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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.

  • Your problem suggests problems with acoustics for the ringing. Have you tried a graphic equaliser between the pa an sound board? – Melloj Sep 10 '16 at 13:40
  • have a read here... good info if you're fuzzy on the various points at which you can adjust signal level... sound.stackexchange.com/q/23801/6957 – JoshP Sep 10 '16 at 14:17
  • I'm guessing your system is analog right through - so try this & report back. Wind up the main amps a lot, drop the input gains until you're at the same perceived level. Leave your channel volumes as they are but maybe pull back the master outs a little.. That ought to reduce your feedback. I'm guessing that your early stages are adding some unwanted compression/distortion to your later stages, which are almost running at idle. This may lift your noise-floor, so juggle until it's acceptable. – Tetsujin Sep 10 '16 at 19:34
  • @Melloj We have parametric eq on the desk. I haven't been successful at using it to reduce the ringing. Possibly I just don't have the right ear for this. I believe there's also some eq in the amp driver that's been calibrated to the room but that's not something I'm prepared to tinker with as it's hidden away behind various menus. – Carl Sep 12 '16 at 15:19
  • @JoshP Thanks for the link. I think I'm clear on the definitions but I don't think any of my gains are high enough to distort the signal. – Carl Sep 12 '16 at 15:22
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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.
  • Thanks for the suggestion (similar to @Tetsujin in the comments). I think this is similar to what we had previously: amps turned up, channel gains pretty low. I don't think the sound was any better but I'd have to try again. The big problem was that the bar graphs on the desk were constantly at zero with the occasional flicker if someone dropped a mic. Surely that's not how the desk is meant to be used? – Carl Sep 12 '16 at 11:28
  • @Carl Yeah I can see some overlap between my answer and the comment by Tetsujin. I was not aware of his comment when I wrote the answer (he should have made the comment an answer really). If the sound or feedback issues were not better back then (with the more optimal gain settings) there is really no point in your speculation about about preamp gain vs fader (but try it out again and see if it works) - and you should move on to some of the other points - and perhaps simply consider dropping a feedback destroyer in between the mixer and the P.A. – Michael Hansen Buur Sep 14 '16 at 8:07
  • I'm trying to understand why you suggest that optimal gain settings are "channel gain low, amp gain high" when everything else I've read says that the best gain structure is "channel gain high enough to hit 0dB, amp gain wherever it needs to be to get the right volume". So far, all the explanations seem to assume there's some unreasonably high gain leading to distortion but that's not the case according to the desk meters. I still don't see any theoretical difference between adjusting channel gain, faders or amps as long as there's no distortion, and yet in practice they behave differently. – Carl Sep 14 '16 at 9:24
  • @Carl True, the assumption is that the feedback issue could be due to a too hot channel gain setting and that the power amp and mixer volume is not full up; Of course you can and should use the faders too - otherwise you may end up with an undergained and noisy result or an output that is simply too loud. I simply present the method here as a way for you to check if the preamp (and its gain) itself could be a part of the problem. The optimal (and operational gain) is probably a setting that peaks around -6 .. -3 db (leaving plenty of headroom). Then you use the channel fader and attenuate – Michael Hansen Buur Sep 14 '16 at 10:50
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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.

  • Good links. Thanks. Note that the first video for setting gain structure (and plenty of others) describe exactly how we set up our desk, but this is the opposite of what was suggested by a couple of others here (i.e. set amps high and lower channel gain). It might be worth ringing out some of the mics, although I'm not sure how beneficial it is for a lapel mic which is going to be heavily affected by how it is worn. Note that the main speakers have already been "rung out" by a pro and this is configured in the amp driver (I think that's what it's called). – Carl Sep 13 '16 at 17:05
  • @Carl, when you say they are already "rung out by a pro" on the "amp driver" then you might want to get more details of that, because even the slightest variation in room, placement of objects, placement of microphone and other variations can cause that "ring out" to be invalid. Try to get more info on the system and update your original post, so more precise answers can be given. Bottom line, if the system is "rung out" and you do have "feedback" then the "rung out" is no longer valid and has to be re-done.... I have also updated the answer with some generic info on gain and faders. – Edwin van Mierlo Sep 15 '16 at 8:58
  • We have something that looks exactly like a dbx DriveRack PA+ although I don't know if it's that specific model. Apparently it includes some EQ and feedback elimination but from the design I suspect it's not meant to be modified very often (everything is hidden away behind what looks like a complicated menu system). I'm not aware of the room having changed much since the system was installed a few years ago but inevitably the microphones get moved around a bit. Apart from the DriveRack, we don't have a graphic eq, only the parametric eq on each channel to play with. – Carl Sep 15 '16 at 9:11

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