Many microphone users want to insert a mute button in-line with a mic cable. With 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.

Figure 1: Microphone on/off switch Popless Mic On/Off Switch

The cause is phantom-power imbalance. If the phantom DC voltage on pin 2 is higher or lover 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. It is described in Figure 1.

This is essentially a low-pass filter, with a custom frequency down 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. Thanks for your suggestion, Bob!