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Author Topic: Incorrect Gainstructure  (Read 3937 times)

Dennis Berglund

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Incorrect Gainstructure
« on: May 11, 2011, 12:35:35 am »

Hello folks, just a quicky question.
Since im running my rig for dj gigs (24-48 hour Techno parties) and i have no chance in the world to stay awake to babysit the rig i was thinking that incorrect gainstructure could do the trick since i don't have any brickwall limiters.

But what is the worst thing that can happened if i set it up like this, will i loose dynamics or will it sound poor?:

1. Set dj mixer to clip
2. Amplifier attenuator are set fully clockwise. (Amp is clipping)
3. Adjust the gain to the amps in the driverack crossover (Amps are not clipping anymore)
4. Turn down dj mixer so its not clipping anymore
5. I set the limiters on the Driverack to save the rig in case the shit hits the fan.


What are the bad sides using the crossover gain to prevent amps from clipping?
I doubt the ppl or the djs will start messing with the driverack, its more likely the would mess with the attenuator if i set a proper gainstructure if they want it louder, thats why i want it set fully clockwise.

Cheers
« Last Edit: May 11, 2011, 12:40:11 am by Dennis Berglund »
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Dave Neale

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Re: Incorrect Gainstructure
« Reply #1 on: May 11, 2011, 01:20:30 am »



you are exchanging s/n ratio for increased headroom.  Given the noise floor at a dj gig, I would consider optimal s/n a low priority compared to ensuring some idiot doesn't damage my gear.  You'll be able to dial in as much headroom as you want at the crossover. 

Dynamics will be up to the performer.  If they want to push their mix into the limiters and leave it there, there is not a whole lot you can do about it. 

I think that it is a good solution for you based on the reasons you have cited.

You didn't specify what amplifiers you are using.  Is it possible you are overlooking any clip protection that is built into your amps?

Also, you should consider bidding your events to include a second hand that can keep an eye on your system when you need to eat, sleep, etc.  I seldom leave a system unattended if I can help it, and never at "underground" events. 

I couldn't afford to buy my gear the first time...

I'm sure you've got a trustworthy friend that you can train to watch for red lights, that would love to push some gear and watch the girls dance all night long for some pocket money.
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Frederik Rosenkjśr

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Re: Incorrect Gainstructure
« Reply #2 on: May 11, 2011, 07:11:52 am »

It doesn't matter if it's the amp that's clipping or something earlier in the chain. The amp could be passing on a badly clipped signal just within it's own normal operating space and the output of the amp would about as damaging but without a single flicker of the clip lights.

The only way to make your system completely fool proof with or without a limite (but without any sophisticated solutions like d&b system or something) is to make sure that the speakers can handle whatever comes out of the amp - no matter what it looks like, meaning it has to be able to handle a perfect square wave indefinitely.

This will imply setting your limiter way lower than the system can actually play in peaks. Figure out what your speakers continuous power rating is, figure out what that means (what crest factor was used in obtaining that rating) and set your limiters so that a square signal (with a crest factor of 1 / 0 dB) can not exceed this figure.

This will mean that your rig will be a lot quieter than it can be with live audio material and high crest factors, but it should be safe. In this case you need to bring a lot of rig for the gig. Typically, if your speakers handle peaks of, say, 3200 watts the "program" rating would be 1600W (-3dB), the "Continuous" rating would be 800W (-3dB again) and that would be a crest factor 2 measurement, meaning you'll have to 3 dB lower again to protect yourself against all types of signal. So now your rig is 9 dB down from what it can actually do on live material.
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Brad Weber

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Re: Incorrect Gainstructure
« Reply #3 on: May 11, 2011, 08:38:04 am »

But what is the worst thing that can happened if i set it up like this, will i loose dynamics or will it sound poor?:

1. Set dj mixer to clip
2. Amplifier attenuator are set fully clockwise. (Amp is clipping)
3. Adjust the gain to the amps in the driverack crossover (Amps are not clipping anymore)
4. Turn down dj mixer so its not clipping anymore
5. I set the limiters on the Driverack to save the rig in case the shit hits the fan.


What are the bad sides using the crossover gain to prevent amps from clipping?
I doubt the ppl or the djs will start messing with the driverack, its more likely the would mess with the attenuator if i set a proper gainstructure if they want it louder, thats why i want it set fully clockwise.
As Frederik said, no matter where in a signal chain the signal is clips it is still clipping.  While one device clipping may be better than multiple devices clipping, many people actually set their systems so that the one device to first clip is the amplifier, that way you have a little warning before everything else starts clipping.
 
