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In Reply to: Help: How to set gain leve?? posted by cktc on December 15, 2004 at 11:19:51:
The gain setting isn't causing the feedback. You need either a parametric or graphic EQ to notch out the feedback frequencies. Gain on a power amp is usually set to wide open. If you only have one speaker that's problematic. At least two, one to either side of the podium, is the minimum, and a distributed system is better.
Follow Ups:
> Gain on a power amp is usually set to wide open. <That shouldn't be. Most situations require considerably less than maximum gain. If power amp gains were supposed to be wide open there wouldn't be any gain knobs in the first place.
-Bob
I have to speakers, one on each side. Do you mean I have to find out the feedback fequency and then tune it down on the EQ on mixer.
You can do it yourself by seeking out the offending frequencies by trial and error or get a feedback eliminator, which is an EQ with a microprocessor inside to do the job for you. The EQ on the mixer, assuming it is less than 31 band, has too wide a bandwidth to notch out offending frequencies without killing the ones you want to keep.Bob, I've always been in the habit of cranking my power amps to full so that the output stage of the board isn't overloading either itself or the input stage of the power amp. Problem with that?
Hi Bill,Most pro power amps at full gain hit full power at somewhere from about 0.7 to 2 volts RMS (~ -1 dBu to +8 dBu) of input, depending on the model and the load. Most line level pro gear can put out +18 dBu or higher. If I run my power amps at full gain, I would have to reduce the signal somewhere so that the maximum levels don't exceed those lower points that would clip the amps. That would sacrifice about 10 to 20 dB of potential signal-to-noise between the point of reduction and the power amp gain control.
That may not be too terrible, depending on how much gear is operating with reduced signal levels and how good it is. If you dial back the levels on the mixer, for instance, and then run the signals through a slew of processing gear, you have a greater potential for an intrusively high noise floor than if you, say, turn down the gains on the various bands of a crossover that feeds the amps directly.
If I'm driving amps with a piece of consumer or hi-fi gear that can only put out a maximum of a volt or so, then I would probably need to turn the amps all the way up.
Syn-Aud-Con (www.synaudcon.com) has published some good procedures for optimizing gain structure.
-Bob
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