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Author Topic: Still confused about gain structure  (Read 2158 times)

Eddie Vansell

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Still confused about gain structure
« on: September 20, 2007, 10:31:24 pm »

After reading many posts on the forum concerning gain structure, I finally realized that my church's problem with system noise was probably mainly attributed to bad gain structure. I'm pretty new at this so I need help understanding completely. I followed the instructions in the back of my DBX driverack on how to optimize gain structure. From my understanding of the manual, DBX's goal is to have your driverack and your amps to clip at the same point. (I probably need to mention that the system is running lows to two 18" subs and the highs and mids are running to a LCR cluster with three 12" cabinets) I disconnected the speakers from the amps and ran pink noise through the board and the driverack's crossover, turning up the pink noise's channel gain until the board and the driverack reached just below clip. Although the board read just below clip and the driverack's high level read just below clip, the low level meters did not, so I turned up the output on the driverack's low level until it matched the high level. I then turned up the sub amp and the cluster amp's sensitivity knob until their levels read the same as the board and driverack. (it was only two clicks until both amps reached the max level) I turned off the pink noise, set the limiters, and EQ'd the system. The result was no system noise. I EQ'd the most low end frequincies I could without clipping. My question is, if there still isn't enough low end as desired, then do I need more power? Why did my amps only turn two clicks before clip? The driverack and the amp meters read pretty closly, but did I kill all of my headroom? Is the system as loud as it can get, or would it be louder with the amp sensitivity knobs turned a little higher? After I EQ'd is it okay for my low sub meter on the driverack to be going above unity, does this mean I could have more headroom? If my mids and highs are louder than my subs, does it mean I could run a smaller amp on the cluster? I don't want to underpower the cluster, so should I just attenuate the highs and mids by reducing their bands on the EQ? Sorry for all the questions. I don't expect for all of them to be answered. I've just confused myself and the more I think the more messed up I get. Thanks guys.
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Don Boomer

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Re: Still confused about gain structure
« Reply #1 on: September 21, 2007, 09:25:17 am »

I've just confused myself and the more I think the more messed up I get.

Hey Eddie ... first the good news.  Nobody goes to jail over less than perfect gain structure.  Setting gain structure is a compromise.  It has to be because the program material is constantly changing.  it would be easy to set it if you could freeze time and set it for an instant in time ... but your music keeps going up and down.

Setting your gain is a compromise between distortion on the high side and system noise on the low side.  If you are not particularly bothered by either in your current configuration then you are probably close.

From my understanding of the manual, DBX's goal is to have your driverack and your amps to clip at the same point

Yes and everything else in your system, mixer, eqs, etc.  The big gotcha here is that while this will optimize your sn ratio it will not be very forgiving of anyone driving the board into clipping.  And even if you don't see clip lights that is sometimes a difficult thing to tell.  It also minimizes the capabilities of the limiters in your power amps so make certain your limiting is set up correctly in your driverack.  The formula for this is:
AMPLIFIER ATTENUATION(db) = AMPLIFIER SENSITIVITY(dbu) - Max output from Driverack(dbu)

So are you actually having a specific problem with the system?
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Don Boomer
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Eddie Vansell

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Re: Still confused about gain structure
« Reply #2 on: September 21, 2007, 12:41:36 pm »

The problem to begin with was that there was a huge amount of system noise. I noticed that it might be a gain structure issue when meters weren't hitting unity gain. The confusion that I caused myself is listed in the numerous questions that I posted in my first comment. I guess my question in a nutshell is did I kill all of my headroom and by doing this am I limiting my power output/total loudness.
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Don Boomer

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Re: Still confused about gain structure
« Reply #3 on: September 21, 2007, 02:29:16 pm »

I don't think you are asking what you really want.

By making tour system noisey you increased the headroom.

Is what you want to know how to get the noise out?  What amps are you using?  where do you have the sensitivity knobs set?

Basically turn your mixer up and your amps down.  The position of the knob on your amps does not limit the amount of power your amps can develop.

I thought the driveracks had an automatic setup feature.  Did you try that?  what happened.

Here's a white paper I wrote for  a Peavey product (that is similar) most everything holds true ... try reading this ...

http://www.peavey.com/assets//literature/additional/00420060 _7.pdf
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Don Boomer
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Kevin Pippen

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Re: Still confused about gain structure
« Reply #4 on: October 27, 2007, 10:56:13 am »

Hi Eddie,

Here is a good place to start: http://www.dbxpro.com/Forum/viewtopic.php?t=959

Kevin
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Gary Creely

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Re: Still confused about gain structure
« Reply #5 on: October 27, 2007, 04:03:56 pm »

Gain structure is a funny thing. I have found that their is how it should be, and then sometimes how it needs to be. It is difficult to get it right with a formula, because their are factors in play that sometimes require you to tweak from what is correct.

For instance some cheaper amps are really noisy, and you may need to tweak the gain to reduce that. I think DBX drive racks are noisy, so I usually tweak the gain to get around that. The list goes on and on as to why you could need to change from what is technically correct.
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Gary Creely
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Ivan Beaver

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Re: Still confused about gain structure-dynamic range
« Reply #6 on: October 27, 2007, 05:49:31 pm »

You do not increase headroom in a system by having a poor gain structure.  You increase the "apparent" headroom in the mixing console, but that is unusable "headroom" because as soon as you get past a certain output level (typically around 0db, the amp will hit their max output.

So, so what if the mixer can go louder, all the system will do is distort.  Personally, I don't care for distortion in my sound system, but to each his own.

This is an old road dog complaint (This console has no headroom) when they have to push it harder.  It has NOTHING to do with the console and EVERYTHING to do with the rest of the system.

Nothing you do anywhere in the chain, adjsutment wise, will give you any more headroom or output in the system.

A properly setup system will however have a greater dynamic range than one that is setup so that the console 0 meter is max amplifier output.

You cannot increase the max output (unless you change amps or loudspeakers) but you can lower the noise floor, making for a system with more dynamic range.
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Re: Still confused about gain structure-dynamic range
« Reply #6 on: October 27, 2007, 05:49:31 pm »


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