If you haven’t done so already, please read our article on Amplifiers and also the one on Current before reading this one.

Let’s get one thing straight before starting with the article. A lot of people call this adjustment “Gains”, but its actual name is “Input Sensitivity”.  Remember, what you know as “Gains” = “Input Sensitivity”. Got it? Good.

The basic job of the amplifier is to take a small signal and enlarge it. All Amplifiers have a certain sensitivity for their input signal, which they will enlarge to the best of their ability. The best way to explain this is with a quick example:

First, lets take a look at the ideal example, where the source, and the amp are perfectly matched:

Say you have a source that produces a signal of 500mV (That’s half a Volt or 0.5V in case you were wandering) when the volume is turned all the way up. So this device will produce output from 0V to 0.5V as you move the volume knob from MIN to MAX.

You connect this to an amplifier rated at 400W across a 4Ohm Load, with an maximum input sensitivity of 500mV. At maximum power this amplifier will produce 400W when connected to a 4Ohm Speaker. Using the equation that relates Power and Resistance we can calculate the output voltage at maximum power.  In this case it turns out to be 40V. (If we lost you here, read the examples at the end of the Current article).

So the amplifier will produce 40V at maximum power.   This means that when the source unit’s volume is turned all the way up, it will produce 0.5 volts, which the amplifier will multiply 80 times producing the output of 40V.  That’s great. We have just determined that under normal conditions our amplifier enhances the incoming signal by a factor of 80. (By the way, this enlargement factor is what is actually defined as “Amplifier Gain” and it’s defined in decibels or dB)

But what happens when the same amplifier is connected to another source unit that will only produce 0.3V when its volume is set to MAX?

When you turn the volume to MAX, the amplifier will take that input signal and multiply it by 80. This produces an amplifier output of 24V  (0.3V times 80), which translates into 144W at Maximum volume. This means that we are not even using half of the amplifiers potential at MAX volume.

Let’s look at the opposite case now. What happens when your source unit produces 0.75V at MAX volume?

Following the same process this produces an output of 60V that creates 900W when connected to our speaker.  This of course is well beyond what the amplifier can sustain, and it won’t be long before you see smoke coming out of it (A number of amplifiers have safety shutdowns and/or will clip the signal to avoid a certain death).

What is an Amplifier manufacturer to do?  Provide adjustable input sensitivity of course!

The job of the Input Sensitivity adjustments, (or “Gains” as you will see them labeled all over the place), is to match the input signal so than  MAX volumes translates into maximum output power, without overpowering  or under-using the Amplifier.

This way, when we have a source that produces less than what the Amplifier likes to see as maximum, we would turn the sensitivity up, so a smaller input voltage will create a greater output. Likewise, we would turn the sensitivity down so a high voltage source won’t drive the amplifier over its limit.

How to set gains properly?

From what we’ve seen so far, the ideal setting is when the input sensitivity is set to drive the amplifier at full power (Without going over) when the source volume is set to MAX.  This works great in a lab, when testing with a reference tone at a constant level.

In real life things works slightly different.

Music is very dynamic, with ups and downs, loud and quiet passages, and different recordings can have different volume levels. (Think a classic music CD Vs a Megadeth CD).

When we set our input sensitivity we want to have a little bit of wiggle room, so that we can turn the volume up a little bit more on the quiet tracks, and down on the louder tracks.  The general rule of thumb is to set them so that ¾ of the Max source volume can fully drive the Amplifier; giving you the desired wiggle room.

You have three ways of setting the gains.

The best method is using an oscilloscope, and watching the signal as it begins to distort when you reach the limit. While technically this is the most precise way, it’s unfeasible for pretty much anyone due to lack of proper equipment.

The worst method is by using your ears. You basically start with your source volume at ