Trim vs Fader

[jagpunk]

What's the difference between input trim and the fader? They're both preamp, right? I know that by rule, you should set the the fader to 0 and adjust the input trim knob from there when balancing levels, but why? Does it have something to do with noise/distortion?

[regebro]

Yes, thats it exactly. The input trim is use to set the level of the signal so that it is as high as possible without distortion. This is done to minimize noise.

[Blue Bear Sound]

Originally posted by regebro
Yes, thats it exactly. The input trim is use to set the level of the signal so that it is as high as possible without distortion. This is done to minimize noise.

Well... yes... but that's not quite clear. The idea is to use the trim control to make the signal levels at each channel consistent coming into the board.

Let's say you have a vocal going to channel 1 and a keyboard going to channel 2. The vocal mic (assuming you're not using an external pre) will be generating a low-level signal. You set the FADER at the nominal "0" level (because that's your fader starting point for best s/n), then you adjust the TRIM control until the signal hits 0 on your meters. Then, you follow the same process for channel 2 - FADER at 0, play the keybaord and adjust the TRIM until you hit 0 on the meters.

What you have just done is adjusted the relative levels of 2 different signal sources so that they have the same frame of "level" reference coming into the board. If you look at where the TRIM level is for channel 1 (in the above example) versus channel 2, you'll find the mic probably required a significant increase in the TRIM setting (more clockwise), whereas the keyboard required much less (more counter-clockwise). But as far as the board is concerned, you've "evened-out" both signals to make best use of s/n and headroom levels.

Bruce

[skippy]

Just adding a little more information to what Bruce and Regebro had to say: they're dead on, of course.

The other thing is the concept of "gain structure", which supplies the "why" for the things the other posters have said. This basically means that you want your signal to experience all the gain that it is ever going to need right in the mic preamp, before anything else ever touches it. Let's say that your mic level signal needs 60dB of gain to give you a 0dB (reference) level at the mixer's output. The mic preamp is designed and optimized to give you lots of gain with minimum noise. So if you set the trim for 60dB of gain, you can set the channel fader and master fader at 0dB (unity gain), and your levels are set. Happy, happy.

On the other hand, you could set the trim for 40dB, and then run the channel fader up against the stops at roughly +10, and the master at roughly +10 as well(the levels don't add linearly, of course, but bear with me for this example: the numbers aren't that important). It may seem like this achieves the same result, but think about it: the mic preamp produces some noise level. The channel fader makeup amp that amplifies the whole signal, *including* that noise, by +10dB. The master fader makeup amp then multiplies the (preamp noise*10dB) by another 10dB! Any way you slice it, this results in more noise than you need.

Bottom line is that you want to set the trims so that you get all the gain you're ever going to need right at the mic preamp, and not at the channel fader or summing buss makeup amps. They are good, but not as good as the preamp. If you get into the habit of setting levels with the trims, and then riding the gains with the faders (and EQs, and outboard gear) from unity (0dB) *down* whenever possible, you'll minimize the noise, and make the best of your gear.

Most people never really think about how noise stacks up: but adding more gain to an already noisy signal is by definition a hands-down lose.

[Tom Hicks]

For unity gain,most engineers design this "zero" point to locate at about 70 percent of a fader or knob's travel.So if you set your fader for a channel strip at this point,bring up the trim pot until the master out leds are just hitting amber.Repeat for each channel as about.Then subtract 2 dB per channel used from the master out.
This rough guide will give you "max headroom" to make adjustments with minimal clipping.

Tom

[Blue Bear Sound]

That's usually the nominal 0 level on the faders anyways, which, if the board is calibrated properly will correspond to a 0 reading on the meters (which is the start of the amber indicators!) Funny how it all falls into place!!!

Bruce

[jagpunk]

Thanks everyone, for explaining on what I already inferred, but with all the details that make my thirst for knowledge temporarily quenched.

Most of what you guys said what I already kind of figured, but now I know EXACTLY why. I appreciate your posts.

[Tom Hicks]

Bruce
Your moniker has changed recently.New professional affiliation?Wasup?

Tom

[Blue Bear Sound]

No Tom, I was always adding "Blue Bear Sound" to my signature anyways... I just got tired of the nonsensical BVALERIA - when I registered, I didn't realize that the account name was going to be the displayed name... I finally got fed up enough to re-register.

Professional affiliation, hell yeah - you bet!!! (But I could disappear if you guys don't want me here - I'm not like FT and co. hanging around when I'm not wanted!!!)



Bruce

[The Green Hornet]

Yo Bruce:

DON'T DISAPPEAR. Everytime you post, I learn something.

Green Hornet

[Blue Bear Sound]

...no worries -- I was kidding!!!

(I like it here!)



Bruce