When you note adjusting the DriveRack 'gain' are you adjusting the analog output level after the D/A conversion or the digital signal level prior to the D/A?  If you're adjusting the analog output level then that is virtually the same effect as adjusting the amp input attenuators, however if you are adjusting the digital signal level then that is different.  Depending on the DriveRack model you may also be able to adjust what analog output level 0dBFS in the digital realm represents.  I believe that you can also do the same with the inputs, setting what incoming dBU analog signal level equates to 0dBFS after the A/D conversion.
 
Limiters may control the maximum levels but the more you push into the limiters, the closer the average level may become to the peak level.  If you're assuming the DJs may be running into the limiters all night then I agree that you may want to consider the impact that has on the long term power being delivered to your speakers.
 
An old trick that may work for you was to set proper gain structure, determine the attenuation needed to get the desired levels and then insert a fixed pad or attenuator of that value before the amp (maybe hidden in a rack).  That way you can set the system for good gain structure and run the amps wide open.
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Dave Neale

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Re: Incorrect Gainstructure
« Reply #4 on: May 12, 2011, 02:47:39 am »

You might also consider having a small console set up side stage and using it as the input to you system, rather than patching the dj mixer directly into your PA.

You can control their levels, squeeze them as needed with an inserted comp, and clean up their EQ BEFORE it hits your system inputs.  I'll often patch an announce mic for them as well.
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Chris Gruber

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Re: Incorrect Gainstructure
« Reply #5 on: May 13, 2011, 01:22:17 am »

You might also consider having a small console set up side stage and using it as the input to you system, rather than patching the dj mixer directly into your PA.

You can control their levels, squeeze them as needed with an inserted comp, and clean up their EQ BEFORE it hits your system inputs.  I'll often patch an announce mic for them as well.

+1. A MixWiz and a stereo comp with a high ratio(10.1+) will solve a lot of problems. Tip: Grab some gaff and tape the master knob or fader on the dj mixer where you set it. A couple layers does the trick. I always ask first but after explaining the why's and that it won't leave a residue, no one has said no. Between fixing the max output of the dj mixer and the compressor you shouldn't have to drastically alter your dsp settings. Good luck!
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Dennis Berglund

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Re: Incorrect Gainstructure
« Reply #6 on: May 14, 2011, 03:53:26 am »

Thanx for all tips :) I'm thinking of getting a small console quite soon, can't afford more shopping at the moment since im saving up for a van to carry the stuff.

I don't have the best stuff at the moment but i guess its a good start and i hope it will pay itself.
Or at least i will make it last to pay itself. :)



Im running my 4 dynacord corus 2.18 with 2 t-amps 2400 mk-x and the 4 jbl jrx 125 tops are driven by 2 peavey pvi-3000 amps. The crossover is a DBX Driverack PA and djmixers i have are Pioneer Djm2000 and Ecler evo5.
The amps and mounted in a 12 unit rack with the driverack, power distribution and an I/O panel.

The most expensive parts in my setup are the dj mixers so i don't really want to put gaff tape on it.
The ecler mixer is designed by the same guy who designed Ferrari so putting gaff on that one wouldn't feel good. It has a built in limiter, but im not sure how good it is, maybe i should try it out a bit more and see how good it works. I hope it's a brickwall.


Calculating crest factors and Db's is really far beyond my understanding but id love to learn it.


How good is the limiter in the driverack?
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Bob Leonard

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Re: Incorrect Gainstructure
« Reply #7 on: May 15, 2011, 10:04:21 am »

But what is the worst thing that can happened if i set it up like this, will i loose dynamics or will it sound poor?:

1. Set dj mixer to clip
2. Amplifier attenuator are set fully clockwise. (Amp is clipping)
3. Adjust the gain to the amps in the driverack crossover (Amps are not clipping anymore)
4. Turn down dj mixer so its not clipping anymore
5. I set the limiters on the Driverack to save the rig in case the shit hits the fan.


What are the bad sides using the crossover gain to prevent amps from clipping?
I doubt the ppl or the djs will start messing with the driverack, its more likely the would mess with the attenuator if i set a proper gainstructure if they want it louder, thats why i want it set fully clockwise.
As Frederik said, no matter where in a signal chain the signal is clips it is still clipping.  While one device clipping may be better than multiple devices clipping, many people actually set their systems so that the one device to first clip is the amplifier, that way you have a little warning before everything else starts clipping.
 
