AES Journal Forum

We're glad to see more discussion of this problem, and Ros Bortoni's paper gives good solutions that more console and mic preamp makers should observe. However the problem occurs not only when one side of the mic cable is shorted due to a fault, or when a line level output is connected with a mic input. It is also experienced when a microphone is plugged in to phantom-powered input, or disconnected from the input, while the phantom power is on.

The additional problem is that XLR connectors are not precisely symmetrical. Either pin 2 or pin 3 of the microphone cable connector will contact its mate on the preamp first, with the other contact being made a fraction of a second later. If the microphone draws very little current, this only results in a loud thump in the output because the transient current, hence the voltage spike, is limited. But if the microphone draws a lot of current as soon as it's connected (many currently available phantom-powered microphones from a variety of manufacturers exhibit this behavior), this can result in a differential spike of nearly the full phantom voltage across the differential input, with almost as much current as the shorted-contact simulation mentioned in the article. Even with low-current microphones, it can be a problem too when long cables are used; the surge current caused by charging the cable capacitance can be substantial for an instant. Some mic preamps are well protected from this condition, and the methods in the paper provide reliable means to test this. But many high performance mic preamps are not sufficiently well protected. The practice should always be to turn off phantom power and let the capacitors discharge for a few seconds before unplugging or plugging in a microphone cable.