Q. Is there an easy way to match the gain of different channels?

I’ve committed myself to recording a school orchestra in a couple of weeks. Obviously, this will involve using stereo pairs of mics. However, none of my preamps have stepped gain controls and, in fact, most of them have very tiny knobs, so matching the gain on different channels by eye is unlikely to work well. Is there a better way to match the gain across different channels? Would it be better to take a small tone generator and hold it against the front of the mic, or something?

Ceri Jones, via e-mail

SOS Technical Editor Hugh Robjohns replies: 

A tone generator is one solution, if you can guarantee to get it the same distance from both capsules, but it’s fiddly and not that reliable, in my experience.

There are several good alternatives, though, depending on what kind of mic arrays you’re using and how easy they are to get to. It’s also made easier if you have a Lissajous meter display (goniometer) like the DK-Technologies MSD series, and a monitoring system that allows easy access to the side (stereo difference) channel.

A goniometer can help you easily match gain across channels if your preamps don’t have stepped gain controls, but there are also cheaper methods.

The easiest approach is to roughly set the mic gains by ear during rehearsal. Then at the break, when the room is empty and quiet, get someone to stand in the front-middle of the stage and clap their hands repeatedly (or, if they’re not shy, sing a constant note).

On a goniometer you’ll see very clearly the stereo axis of the sound source, so, you can then turn down the louder channel (the side the goniometer trace leans toward) to bring the display back to the centre line. Turning the loud side down maximises headroom, of course, and is a safer way to go than bringing up the quieter side!

If you don’t have a goniometer, a reasonably practical solution is to configure the monitoring to listen to the ‘side’ or stereo difference signal (polarity-reverse one channel and mono-sum them).

If the two sides are equally matched, there should be a deep cancellation null, so by looking at the meters to figure out which channel is louder, wind that down until you pass through the null, and then bring it back up to provide the deepest possible null. Then restore the monitoring to normal stereo. This process works well for continuously variable gain controls that aren’t closely matched, such as those you’re describing.

Frustratingly, though, few monitor controllers have facilities to switch to hear the side signal, and few people appreciate the true value of goniometer metering displays, both making the situation you describe trivially simple to check and resolve. 

Published in SOS August 2010