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Author Topic: Gain structure issue  (Read 2315 times)

John P. Whiteker

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Gain structure issue
« on: December 13, 2020, 03:36:48 pm »

So, we have all X32 channel strips at unity with nominal gain (nothing clipping, nothing too weak)  typically bumping along about -18db.  The issue is that our worship leader prefers the sound of the Crown XTI's run at max volume while running the mains on the mixer about -20db.  My theory and argument is to set the main FOH fader to 0 (unity) and then adjust the amp output volume to what we'd like the room to sound like.  But every time I change that gain structure he feels like he loses some depth or dynamic range.  Which one of us is crazy?  I have some hiss in the mains from running the amps all the way up, so to me, the better signal to noise level ratio is achieved by fixing that gain structure.  Am I missing something?
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Behringer X32
Crown XTI power amps 4002 for mains & 6002 for subs
JBL J-Arry mains
JBL Subs

Tim Weaver

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Re: Gain structure issue
« Reply #1 on: December 13, 2020, 06:38:47 pm »

So, we have all X32 channel strips at unity with nominal gain (nothing clipping, nothing too weak)  typically bumping along about -18db.  The issue is that our worship leader prefers the sound of the Crown XTI's run at max volume while running the mains on the mixer about -20db.  My theory and argument is to set the main FOH fader to 0 (unity) and then adjust the amp output volume to what we'd like the room to sound like.  But every time I change that gain structure he feels like he loses some depth or dynamic range.  Which one of us is crazy?  I have some hiss in the mains from running the amps all the way up, so to me, the better signal to noise level ratio is achieved by fixing that gain structure.  Am I missing something?

What if I told you that you could both have your way?

Since XTi's have DSP you could go into the amp and reduce the input gain by 20dB and then run both the console and the (fake) amp knobs at 0dB.



Those knobs are just for convenience. They don't change anything. They especially don't change the "sound" of anything. In fact they'd be a pretty poor qualioty amp if they sounded different at different gain settings.
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Bullwinkle: This is the amplifier, which amplifies the sound. This is the Preamplifier which, of course, amplifies the pree's.

John P. Whiteker

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Re: Gain structure issue
« Reply #2 on: December 13, 2020, 07:41:14 pm »

What if I told you that you could both have your way?

Since XTi's have DSP you could go into the amp and reduce the input gain by 20dB and then run both the console and the (fake) amp knobs at 0dB.



Those knobs are just for convenience. They don't change anything. They especially don't change the "sound" of anything. In fact they'd be a pretty poor qualioty amp if they sounded different at different gain settings.

That's a good idea.  I could go that route.  However, I used to think those knobs didn't do anything like you say.  But if we crank them up, we definitely get more volume from the speakers.  Not sure what's going on there because I thought they were disabled.  Guess I'll look into that a bit more.
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Behringer X32
Crown XTI power amps 4002 for mains & 6002 for subs
JBL J-Arry mains
JBL Subs

Tim Weaver

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Re: Gain structure issue
« Reply #3 on: December 13, 2020, 08:10:50 pm »

That's a good idea.  I could go that route.  However, I used to think those knobs didn't do anything like you say.  But if we crank them up, we definitely get more volume from the speakers.  Not sure what's going on there because I thought they were disabled.  Guess I'll look into that a bit more.


I mean, um. Yeah. They are volume knobs. Just digital ones in which you can set the range on. So if you turned them up then the speaker will get louder.
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Bullwinkle: This is the amplifier, which amplifies the sound. This is the Preamplifier which, of course, amplifies the pree's.

Luke Geis

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Re: Gain structure issue
« Reply #4 on: December 13, 2020, 10:21:41 pm »

Modern self-powered speakers don't really have a need for gain structuring. The volume knob on the back is literally just that. Most tend to have a nominal/unity gain setting, but that is only as a reference to the standard +4dbu signal level that pro audio uses. The reason most powered speakers go above unity gain is because there are times when you will plug a microphone, iPod, or other devices directly into it that don't have enough gain on their own to get loud enough to be usable. Having a positive gain trim pot available allows you the luxury of not needing a mixer 100% of the time to get enough volume to use it for most anything.

The idea of trying to make the subs clip at the same time the mains does is not really going to work the way you think it will. In theory, assuming the volume pot on all the speakers are set the same and you send the same signal into each speaker, they will all clip at the same time. If you do happen to run with subs on an aux, it will still clip with the same relative signal level. The volume knob is nothing more than an attenuator, if you turn it down, you will just need that much more drive level in order to acquire clipping. Turn the volume knob up and you will just need that much less drive signal to acquire clipping. The gain or overall volume of the speaker WILL NOT CHANGE and will produce the same max output regardless of where the volume knob is set, what will change is how much drive signal must be sent in order to cause clipping. So set them at unity so you always know where the limit is.

