Scientific approach to setting amplifier gains
We have all seen on this forum the helpful posts by Phil, David, and Brian on how to properly set amplifier gains by ear. The issue with this is that these three have been around stereos most of their lives so they can easily hear what max output/borderline distortion sounds like. But for the rest of us, I don’t think it’s as easy.
Which brings me to the purpose of this post. The manual for my M6600 JL Audio amp describes a way to set the gains by measuring output voltage at the speaker wires (with the speakers NOT connected) and setting the amplifier gain to achieve a specific target voltage. This method requires the use of a test CD that contains sine waves at specific frequencies which JL sells for $10. You set the HU volume to about 3/4, insert the CD, choose the track with the appropriate sine wave frequency, then turn up the gain until the voltmeter reading equals the target voltage. The target voltage is calculated using the following formula:
Target Voltage = the square root of rms power per channel times resistance. For example, on my M6600, which is rated at 75 watts per channel at 4 ohms, the target voltage when connecting a pair of 4 ohms speakers to a pair of channels is 17.3 volts. This is calculated by taking the square root of [ 75(rms power) x 4(resistance in ohms) ]. 75 x 4 = 300. The square root of 300 = 17.3.
So you would be looking at a voltmeter instead of listening for distortion or clipping which at least in theory, would seem to be a more accurate and objective way to set the gain. This method assumes that your speakers are rated to handle the maximum rms output of your amp.
So has anyone used this method and if so, could you comment on how it worked for you? Or is everyone doing it by ear? I am an engineer by education and manage an engineering office so the technical method appeals to the way I think :)
Phil/David/Brian - any thoughts on this?
Thanks,
Al