No! They are not hidden volume controls! It is a fact that very
few people, including installers, know how to set gains correctly. Failure to do so yields
higher distortion, a higher noise floor which decreases dynamic headroom, less than
optimum operating conditions for electronic equipment, and higher failure rate for both
the electronic equipment and transducers! So, why don't more people learn to adjust gains
correctly? Because they are too lazy and simply dont care, which is a shame!
Not many people have a scope, so I will give you a couple of
methods using little or no investment.
Suppose we want to set the gains in a system that consists of a
head unit, two way crossover and two amplifiers. We do not have any test equipment
whatsoever. First, turn all the gains to the minimum position. We have probably heard by
now that we want the highest signal voltage possible to keep the signal clean. The reason
the signal is cleaner with higher voltages is because the amplifiers gain is
supposed to be turned down accordingly which reduces noise. For the gain setting, do not
use music. Use a 1kHz Sinewave tone from a test disc. Start with the first gain in the
chain which is the headunit in the dash. Turn it up until the speakers start to distort.
Then back off a bit and remember that this is the limit of the headunit and that you
should never cross this limit since everything after the processor will not be able to
correct the distortion. It will, in fact, make the sound worse in most cases.
Obviously, the next piece in line would be the xover. Every time I
look at a vehicle, the xover is turned down and the amp is turned up. By doing this you
just starved the amp for signal voltage and tried to compensate by turning the amp gain
up. Not cool. Treat the gain on the xover as you did the headunit. Use the same steps.
Increase gain until it distorts and then back off a bit.
If, by miracle, your 4V headunit actually does put out 4V, your
gain should be close to the minimum position. Lets say that your wonderful headunit
does indeed put out 4V. In this case, the amp gain should generally be set to match the 4V
input, which will probably be the minimum position, and will yield full rated power. If a
gain is set in the 50% range, that doesnt mean that the amp is putting out 50% of it
available power. Read this paragraph again... Again...
Now then, lets go to the next level with a little
investment. The only investment will be a small amplified speaker such as the Archer brand
unit offered by Radio Shack. This cool little toy will remove any doubt of where the
clipping starts that causes distortion. Using the Archer is very easy. Simply plug the RCA
outputs from the head unit into the Archers inputs. As you turn up the
headunits volume, the sound will of course get louder. When you clip the output of
the headunit, the Archer will reproduce a noticeable distorted gain in output. When a
signal is clipped, a DC offset is briefly sent through the RCAs. This DC offset contains
smaller sinewaves that are actually 127% more powerful than the unclipped sinewaves! This
translates to about 2.1dB of SPL (though Mother Nature does tend to screw with our success
sometimes). Anyway, turn the gain back down until the sound is normal again. Then move
back to the next signal processor in the chain. It is important to remember that the goal
is to get the gain as high as possible at the front of the chain and as low as possible at
If for any reason the subs are overpowering the components, then
turn the sub amp down more until the right level is achieved. If the subs are still to
loud then turn the gain down on the last signal processor before the sub amp (usually the
xover). If the drivers are all new, you will need to readjust the gains after all drivers
are broken in.