Practical difference between channel gain, fader and amp gain

Question

Why do channel gain, fader and amp gain all seem to affect the sound differently, particularly when it comes to feedback? Is this just a placebo effect? Is it a symptom that something is connected wrong or that our levels are way off? Might any of the equipment be at fault? Or is this a normal characteristic of a sound system?

This is in the context of a church PA system and my attempts to get speech to come across loud and clear. We often get a bit of unpleasant ringing if we turn things up too loud, but only for speech coming from lapel or gooseneck mics. The music and singing is hardly ever a problem. I've never been successful in fixing this using equalisation. Turning things down helps but then we get complaints that people can't hear. We don't have any foldback monitoring on these particular mics so the sound is only coming through the front of house speakers. We do send a feed to an induction loop and to a computer for recording.

I've noticed that turning the channel gain down and pushing the channel fader up sometimes seems to reduce feedback. Recently I've also observed that turning the amps up a bit and decreasing the gain on the desk also succeeded in decreasing the ringing while maintaining a constant volume, although then the desk bargraphs reported that levels were well under 0dB so not optimal as far as I understand.

I usually adjust the channel gain so the bargraph flickers around 0dB, push the fader to 0dB with a bit of adjustment to account for different voices or people standing at different distances from the mic. The amps are set using some recorded music for calibration (adjust gain so desk indicates levels around 0dB then set amp gain so volume is pleasantly loud). The amp gains are relatively low and the channel gains are usually around the middle of the available range. There's no clipping.

Answer 1

In theory there shouldn't really be any difference: Lets say you add some gain at the channel input, and reduce it equally by turning down the fader. Then there should be no difference.

But preamps have their limitations: the more amplification, the greater the chance for clipping and non linear behaviour. Unless that is the exact effect you're going after (as the case is with many guitar amps), you want to preserve as much head room as possible in (at some point things get too noisy or too low volumewise).

That is why you need to ensure maximum gain settings, starting from the destination end (power amp volume) and working towards the source end (the channel gain):

1. Set power amp volume at max
2. Set mixer main volume at almost max - the 0 db setting
3. Set channel fader at almost max - the 0 db setting
4. Now you raise the gain of the mix channel

Note: before you start: turn down all channel gains completely ;-)

I often see people being too causious with the power amp / P.A. volume, and it later on forces them to use too hot gain settings to get the desired volume.. then things start to feed.

EDIT: Of course this does not mean you can't or shouldn't use the mixer faders: the reason I recommend the loud fader settings as a start is for you to test if the preamp gain itself relates to the problem you describe. Optimal and safe gain settings would be something that peaks around -6..-3 db, leaving plenty of headroom for temporary louder peaks. If things overall get too loud, simply use the master fader, and use the channel faders to tweak channel volume during show (at this point you should have the optimal gain settings for each channel from the sound check).

If this does not bring you closer to the goal, you need to look into the following:

1. Mic positioning - is it too far from the source; too close to the P.A. etc.
2. Room acoustics
3. E.Q. and feedback destroyers
4. Super cardioid mics for better background cancelling or even additional cancelling mics
5. A bigger P.A.

Answer 2

While a good answer already given, I approach this a little different. (I would not turn the amp to full volume as step 1 for instance, nor would I raise the gain at step 4, as I would set gain in the gain structure, not to be used of ringing out a system)

First of all, I would set a good "gain structure" in the mixer/board. This to ensure adequate head room for signals and not to amplify (or trying to avoid to amplify) noise.

While I can describe in detail how I set gain structure, a good video exists here

(I have no affiliation with video or creator of such)

Once you have set a proper gain structure, you need to ring out the system. There are usually 2 amplified systems in use, one: "monitors or foldback", and two: "Front of House". Again, there are video's which are much better in explaining this then me typing some text:

Monitors

FOH

(same disclaimer)

As you have described that you are in a church, I actually found this

There are many resources on various websites, both video and text, I would advise to search around.

[EDIT -- after some thoughts]

The title of the post lists a question, which was not answered yet.

Practical difference between channel gain, fader and amp gain

In a typical desk, there are a couple of places where "gain" can be set.

• The "Pre-Amp gain", usually found on top of the channel strip (but not always)
• The "Channel Fader", usually found at the bottom of the channel strip
• The "Group Fader", usually found between channel faders and master fader
• The "Master Fader", usually found at the right of the desk.

Larger desks/consoles will have different layouts, and obviously manuals should be read on how to use the various desks, this is only a general explanation.

What to use them for (practically as the question is)

"Pre-Amp Gain"

The Pre-Amp Gain should be set, so that if:

• the channel is the only channel in use
• the channel fader is set to 0 dB
• (if used) the group fader is set to 0 dB
• the master fader is set to 0 dB

---> then the loudest possible passage of the input signal should not exceed 0 dB on the output meters.

This is setting the "gain structure", optimum signal path, optimum pre-amp gain. Although not mentioned, the [PFL] feature is implemented on many desks which makes this a lot easier to do.

"but what are all those faders for"

To put it simple: to pull back the signal, or attenuate the signal in your mix, to format the end result

• to pull back a single channel, pull back the channel fader
• to pull back multiple channels, assign the channels to a group, assign the group to main, and pull back the group fader
• to pull back main (to lower the volume), pull back the main/master fader

There could be other faders and features on any particular desk, and I would advise to always read the manual, and to have a good look at the block-diagrams to ensure you understand how the signals are routed through your desk.

"but what about the "amp gain""

One of the practices is to set the "amp gain" to the loudest level needed for the venue (estimate noise of people when the venue is full), at a 0 dB output of the desk. This way you can "pull back" with the main/master fader of the desk.

"ah now; that is just all theory, it doesn't work like that"

No, theory and practice are two different things, sometimes you just need to push a single channel a little more, hence when the gain structure is set at a 0 dB level, most desks have room on the fader slider above 0 dB. You do need to realise this is all in dB, so push a fader from 0 dB to +3 dB means double, to +6 dB is four times.

If you find that you are pushing channels during a live show, then you need to learn from this, something in your sound-check did go wrong. I have made many mistakes over the years, and only by doing you learn how to operate this more efficiently. I still learn everytime, no two situations are the same!!

But for what this goes:

• "gain" to push signal to 0 dB,
• "faders" to pull-back and create your mix,
• "main/master fader" to pull back sound level,
• "amp gain" to set maximum sound level needed for the venue.

.... usually it is better to pull back, then to push up: avoid clipping distortion and stay out of maximum ranges of op-amps and circuitry.

And from there it will only get more complicated when you start using inserts, compressors, limiters, gates, various effects, EQ, feedback destroyers... you get the picture...

HTH, Edwin.