Chinn on Trim – Setting Input Trims on a Mixer
Rick Chinn presents a quick and easy way to set input trims on a mixer.
A recent Syn-Aud-Con Listserv thread provoked an interesting discussion about the input stage of a mixer. Rick Chinn’s contribution to the thread touched on the technical and practical issues, and ended with a quick and easy way to set input trims. I include it here with his permission. pb
Let’s say that the gain range of the gain trim control is 60dB to 0dB. Usually this indicates the signal level required at that setting of the gain trim control to 
attain the channel’s design center level. Usually this is around 0dBu, but it could be lower, buying more headroom at the expense of noise (it might also be higher, especially if the equalizer is limited to +/- 12dB). If the maximum level capability of any sub circuit in the channel strip is +22dBu, then for the 0dBu design center level, we have 22dB of headroom. But however good this may seem, remember that the EQ circuitry is typically after the mike preamp and before the channel fader, so that 0dBu nominal level can get eaten into quickly if you apply significant boost via the channel equalizer.
In most mixers, the gain of the channel strip is calculated from input to the output of the fader makeup amp and doesn’t include the insertion loss of the pan pots. Typically the pan pot loss gets combined with the loss thru mixing and made up in the mix amp or the polarity inverter following the mix amp (this is common in large mixers). Sometimes the pan pots have their own makeup gain amps prior to the channel assign switches, but this is a “multiply by N” cost factor item and in lower cost products, there just isn’t room in the factory cost.
The smart way to set gains is to set the channel faders at their unity gain spot, master faders up no more than 5dB below nominal. Now do a rough mix using the gain trim pots. Check levels using the board’s solo system and ensure that nothing is clipping. If so, then you need more gain in the master and less at the gain trim. Ideally you should make the rough mix a bit louder than necessary, so that you can take up the excess in the master. Your system gain, after the console, should be enough to make the system too loud under the circumstance, and low enough that you haven’t compromised the overall s/n ratio.
Setting gains in this manner results in all of your channel faders being in a straight line, which is an easy-to-recall setting, and provides maximum headroom in the input channels. Noise theory tells us that for best system noise figure, the first stage wants to get all the gain, and everything afterwards runs at/near unity. You can’t always anticipate what will happen once the venue fills with people, so that’s the reason for starting with the system master down from unity gain.
I’m sure this will reignite the controversy once more. Think about my method; it makes sense both mathematically and from an operational point of view. I’ve used it for as long as I’ve used mixers with gain trim controls, and my experience predates those by at least 10 years. rc
