Proper amplifier gain setting
Here's how you do it:
Set head unit volume to 3/4 of maximum. 3/4 is a general guideline, some play higher without distorting, and some do not. Turn off all eqs/presets in the head unit.
P = Power in watts
I = Current in amperes
R = Resistance in ohms (effectively the nominal impedance)
V = Potential in volts (Voltage)
Resistance (nominal impedance of your speakers, 2-ohm, 4-ohm, etc.)
Power (desired wattage)
Voltage (we will measure this)
P = I*V (formula for power)
V = I*R (Ohm's law)
So after a little substitution to get Voltage in terms of power and resistance we get
V = square root (P*R)
So, for example, say you have a 4 ohm load presented to a 200 (RMS) watt amp.
V = square root (200*4) = 28.3 volts
This means you should increase the gain until you read 28.3 volts AC on the speaker outputs of your amplifier.
As a source, use a sine wave recorded at 0db at a frequency within the range you intend to amplify. You can get a test CD relatively inexpensively at most car audio shops, or from Parts Express (www.parts-express.com
Additionally, you could use a scope to actually check if the signal is clipping, but I'm not going to get into that here as I doubt very few people have access to that type of equipment.
Put your sine wave cd in your player, and put it on repeat. Be sure all head unit EQ’s are off or set to flat and turn volume to approximately 3/4, and also be sure the gains and Bass Boost (if applicable) are turned all the way down. Put one lead of the dmm (digital multimeter) on the positive terminal of your amp and the other lead on the negative for the same channel. Your meter should be set to AC voltage. Remember ac voltage x ac voltage / speaker impedance=watts. So if your getting 50volts ac @ 4 ohms your amp is producing 625 watts. 50*50=2500/4=625. This little trick will get you in the ballpark on setting gains without having to invest in an oscilloscope and keep your subs from getting to much dc voltage in the voice coil, which always equals fried subwoofer.
This will work for all frequencies, but is easiest to do with subwoofers. Be sure to play the proper sine wave frequency, usually about 40Hz to approximately 250Hz depending on woofer size. Setting the gain for higher frequencies is a little more difficult because you need to know what frequencies your speakers are capable of handling and what frequencies they play at. Those numbers will change for coaxials vs. separates, and bi-amping or not.
Remember to test the internal resistance of your meter prior to measuring your amp. Touch the positive and negative lead from your dmm together and add that number to the total number you get from measuring your amp.