Setting gain controls could be one of the most important things in making a great system sound great.

There is a lot of confusion and bad information going around with different installers on how to set car audio amplifier gain controls.
Car audio guys and installers can be pasionate about thier beliefs on how to do this, even when they are wrong. The bad info and myths make setting gain controls scary to some people.
It is very simple, dont let it scare you!

Although an amp will usually work out of the box without touching the gain controls, and some people just turn the controls all the way up.
There are two reasons to set the gain controls on an amplifier.
1). To match the headunits output signal. (This is easier than some installers make it out to be).
2). To match multiple amps to each other, and or so multiple speakers in the car have matching outputs.

We will cover Matching to headunit, Matching multiple amplifiers, and also some of the MYTHS about setting gains.

Matching Headunit Output

Gain controls on an amplifier are basically just small potentiometers (variable resistors) inline with the RCA input signals that allow you to adjust the incoming signal for a wide variety of headunits.

Most car stereo amplifiers that I have tested are built to reach full power with between 1/2 and 1 volt of input signal. The gain controls allow you to adjust the usually much higher voltage from the head unit down to a point where the amp gets the desired 1/2 to 1 volt.

It's good to have a nice swing on your headunits volume control.

Example: If we hook up a head unit with a 8volt (more or less) output to an amplifier, then the volume might get loud very fast when we start to turn it up... (That's no good!)
Turn down the amp gain so that the headunit has more control in the swing.
We normally dont want it to be either OFF or LOUD, we want it to be variable with many levels of comfortable volume levels between OFF and LOUD.

But on the other hand, if the gain is too low, we may have to turn the volume on the headunit up and up and up before we get to a good listeneing level.
That is no good either...So we turn the gain up!

I like to set the headunit volume control is about 3/4 way up for my normal everyday listening level, this allows me to have a good swing below and above my normal level if I want the music lower or louder.

So simply turn your headunit up to that level and then set the amp gain to a comfortable level.!!

Your done!

It's easiest to do this by ear.
No need to drag out the Oscilloscopes and test tones!
They will do you absolutely no good.

It's funny to me that some of the installers that insist the gains must be set with a test tone and an oscilloscope, do not own an oscilloscope or know how to work one.

Gains on Multi Amp Systems

Now, another reason to adjust a gain control. A MULTI AMP SYSTEM!

If you have more than one amplifier it is possible that one set of speakers plays louder than another!
This missmatch in levels can be because of mounting location, or because the speakers are different sizes or different brands or different speaker sensitivities, or maybe your two (or more) amps are different brands or different sensitivities, either way, you can set the gain controls so the levels into the listening area (car) are the same or sound the best.

Good thing those gain controls are there...

This multi amp adjustment is pretty easy to do by ear, simply have someone sit in the listening area and tell you as you adjust them when the sounds are equal. Simple!

Same kind of thing with gain adjustments on subwoofer amps, just turn them up or down till they sound good and equal with the rest of the systems sound in the listening area.

Myths About Setting Gains

Many installers think a gain turns UP the voltage, not true, the Amplifier gains are resistors, they turn the voltage down and not up, there may be a few amps with some EQ or some active components, but generally and almost ALWAYS the gain only goes DOWN and not UP. Since the headunit usually has a higher voltage signal than the amplifier needs it is a good thing to turn it down for a proper match

Its not rocket science to set the gains.
Gains are like little volume controls,
(I don't know why so many installers are taught that gains are NOT volume controls, when in fact that is EXACTLY what they are!)
its super simple to just set them where the level sounds good to you.

One MYTH is how the gain controls will help to prevent amplifier distortion and amplifier clipping...
That's simply not true, UNLESS you set the gains at a level where the headunit cannot possibly drive the amplifier to full power..
And even if you were to find this magic spot for your gain controls then
(A) you would have to turn that volume control FULL SWING to get your system loud and
(B) since many music sources (or disks) are not all recorded at the same level, its likely that if you have a disk recorded lower then you cant get it loud at all! and if you have a disk recorded louder then you can still surpass your magic spot...
So in reality searching for this magic spot is fruitless! Dont waste your time...

Example: In the early 80s when high fidelity car amplifiers were just starting to make the scene I worked in a high end shop with plenty of test equipment, and with a crazy but intellegent installer that was kind of legendary around these parts...
This installer had heard that the amplifier gain control was to prevent amplifier clipping.. (still widely heard today)..
For a while he set up every car he did where the gain control was so LOW that if you turned the head unit all the way up the amplifier would not distort..
And of course if you did turn the headunit all the way up the system would just barely be getting loud...

Customers would find that some music sources would be recorded at a lower level and the music just wouldn't get loud enough... The installer would tell the customer that a "REAL AUDIOPHILE" doesn't want his music to distort or be that loud!
The customers were NOT HAPPY and came to me to say "Gee Eddie, I don't want to make the other guy mad but can you adjust my system so it sounds good and please dont tell the other guy? " Of course I said yes!

So, you see the only way the gains can be used to eliminate clipping or distortion will also limit your top end volume! And for most of us it is NOT DESIRABLE to do so.

As long as this is not done, it is just as possible to turn your system up to FULL power and beyond to clipping no matter where the gains are set....

All the VOODOO about watching an oscilloscope and looking for a clipped signal is a waste of time and wont get you what you want anyway!  Do it by ear, its simple. Don't let adjusting the gains scare you, they are simply volume controls...