Trim control

[jreskin]

I'm not exactly a newbie, but I think this is a pretty newbie question, so I'm posting here.

As far as the trim (or gain) control on each input channel of a mixer goes, I've always just winged it and set my levels with my ears and my recorder's meters. Nothing wrong with that I don't think, but the other day I got to looking closely at the trim knobs on my console (a Mackie 8-bus). I noticed that for mic input, the trim knob is calibrated from 10 to 60 dB, and for line input, it's calibrated from 10 to -40 dBu (in the clockwise direction, left to right). I can understand that for mics, this means that the preamp is providing anywhere from 10 to 60 dB of gain on top of the mic's signal. But for line input, why does the trim knob's value decrease as you turn it up? That is, as the knob is turned clockwise, line input gets louder, but the value gets lower (closer to -40 dBu). How can that be? Can someone explain what's going on here?


Many thanks!
Joe R.

 

[boingoman]

The nomenclature is odd, but it works like this:

The mic input gain scale tell you how much gain is being applied to the mic signal. Mic signals vary a lot in level, so it makes more sense to just see how much gain you are applying.

The line level scale on your mixer is referenced to 0 dbV or 1 V RMS, and is an indication of how much signal is needed to bring the signal to that level. All the way down, you need a +10 dbV signal to make your channel be at 0 bdV. All the way up, and a -40 dbV signal will bring your channel to 0dbV. I do not know where the idea comes from, but that's how it works.

 

[jreskin]

boingoman,

Thanks! You couldn't have explained it more clearly. That is a weird way of doing it, but I understand it now.


Joe R.