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Live Sound Newbie FAQ

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Donnie Evans:
This one isn't asked all that frequently, but it's a good thread that I think doesn't need to be lost.  

Calculating Limiter Settings
http://srforums.prosoundweb.com/index.php/m/218674/23615/0// /17922/#msg_218674

Too Tall (Curtis H. List):
Andy Peters wrote on Fri, 28 December 2007 13:18
Another FAQ:

Q: "I'm thinking about getting an Aural Exciter (or similar product). What do you all think?"

A1: "Don't bother. They all suck. Spend the money on another microphone."

A2: "Never use these things across the main outputs. They might be useful as on an individual channel insert. But you're better off spending the money on another microphone."

-a


+1
They belong in a studio, if anywhere.

A band hired me to mix their set on another person's PA in a night club.
The guy had a nice system in general, but he used some kind of exciter over the mains.
I switched it in and out a few times, then I made a small EQ change on his 1/3-octave EQ and asked if it sounded the same as his exciter.
He said yes, while scratching his head.

Bennett Prescott:
Can someone find and link to any of the threads regarding "why subwoofers create a power alley, how cardioid subwoofers work, and why it always sucks when I stack my subwoofers left and right, but sometimes not indoors, or on a tuesday"?

Tony "T" Tissot:
I find this a very simple, and informative discussion about amp sizing questions. - Particularly the link to the Peavey tech. note.

http://srforums.prosoundweb.com/index.php/m/335411/35327/0// /15490/#msg_335411

You'll learn - under-powering is not the culprit for driver damage. Expecting "Champagne" results from a "Beer" budget is.

My favorite quote: "Inexperienced operators often try to use a system that just doesn't have enough power (speakers and amps and AC power) to do the job properly, not the power of the speakers themselves. This results in pushing the system way too hard and making failures more common."

John Wilder:
How about...

How does the amp get full output power when the "volume" knobs aren't turned up all the way?

The knobs on a power amp are NOT volume knobs. They do not alter the amplifier's power gain in any way.

They are "input attenuators" and they are a matching device. They are used to "pad" the input sensitivity so that the input sensitivity of the amplifier matches the mixer's "max before clipping" output.

Most power amps have an input sensitivity of 1-2Vrms with no input attenuation. However, a typical +4dB mixer can kick out about 8-10 volts before the mixer clips...WAY too much signal for the amp input with the attenuators up all the way. So the input attenuators give you an "adjustable pad", allowing you to match the input sensitivity of your power amp to your mixer's "max signal amplitude before clipping" value, allowing you to get max possible gain from your mics and from the mixer to the amps without the system being so sensitive.

You'll still get full power output...again, the power gain is a fixed value and cannot be altered. It's just gonna take a much bigger signal from the mixer to get that max output, which is a good thing. Benefits - lower noise floor, amps aren't near as sensitive/feedback reduction/elimination, ability to get max possible gain = highest signal/noise ratio.

Now if your amps are not reaching full output power before the mixer clips, you have your attenuators turned down too much. Open them up some. If the amp is hitting clip before the mixer is (i.e. you can still get more gain from the mixer without clipping the mixer itself yet the amp is already clipping), turn the attenuators on the amp down. The trick here is to get the mixer and everything downstream of it to hit its clip point at exactly the same time. This is known as setting a proper "gain structure".

With the speakers disconnected, if you were to inject a pink noise signal into a mixer channel, set the channels EQ flat with low shelving filters off, channel and main faders at "0" (unity gain), then slowly increase the gain/trim of that channel, everything downstream of the mixer including the amps should hit clip at exactly the same time the channel's "Clip" indicator just starts to indicate clipping.

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