Gain Stages

starbuck26

starbuck26

New member
Hey Guys...

I've been doing a lot of live recording lately, and when recording a live band (while I'm playing in it) becomes fairly complicated. I find I usually wind up trying to walk myself through all of these separate gain stages and wondering if any of you had a technique or approach to dealing with it.

So, the drums go straight into a mixer, and use the preamps there. The guitar usually goes through a Presonus preamp, and the vocals through an ART tube pre. I send everything out of my mixer into a Fostex MR-8. So, with respect to guitars and vocals, they're essentially traveling through three separate gain stages. First pre-amp, mixer, and then the internal preamp on the MR-8. Couple this with the problem of recording on location with headphones instead of monitors and....... clipping, somewhere, sometime. I don't hear it when I'm recording, but I hear it later. And it drives me insane.

The question is two fold. How do I chase this out in the original mix... and, since I'm in the market for a new multitrack recorder... is there one on the market where I can bypass the built-in pres? I would much rather use a dedicated pre--I'm starting a small collection--for each channel.

Perhaps I am overthinking it.... :confused:

Cheers.
 
Without knowing where you are setting your gains/trims, I can only guess at an answer here.

A lot of people tend to misunderstand the use of input gain or trim. It is intended to boost the incoming signal only if it is lacking in strength. On any piece of gear, you want your incoming signal to be strong enough to activate the channel circuitry but not saturate it to the point of distortion. On the other hand, if it is too weak then the circuit will likely add background noise such as hiss. You should seek a happy medium.

If the output signal level of the preamp is strong enough the mixer channel input gain can be turned down to zero or off. The same goes for the MR-8 trim knobs. If the incoming signal from the mixer is strong enough, leave it set fully counter-clockwise. Only increase these when it is absolutely needed to keep the signal up to a healthy usable level.

This is what we call gain structure. The art of keeping the signal level the same as it is passed from piece to piece throughout the system.

The second part of your question can be answered sort of the same way. By leaving the input gain turned all the way down, you are essentially bypassing the built-in preamp.

Another thing to consider is that most people sound check at a lower volume than when they really sing. Leave yourself a little headroom.

Hope this helps,
RD
 
If you're hearing clipping on the recording that you didn't hear while performing. It's probably either one of two things...
  1. You're monitoring at the wrong point of the signal chain
  2. Not enough isolation in your headphones to hear the clipping
Make sure that you're monitoring from the output of your computer interface while you're perfroming... If you're listening to the output of the board (without returning the recording interface output), you're not going to hear any clipping through your A/D conversion into the box. If you are monitoring at this point, then you should probably invest in a powerful headphone amp with some cans...
 
Thanks!

That is excellent information. I did not realize that the MR-8's pres were bypassable in that way. So, excepting my drum tracks which use the mixer's preamp, I sould only be adjusting the dedicated pre for that channel. The other pres should be fully counter-clockwise?

I'm assuming I should be monitoring from the MR-8 headphone out, and not the mixer.

Problem is, I play in the bands I record. So when recording live I don't really have the benefit of a space where I can listen to the output in a seperate setting from where it's happening. Perhaps I should just take a couple of hundred foot xlr cables and train a monkey to record from accross the street and communicate by cell phone...

:D
 
ummm....

I don't know... I wouldn't be so sure...

I've seen some pretty talented monkeys in the new york times science section... :cool:
 
starbuck26 said:
I don't know... I wouldn't be so sure...

I've seen some pretty talented monkeys in the new york times science section... :cool:
Click to expand...
But not in your studio...

RawDepth gave you some good advice... turn down the gain, and if you're not available to ride gain or monitor accurately while tracking; turn it down somemore...

And I hate to beat this to death, but to me it's very important in gain staging...

  1. The amplifier stage of a preamp does not have a variable output
  2. Whatever the preamp sees at it's input is amplified only by the spec'd gain of the amplifier circuit
  3. So... you control the output of the amplifier by controlling the input
  4. The Input to the amplifier is controlled by the gain pot... which is a variable attenuator
  5. So... by decreasing attenuation at the input, you are actually increasing the input to the amp... which is then reflected on the output... which increases directly proportionally to the change at the input

Instead of turning up the output of the preamp when you turn the gain knob, you are actually trimming (or attenuating) the input, with the same net result.

Too much trim, or input too low... signal is closer to the noise floor and pre amplifies that with the signal... Not enough trim, or input too hot... you overdrive the amplifier input, signal clips off at highest output of spec'd gain on the amp circuit... you get square waves, distortion...

SO RD was right on the money when he says that gain staging is a compromise... the same compromises go into designing these amplifier circuits... and is one of the reasons they sound different than each other...
 
The MR-8 is 16 bit, no?
When i had one of the first generation, i really found the limits of the dynamics quckly, to get a decent track you really needed to have signals sitting in a very tight dynamic range of you would be either clipping on one end or pull of noise from the pre's in the MR-8 on the other end.

I would recomend upgradig to a 24 bit interface/recorder, you will be very impressed with the increased quality and fexiblity.

Daav
 
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