USER BEWARE!: Will ‘hot-plugging’ a microphone kill your pre-amp?

‘Hot-plugging’ a microphone – that is, plugging or unplugging a microphone while phantom power is on – will damage the performance of some preamplifiers, and may completely destroy them. The damage usually begins as a rise in preamplifier noise. Every ‘hot-plug’ event can add further damage to the noise performance of the preamplifier.

Defective microphone cables and microphones can create severe phantom-induced damage. If the mic cable shorts between one of the signal conductors and the reference conductor / shield (ground) while phantom power is on, it can be devastating to the life of the pre-amp. It is important to find out if your mic pre-amp is susceptible to this type of damage, as many are.

The reason phantom power can cause so much damage is because the + and – conductors (the XLR input pins) each have a voltage of +48 Vdc when phantom power is on. These conductors are often isolated from the pre-amp components (op-amps / transistors / tubes) by DC blocking capacitors. These DC-blocking capacitors are charged to 48 Vdc and store enough energy to damage the protection components and/or the sensitive components in the input stage of the preamplifier. When a microphone is ‘hot-plugged’, or when there is a short between the signal conductor and the shield (ground), the signal conductors are instantaneously dropped to 0 Vdc. This causes the capacitors to transfer the 48 Vdc potential to the pre-amp side. The pre-amp will be slammed with -48 Vdc. This can cause severe damage to many pre-amps.

The PRE420 (www.pre420.com) is one mic pre-amp that is built to withstand ‘hot-plugging’ and cable-shorting without any damage to the PRE420’s performance. When John Siau (Director of Engineering at Benchmark) was designing the PRE420, he decided to design a bullet-proof protection network that would make the PRE420 immune to ‘hot-plugging’. This network absorbs all of the stored energy and prevents damage to the sensitive input transistors whenever the 48 Vdc shorts to ground. The PRE420 can absorb an unlimited number of +48V signal short-circuit events without degrading the performance of the pre-amp.