Q. Do I need a mic with a HPF and pad?

By Hugh Robjohns

I've just been looking at buying a cheap condenser mic and I was wondering how useful it is to have the low-frequency cut and 10dB pad options that are on some mics. I assume the low-frequency cut is there to cut out bumps and rumble from knocking the mic stand. Is this still a problem when using a shockmount? How essential is it to have these facilities on a cheap mic?

Nick Bramwell

Technical Editor Hugh Robjohns replies: The 10dB (or sometimes 20dB) pad on capacitor mics is intended to prevent the head amplifier (the part of the mic which amplifies the signal picked up by the diaphragm) from overloading when the mic is placed in front of very loud sound sources. Obviously, if the head amp overloads there is nothing you can subsequently do at the desk or preamp to rectify the problem. If recording loud sources like close-miked drums or brass instruments is something you intend to do, then a pad switch will be very useful.

 
Low-cut and 10dB pad switches, as found on the Groove Tubes GT67 and GT55, are a useful feature, though not essential.

The low-frequency cut is a more complex matter. Different manufacturers intend this 'low-cut' filter to serve different functions. Some do indeed provide a high-pass filter (HPF) intended specifically to reduce low-frequency vibration and rumbles. Again, in severe situations, low-frequency energy picked up mechanically by the mic can cause overloads in the head amplifier, and hence building the filter into the mic amp circuit can be very beneficial. In more typical situations, though, the HPF on the mixing desk, preamp or recording channel is usually as effective.

However, it is worth being aware that some manufacturers provide a 'low-cut' filter which is intended instead to help compensate for the rise in bass energy when a source is close miked — the proximity effect. In this situation, the 'low-cut' filter will not be so effective in removing mechanical rumbles and the like. So, it is worth finding out what kind of 'bass-cut' filtering is provided — it should be listed in the manufacturer's specs.

If you're using a decent elastic suspension shockmount, you may well find that you don't need to use the HPF. There's a good case for not using it unless you have to — unlike head amp overloads, which you're stuck with, the decision to filter out low frequencies can be made later on, should it be necessary.

Published November 2003