Setting your amplifier's gain control is a very important step to take to aid in getting proper, regulated, non clipped sound out of your amps.  Being as amplifiers in boats are always fighting heat, taking the few minutes to do this is going to make you much more comfortable with cranking the volume on those hot days, in knowing that you are not gong to cause overheating or damage to your drivers due to clipping.  While the gain control is very much like a volume knob, you should never treat it as such.  The gain control is used to match the signal levels of the head unit to the amplifier.  You know how a head unit list out 3vrms or 5vrms on the line outs?  That has to do with the signal level that travels from those to the amplifier inputs.  The gain control plays the role of a pop-off valve or regulator and adjusting it sets the level of the signal, matching it to the amplifier.  DO NOT MAKE THE ASSUMPTION THE INSTALLER DID THIS.  More likely than not, the gain was set at the shop using the ear method, which I will cover later in this article.

I am not going to get into clipping in this article.  For more information on clipping and the things that cause it, visit http://www.bcae1.com/.  What I am going to get into is how to prevent your head unit and amplifier from delivering clipped signals to your stereo by way of gain control.  I am going to cover two methods here.

The Multimeter Way

This is the preferred method for getting your gain as accurate as possible without an oscilloscope.

Required Tools:

• Test Tone CD (use the tones below)*
• Multimeter with AC Voltage display, preferably digital with a back light
• A screwdriver/allen wrench/??? to loosen and disconnect terminals on your amp
• Access to your amplifier/s
• *Burn an audio or mp3 cd (depending on your source unit) using the tones below.  These tones are the ones I use for configuring my stereo.  You may want to use different tonal ranges depending on the types of music you prefer.  I listen to more Slipknot than Snoop Dogg, so please use these as a reference point.  You will likely get very good results for all types of music with these.
  
  
  • 35Hz for large ported subs (your box should have a tuning....use a tone for that)
  • 45Hz for sealed smaller sub-woofer
  • 950Hz for midrange (if isolated from highs)
  • 11kHz for midrange (components on same amp)
  • 14kHz for highs (if on a separate amp)
  
  
• You need to pull out your manual on your amps and do a little math with Ohm's Law.
  
  
  • Find the page that list the amplifiers output. 
  • Find out the corresponding number for that number in Volts for your given load (impedance/ohms)
  • The formula to do so is: Voltage output = sqrt(RMS power X impedance of speaker or load)
    Say the amp provides 100WRMS into a 4 ohm speaker:
    Voltage = sqrt(100W X 4 ohms)
    Voltage = sqrt(400W*ohms)
    Voltage = 20V
    Use your amplifer ratings.
    If you have a 2 channel amp bridged, test one channel using the 4ohm setting or test it bridged using the 2ohm setting..
  
  
• A helping hand is almost a requirement for most installs, as you need to be at the controls and at the amp simultaneously.

Procedure: 

Double check that radio and amps are powered off

At the amp, disconnect one half of your speaker terminal lead on whatever channels you are checking.  You only need to do left or right, and only each input.  Remember you are checking the input levels and adjusting, so no need to go to different outputs that are on the same input signal.  You can remove either (+) or (-) speaker terminal.  Go ahead and turn the gain all to minimum.  Some amps have a sensitivity switch.  If yours does, set it to low or minimum now.

Power up the head unit and turn the volume to minimum.

On the head unit, turn off all loudness, eqs, etc, etc.  Of you have an eq installed, or a line level control, bypass it for this test.  You want to get the raw signal out from the head unit as a baseline.  If you have a sub volume control on the head unit, set it to default as well.

Insert the test tone for the driver you are setting, set it to repeat track, and head back to the amp/amps.

Have someone turn the head unit volume to about 75%-80% of max.  This will be about a maximum volume before the source unit begins to clip.

Place your multimeter's leads on the speaker terminals with it set to AC voltage, and adjust the gain until you get the correct voltage found from the formula above.

Complete for all other amps or inputs.  Remember to change tracks for each.

The Ear Method:

Turn the gain on your amps all the way down.

Find a cd that has music you listen to 99.9% of the time you are by yourself.  Put it in the player and turn the volume up to 75%-80%

Crawl down into the amp area and turn the gain up until you hear clipping or distortion.  Turn the gain down a little from that point.

Repeat for all other inputs.

Final Settings:

Once you are done, put in your favorite music and make adjustments by turning the levels DOWN, never up, if you have line level controls, then use them as the max should be equal to bypassing them.  The setting you had was a max level and you do not want to go above it.  Once you have your sound stage set correctly, you can begin to tweak EQ, bass boost, etc to your liking.  Remember to not turn the head unit volume up louder than 75%-80% of max.

The most important thing to remember is that it is YOUR boat and YOUR music.  You are the one that needs to be happy when you turn up M.C. Hammer, Depeche Mode, or Boy George (for the hot boater converts), so set it to sound good to you.

As always hit me up with questions about this article.