properly setting amp gains.

There are a couple of ways to setting gains. For precise gains there are two options really: 1) Use an oscilloscope, run a tone at desired volume, and SLOWLY increase gains until a DC signal shows up on the screen. 2) Use a preset microamp speaker (1 watt from radioshack ~$12) and increase the gains until it squeals.

To set it by ear, yes a lot of preference is involved. For many amps, the factory settings are usually preset at the right levels (at least that is what we find for our kickers and mtxs). For those who want to adjust the settings still, get your favorite song that you know really well and play it repeatedly at the level you usually listen to it. The gains at this point are all the way down. Slowly up the gains until you hear the amp start to mess up the song then back down on it until it sounds right again. For a majority of amps, you'll end up setting the amps at 2/3 - 3/4 from full.

Ganbatte ne!

OK, here's the correct way to set it by ear:

Turn all the gains down, as well as any bass boosts on the amplifier.  Now grab a good quality recorded CD and start turning up the volume until you hear distortion in the speakers.  This is the point where the head unit starts to clip the signal.  Now turn the volume down until the distortion goes away.  From here, turn the gain up until the speakers the amplifier is controling start to distort, then turn it back down till the distortion goes away.  Now your gains are set correctly. 

Every amplifier/head unit combination will have different proper gain settings, so you cannot expect 2/3-3/4 to be accurate.  And I have NEVER seen an amplifier come "factory preset" at the right levels as there is no way for the manufacturer to know what the input voltage will be. 

Edit:  One more thing to add, if you have anything else in the signal path with a gain control, use the above technique to set it's gain first, then move down the chain toward the amplifier.

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