad or circuit.

Today, in my PA I use the same industrial-grade switch pad with a simple signal generator to feed an 800Hz tone into the key insert on a Drawmer 201 noise gate. The use of a noise gate allows a precise adjustment of the attack, delay and amount of signal reduction during gating; a total mute is neither necessary nor desirable.