Popless Mic On/Off Switch

Many microphone users want to insert a mute button in-line with a mic cable. When you use a dynamic mic, all you have to do is short XLR pins 2 and 3 together to mute the mic. But with a condenser mic, shorting pins 2 and 3 can cause a click or pop.

The cause is phantom-power imbalance. If the phantom DC voltage on pin 2 is higher or lower than on pin 3, you get a pop when you flip the on/off switch. Bob Stadtherr, of Bob Stadtherr Engineering, kindly supplied us with a circuit to prevent this problem. Stadtherr describes the circuit shown below:

popless mic on-off switch

This is essentially a lowpass filter, with a cutoff frequency around 1 Hz (assuming a 150 ohm mic impedance). The 10K resistor allows the capacitor to charge to the DC voltage between the lines, so when the switch is closed, no change to the DC levels occurs.

Pin 2 might be positive relative to pin 3, or vice versa. So the capacitors should be non-polarized. To create the 1000 uF capacitor, you could use two 470 uF polarized capacitors in parallel, wired in opposite polarity.

Thanks for your suggestion, Bob! The figure below shows how to add an LED to this circuit. Use a DPDT switch and a 9V battery. It can be wired so that the LED comes on when the mic is on, or when the mic is off. In the figure below, the DPDT switch is shown pictorially rather than schematically. The components in the figure below are the same as those in the figure above.

popless with led

Some people have had better luck using a switch in parallel with a potentiometer, both across pins 2 and 3. You adjust the potentiometer until there is no voltage difference between pins 2 and 3, then throw the switch to short pins 2 and 3 together.

popless mic on-off switch with potentiometer