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Author Topic: Live Sound Newbie FAQ  (Read 27748 times)

Dave Dermont

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Re: Live Sound Newbie FAQ
« Reply #10 on: October 14, 2009, 09:09:09 am »

John Wilder wrote on Mon, 12 October 2009 02:36

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.



Actually, them there controls on the amp are NOT input attenuators. They are gain controls.

See this post by Bob Lee.

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John Wilder

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Re: Live Sound Newbie FAQ
« Reply #11 on: October 15, 2009, 12:08:48 am »

True...if you look at the actual circuit itself you'll see that yes they are in fact gain controls for the amplifier's "pre-drive" section. The power amp section itself has a fixed amount of power gain and it requires X amount of input signal to reach full power out, and the required amount of signal to accomplish that is determined by the fixed power gain of the amp itself. However, the input signal at the input of the amp will determine how much gain the pre-drive needs to add to the signal to reach that required value. The more clean signal you have from the board, the less gain required from the pre-drive section to get the amp to swing to full output voltage, which is why the controls are never ran full up.

However...there was a very legit reason for leaving that fact out of my explaination.

When teaching a 'newbie' (after all, this is in fact FAQ for the live sound newbie), I don't prefer to teach them that concept as it overcomplicates things for the novice. It's much simpler to comprehend for the novice by simply explaining them as adjustable input pads.

Another reason why you don't explain it to the novice like that is because to most novices I've encountered, in their mind the term "gain" = "volume" and that's not the case. I've found that when teaching someone from scratch, my first goal is to get the false pretense that gain=volume out of their heads...it leads to the "I want more volume so I add more gain" mentality that leads to disaster everytime. I always teach that gain is a measure of amplification factor and that the channel's preamp in most "club/weekend warrior" setups has more than enough gain to drive the signal through the mixer and driverack all the way to the amplifier without having to add it downstream of the channel preamp at different points in the signal path. It's always much better to add as much clean gain as possible at the mixer channel's trim control upstream of the driverack and amplifier and only have to add a little at the amplifier, than it is to set your mixer channel gain low, then have to add additional gain at the amp itself by running the amp's controls full up.
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Pat ONeill

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Re: Exciters, Enhancers, and Similar Products
« Reply #12 on: January 20, 2010, 08:10:02 pm »

Spend the $$ on compression......much more useful! Very Happy
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