Monitors – Board Unity Gain vs Amplifier Output Gain

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Today’s post is regarding monitors.  At North Ridge Community Church, NRCC, we run our three separate monitor mixes from the front of house.  These monitor mixes are fed via the pre-fader aux sends, go through a graphic equalizer, amplified by a Crest Audio amplifier and then show up to our JBL floor wedges.

This is a very normal way to do monitor mixes and all is great, until… I discover how it is truly set up.

  • The amplifiers in the amp room are all turned up to full.
  • The output of the board aux sends are not full signal.  In fact they don’t have enough signal to light up the -21dB light on my equalizer.
  • 100-105dB volume normally output by monitors.
  • AC hum present when system is turned on.

Because the are turned up so loud, the aux send knobs are in the 9 o’clock to 11 o’clock range.  While this doesn’t sound that bad, on the Allen & Heath ML4000 the unity gain (+0dB) for aux sends is at 3 o’clock and the knobs are a logarithmic potentiometer.  They act just as a normal fader does on the sound board, where there is more resolution in the higher areas of the fader.  Basically if you move the aux send knob from 9 o’clock to 10 o’clock you would increase the volume by 10dB.  If you were to move the knob from the 2 o’clock to the 3 o’clock position you would increase the volume by 1-2dB.

Found at http://www.bigmuffpage.com is a perfect graphic for explaining the logarithmic taper of the potentiometers that are used in aux sends.

Another thing that is happening by having such a low output from the board and the amps turned up high is easily explained in the graphic below that I made:

By having the board turned down and the amplifer turned up to full, you have made your signal to noise ratio very small. But when changed to a full board output and to a lower gain on the output of the amplifier you will have a high signal to noise ratio giving you less noise in the monitor system and better clarity.

As shown in the above graphic, by having the board aux output turned down and the amplifer turned up to full, you have made your signal to noise ratio very small. By keeping it this way you amplify the noise along with the small signal sent from the board.  This makes for a noisy system. But when changed to a full board output and a lower gain on the output of the amplifier you will have a high signal to noise ratio giving you less noise in the monitor system and better clarity.

By keeping your board output in a unity gain region you have higher resolution on the aux knob giving you an easier time mixing the monitors.  Also this leads to when you do an AFL (after fader listen solo) you don’t need to turn your headphones up to hear it.

After turning the amplifiers down and turning the board up I was able to keep the same overall volume with the monitors but now there is no audible noise in the room from the system being on.  If you find yourself having a hard time getting a consistent monitor mix or a noisy system, you might want to double check your amplifiers output gains to make sure they are receiving a unity gain signal.

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2 thoughts on “Monitors – Board Unity Gain vs Amplifier Output Gain

  1. In your YouTube channel,I noticed that all your amps where turned up during the amp room tour. Doesn’t this same principle apply???

    • It would, but in this case the EAW system processor relies on the amps at full gain to correctly compensate the use of different output gain amps for the tri amped EAW KF650e’s.

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