When you note adjusting the DriveRack 'gain' are you adjusting the analog output level after the D/A conversion or the digital signal level prior to the D/A?  If you're adjusting the analog output level then that is virtually the same effect as adjusting the amp input attenuators, however if you are adjusting the digital signal level then that is different.  Depending on the DriveRack model you may also be able to adjust what analog output level 0dBFS in the digital realm represents.  I believe that you can also do the same with the inputs, setting what incoming dBU analog signal level equates to 0dBFS after the A/D conversion.
 
Limiters may control the maximum levels but the more you push into the limiters, the closer the average level may become to the peak level.  If you're assuming the DJs may be running into the limiters all night then I agree that you may want to consider the impact that has on the long term power being delivered to your speakers.
 
An old trick that may work for you was to set proper gain structure, determine the attenuation needed to get the desired levels and then insert a fixed pad or attenuator of that value before the amp (maybe hidden in a rack).  That way you can set the system for good gain structure and run the amps wide open.

Just a minor correction. A clipped input is not the same as an amplifier pushed to clipping. An amplifier pushed to constant clipping can and will develop 2-10 times it's rated output power, the reason voice coils are burned. A clipped signal can be a distorted guitar, MOOG, screaming DJ, etc.. There should be no damage from these events unless they also force the amplifiers to high level clipping.
 
This old, but not outdated, paper from JBL gives a simplistic look at clipping and not enough rig for the gig.
 
http://www.jblpro.com/catalog/support/getfile.aspx?docid=246&doctype=3
 
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Brad Weber

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Re: Incorrect Gainstructure
« Reply #8 on: May 18, 2011, 07:35:46 am »

Just a minor correction. A clipped input is not the same as an amplifier pushed to clipping. An amplifier pushed to constant clipping can and will develop 2-10 times it's rated output power, the reason voice coils are burned. A clipped signal can be a distorted guitar, MOOG, screaming DJ, etc.. There should be no damage from these events unless they also force the amplifiers to high level clipping.
Clipping a signal affects the signal similarly, including increasing the average level and total energy of the signal, regardless of where in the signal path the clipping occurs.  Look at Chuck McGregor's comments in this, http://www.prosoundweb.com/article/clipping/.
 
A simple example would be a speaker powered with an amp that is rated at the Program or even Peak rating of the speaker.  Clipping of the signal feeding the amp anyhwere in the signal chain results in the average level of the signal increasing while the peak level stays the same, a lower crest factor.  With a heavily clipped input the crest factor of the signal could be less than the 3-6dB assumed in the Program and Peak power ratings of the speaker, with the result that the average level fed the speaker could exceed the long term rating even if the peak level is never exceeded.  So the amp never clips since its peak level is never exceeded, however the voice coil of the speaker burns out due to the amp being fed a clipped signal.
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Bob Lee

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Re: Incorrect Gainstructure
« Reply #9 on: May 25, 2011, 05:30:59 pm »

Just a minor correction. A clipped input is not the same as an amplifier pushed to clipping. An amplifier pushed to constant clipping can and will develop 2-10 times it's rated output power, the reason voice coils are burned. A clipped signal can be a distorted guitar, MOOG, screaming DJ, etc.. There should be no damage from these events unless they also force the amplifiers to high level clipping.
Clipping a signal affects the signal similarly, including increasing the average level and total energy of the signal, regardless of where in the signal path the clipping occurs.  Look at Chuck McGregor's comments in this, http://www.prosoundweb.com/article/clipping/.
 
A simple example would be a speaker powered with an amp that is rated at the Program or even Peak rating of the speaker.  Clipping of the signal feeding the amp anyhwere in the signal chain results in the average level of the signal increasing while the peak level stays the same, a lower crest factor.  With a heavily clipped input the crest factor of the signal could be less than the 3-6dB assumed in the Program and Peak power ratings of the speaker, with the result that the average level fed the speaker could exceed the long term rating even if the peak level is never exceeded.  So the amp never clips since its peak level is never exceeded, however the voice coil of the speaker burns out due to the amp being fed a clipped signal.

The only reason a clipped signal would be dangerous to a loudspeaker would be if it is too much power. That would be true of a clean signal as well, though. The issue is the power, not the shape of the waveform. An amp clipping is putting out power in excess of its rating--up to 2◊ more in extreme cases. A clipped signal fed into a power amp need not result in an excessive level on the output unless the amp's gain is set that way.
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Bob Lee
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Re: Incorrect Gainstructure
¬ę Reply #9 on: May 25, 2011, 05:30:59 pm ¬Ľ


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