The idea of using the volume knobs to balance the PA to me is not a good idea. In most cases, the tops will get louder than the subs if using a 1:1 ratio ( 1 sub for each top ). And even if they do achieve the same SPL, in most cases you will want more sub than the tops, so again you would have to turn the tops down to balance the PA. I don't like using the volume knobs to do this, I prefer adjusting the drive level to the speakers instead. This way you know that if you reach a certain metered level, it will be ok or not. If you tune the tops down to get the sub balance, you will probably find pretty quickly that you are running out of headroom for the subs when you get the level you need from the tops.

Most modern powered systems that are run with a 1:1 ratio ( again, 1 sub for each top ) only have enough sub output to get the point across, but NOT crack ribs or rattle houses to the ground. You typically need 2x to 4x the amount of power or more going to the subs as you do the tops to achieve a level of sub power that really says I mean business. So if you have 2 tops ( one left and one right ) you would want at least 2 subs per side to as many as four subs per side to really say hello. If you don't have that number of subs or that amount of power, you MUST accept the compromises that come with it. If you must have " the balance " then you will have to accept that your PA will not get as loud as you may like. Or you will just have to live with the fact that you don't have enough sub output.

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Tim Weaver

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Re: Gain structure issue
« Reply #5 on: December 13, 2020, 11:07:49 pm »

What powered speakers?
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Erik Jerde

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Re: Gain structure issue
« Reply #6 on: December 14, 2020, 12:56:34 am »

So, we have all X32 channel strips at unity with nominal gain (nothing clipping, nothing too weak)  typically bumping along about -18db.  The issue is that our worship leader prefers the sound of the Crown XTI's run at max volume while running the mains on the mixer about -20db.  My theory and argument is to set the main FOH fader to 0 (unity) and then adjust the amp output volume to what we'd like the room to sound like.  But every time I change that gain structure he feels like he loses some depth or dynamic range.  Which one of us is crazy?  I have some hiss in the mains from running the amps all the way up, so to me, the better signal to noise level ratio is achieved by fixing that gain structure.  Am I missing something?

If you want to get this done just on the console then run your stereo bus to a matrix and then feed the amp from the matrix.  Set amp to full, set master to unity, trim matix sends to where you want the level to be.  That way you can still get very discrete control with the master and keep all your channel faders close to unity too.
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MarkDubosky

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Re: Gain structure issue
« Reply #7 on: December 14, 2020, 09:51:02 am »

So, we have all X32 channel strips at unity with nominal gain (nothing clipping, nothing too weak)  typically bumping along about -18db.  The issue is that our worship leader prefers the sound of the Crown XTI's run at max volume while running the mains on the mixer about -20db.  My theory and argument is to set the main FOH fader to 0 (unity) and then adjust the amp output volume to what we'd like the room to sound like.  But every time I change that gain structure he feels like he loses some depth or dynamic range.  Which one of us is crazy?  I have some hiss in the mains from running the amps all the way up, so to me, the better signal to noise level ratio is achieved by fixing that gain structure.  Am I missing something?

You are correct, in theory.

However, depending upon how the amplifier input attenuator is configured, the performance could be degraded as the worship leader describes.  It shouldn't happen that way, but some attenuators have non-linear errors at less than "fully open".

In this case, either lower the amp gain internally, use an outboard processor to electronically attenuate the signal before it reaches the power amplifier, or perhaps use a step-down transformer, such as the Radial JDI duplex, which can take a balanced professional input and convert it to balanced microphone level.  If you go this route, be sure that you have enough gain to compensate for the transformer attenuation.  Jensen transformers also makes a step down line level transformer that may help.

Just some possibilities
« Last Edit: December 14, 2020, 09:55:47 am by MDubosky »
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Kevin Maxwell

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Re: Gain structure issue
« Reply #8 on: December 14, 2020, 10:46:13 am »

If you want to get this done just on the console then run your stereo bus to a matrix and then feed the amp from the matrix.  Set amp to full, set master to unity, trim matix sends to where you want the level to be.  That way you can still get very discrete control with the master and keep all your channel faders close to unity too.

That is what I do. On installs that have the X32.

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brian maddox

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Re: Gain structure issue
« Reply #9 on: December 14, 2020, 03:13:24 pm »

If you want to get this done just on the console then run your stereo bus to a matrix and then feed the amp from the matrix.  Set amp to full, set master to unity, trim matix sends to where you want the level to be.  That way you can still get very discrete control with the master and keep all your channel faders close to unity too.

I've used this workflow for decades. Works very well.
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Re: Gain structure issue
« Reply #9 on: December 14, 2020, 03:13:24 pm »


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