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  #1 (permalink)  
Old Saturday, February 25th, 2012, 08:06 AM
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Amp Attenuators

Is there any reason why you would attenuate your amps besides for change in distance from the speaker to the audience?

We have 4 CX1102 amps powering Renkus TRC121 boxes. All the amps are in stereo and have external EQ and limiting. The boxes are not bi-amped. The TRC has a 700w program rating and the CX1102 has a 700w 8 ohm stereo rating.

Would the contractor purposely hold the system back to limit the output levels that the sound operator could produce?
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Old Saturday, February 25th, 2012, 08:27 AM
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If you are talking about the knob on the power amp, yes, it is usually not at max. It is part of the gain structure. in short, you turn it all the way down, You set up the board so that with a average input on a channel, that channel is outputting 0 db. Example, Put a boom box in front of a mic. Set that ch input at min, set the fader to 0, now bring up the input untill that ch is at 0. Next the main fader is brought up to zero. (If you don't have meters on each ch you may need to bring the main up to zero first.) Now the output of the board is zero db (average) Now you go to the power amp gain and bring it up to as loud as you will ever want it.

In your case the contractor probably did this and more. He would have set up the board as described, then he would have put pink noise through the board and adjusted the 4 amps to get the most even coverage for the entire room. He would also adjust the external EQ to give the most accurate reproduction of the pink noise in your room. Finally he would check his work with music and speech through the system.

The system is not held back, it is tuned to the room. If you have already changed them, call the contractor back.

A big advantage of proper gain structure is a lot less hiss and hum in the system. Another advantage is that you can use the meters.

I gave a short version, there are usually more steps.
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Old Saturday, February 25th, 2012, 08:36 AM
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I figured it was part of proper gain structure.

Why not gain down the outputs going to the amps at the processor vs using the attenuators on the amps? This is what the same contractor did at our student building so was just wondering. In the student building, the DriveRack takes care of everything and has different attenuation for each output to the amps and the amps are maxed out. (except for one delay speaker)
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Old Saturday, February 25th, 2012, 08:41 AM
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Depending on the design of the processor/DSP, reducing the output gain at the processor also reduces the headroom and increases noise.
By attenuating at the amplifier, the gain structure upstream is optimized for best signal to noise ratio.
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Old Saturday, February 25th, 2012, 10:59 AM
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Quote:
Originally Posted by cmchamp View Post
Depending on the design of the processor/DSP, reducing the output gain at the processor also reduces the headroom and increases noise.
By attenuating at the amplifier, the gain structure upstream is optimized for best signal to noise ratio.
To expand on this, some DSP devices provide analog attenuation after the D/A conversion and in those cases attenuating the analog signal there is similar to attenuating it at the amp input. However, for many DSP devices, especially lower cost ones, the output attenuation is digital attenuation before the D/A conversion and attenuation applied there can affect headroom and noise.

There is a third type of DSP that allow you to set what analog output level a 0dBFS digital signal represents during the D/A conversion. These are less common and one impact is that any output metering may then no longer represent what you think it does.
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Old Saturday, February 25th, 2012, 10:16 PM
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If the system is set up with the amps cranked, no one can render the system limiters useless by messing with the amps. To me, that is reason enough to always crank them, and back the gain off at the tail end of the DSP (if needed).
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Old Sunday, February 26th, 2012, 10:22 AM
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Quote:
Originally Posted by Tim Padrick View Post
If the system is set up with the amps cranked, no one can render the system limiters useless by messing with the amps. To me, that is reason enough to always crank them, and back the gain off at the tail end of the DSP (if needed).
Agreed, but only if the DSP in question allows you to adjust the analog output gain and not just the gain while still in the Digital domain.
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Old Sunday, February 26th, 2012, 01:12 PM
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Quote:
Originally Posted by Tim Padrick View Post
If the system is set up with the amps cranked, no one can render the system limiters useless by messing with the amps. To me, that is reason enough to always crank them, and back the gain off at the tail end of the DSP (if needed).
But, how many amps are going to work like this, for every application? The contractor may have spec'd the same amps for redundancy, for fills and mains, highs and lows. Not all amplifiers have the same gain. The pot is not always an input sensitivity control. The gain and pot function is manufacturer specific.

If someone is going to jack with the system, and they ALWAYS do, thinking they know more than the installation contractor, they are going to jack with the processors just as much as the amps. So, I do not buy that logic at all. I like using other products with levels of security, or blank face IP addressable boxes so people cannot mess around with it.

I will say, that I recently installed a QSC line array in a church with a very fine speaker system. The know-it-all, always tinkering, tin-eared, sound man replaced the dedicated speaker processors with a dbx driverack. The driverack was insufficient in DSP of course, but also could not function the same way. The system processor removed used sense lines from the speaker systems. It was inserted between the amps and speakers. This guy destroyed a great sounding system by tinkering. The new, young, not so educated, music director came in and forced the purchase of the new speaker system, all because of a lack of knowledge and experience.

