If you have a car stereo, you probably want to get the most sound out of your system, including your amplifier. Setting the amplifier's gain is an important step to achieve the most performance, as well to protect your equipment. You can ensure the amplifier's gain is correctly set for the stereo by matching the amp's input level with the stereo's output level. With a multimeter, as well as a little math, test the amplifier's AC voltage output and set the device's gain to the correct level for the system.
Measure the resistance of the speaker that you'll be connecting to the amplifier using a multimeter. Set the multimeter to measure ohms. Place one probe of the multimeter on the positive lead and the other probe on the negative lead of the speaker. Note the ohm resistance.
Check the amplifier's manual to find out the recommended wattage output at the resistance ohm load you found your speakers to have.
Calculate the desired AC voltage output for the amplifier. This equation can be solved as: voltage equals the square root of wattage times resistance. For example, if your manual said the amplifier should put out 50 watts at four ohms (the resistance of your speaker), solve the equation by multiplying 50 watts with four ohms, which is 200. Next, use a calculator to find the square root of 200, which is 14.142. This is the desired AC voltage output for your amplifier.
Disconnect your speakers and subwoofers from the amplifier you're setting the gain for. The amplifier should still be connected to an electrical source.
Set the stereo to 85 to 90 per cent of its maximum volume.
Insert the multimeter probes into the output terminals of the amplifier, placing the positive probe in the positive terminal and the negative probe in the negative terminal.
Play a 60 Hz test tone CD on the stereo.
Adjust the amplifier's gain knob, while keeping an eye on the multimeter. When the multimeter reads the desired AC output voltage reading (which you found through the math equation), stop adjusting the gain knob. The amplifier's gain is now correctly set.