Churches waste significants amounts of money, ignorantly. That money should be used to do better things than for church show-biz IMHO.
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Old Sunday, February 26th, 2012, 07:13 PM
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Quote:
Originally Posted by pdc View Post
But, how many amps are going to work like this, for every application? The contractor may have spec'd the same amps for redundancy, for fills and mains, highs and lows. Not all amplifiers have the same gain. The pot is not always an input sensitivity control. The gain and pot function is manufacturer specific.
Virtually all professional power amps will produce rated output from a very low input level when the amp level control is turned all the way up. The exact sensitivity usually is somewhere between 0.5 volts (-3.8 dBu) and 2.5 volts (+10.2 dBu). Most equipment that drives the amps has an output capability between 7.9 volts (+18 dBu) and 25.1 volts (+28 dBu). Obviously attenuation is needed someplace.

Many folk turn the gain down on the power amp. This may or may not work as desired since there is almost always circuitry before the level control, and that circuitry might still overload no matter where the level control on the amp is set.

Quote:
Originally Posted by pdc View Post
If someone is going to jack with the system, and they ALWAYS do, thinking they know more than the installation contractor
Agreed. You can never stop people from messing with the system.

However, if the attenuation was taken before the amp in a suitable DSP, then the worst they can do is turn _down_ the levels.

If the amp level controls were turned down, they they can turn the levels up and blow up the speakers. It also takes a trip, often across country, to fix such a messup. If the amp's correct setting is full up you can tell that to your customer over the phone and save you both the time and expense of a cross country flight. Been there done that, don't ever want to have to do it again!

Quote:
Originally Posted by pdc View Post
they are going to jack with the processors just as much as the amps. So, I do not buy that logic at all. I like using other products with levels of security, or blank face IP addressable boxes so people cannot mess around with it.
Since virtually all professional grade DSP products need a computer and software (and maybe also a password) to adjust, I do not see that as an issue. At least in my world of professional grade installed sound systems, DSP with front panel controls and programming virtually do not exist.

Quote:
Originally Posted by pdc View Post
The know-it-all, always tinkering, tin-eared, sound man replaced the dedicated speaker processors with a dbx driverack.
When someone is willing to rewire a sound system, they can defeat any protection scheme.

Quote:
Originally Posted by pdc View Post
Churches waste significants amounts of money, ignorantly.
The only solution for that is education.
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Old Sunday, February 26th, 2012, 08:11 PM
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I totally get the limiter/sensitivity conundrum.

No, not all products need a password, and just about all Crown and dbx products have no password or lock capabilities. The Crown amps that do merely have a lock/unlock feature that anyone can do.The processor in question (one of the ones brought up in this thread) was a Driverack. There is no way to protect settings in that thing. EV DCOnes do not have password protection. None of the Harman, EV, Ashly, Symetrix, etc requires a password. A computer is not required to set up any of the dbx, EV processors. Of course, your idea of pro and mine are likely the same, and using Media Matrix, higher end Symetrix, EV NetMax, Q-Sys, etc are not what we are talking about here.

As for customers changing things, this is where documentation, education and rack security come into play. We document settings on the gear itself, in addition to a binder. We also install contractor series amps most of the time with removable knobs and/or cover plates (QSC for example). Some of the Crown install amps have no pots at all on the front. If someone is going to get into our rack, they must have a key. If they have a key, they must have permission from someone in charge. Once the person in charge authorizes access, we get some billable hours. It is on them. You cannot protect against a poor lack of common sense no matter what. In the old days when we were installing Crown DC series amps, we used a little thing called epoxy to keep pots from moving.
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Old Monday, February 27th, 2012, 08:09 AM
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The 'old school' approach, and one that still works well, was to setup the system gain structure, set the amp input attenuators for the desired level and figure out what attenuation was required. Then you would insert pads with that attenuation value just before the amp inputs and turn the amp controls all the way up. That provides the desired levels without having to know if the processor is controlling the digital or analog levels, helps avoid overdriving the amp input, allows the amp controls to be set a known and easily reset level and, especially is you put the pads somewhere not easily accessible, limits people playing with things in a way that could damage something.
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Old Tuesday, March 6th, 2012, 04:34 PM
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Quote:
Originally Posted by Frank DeWitt View Post
If you are talking about the knob on the power amp, yes, it is usually not at max. It is part of the gain structure. in short, you turn it all the way down, You set up the board so that with a average input on a channel, that channel is outputting 0 db. Example, Put a boom box in front of a mic. Set that ch input at min, set the fader to 0, now bring up the input untill that ch is at 0. Next the main fader is brought up to zero. (If you don't have meters on each ch you may need to bring the main up to zero first.) Now the output of the board is zero db (average) Now you go to the power amp gain and bring it up to as loud as you will ever want it.

In your case the contractor probably did this and more. He would have set up the board as described, then he would have put pink noise through the board and adjusted the 4 amps to get the most even coverage for the entire room. He would also adjust the external EQ to give the most accurate reproduction of the pink noise in your room. Finally he would check his work with music and speech through the system.

The system is not held back, it is tuned to the room. If you have already changed them, call the contractor back.

A big advantage of proper gain structure is a lot less hiss and hum in the system. Another advantage is that you can use the meters.

I gave a short version, there are usually more steps.
If the system was installed years ago, and a previous sound guy tinkered with it until it sounds like a 99¢ AM radio, does it require a professional sound technician to reset it, or could the current sound guy, repeat these steps and get at least most of the way there. At least until next years budget
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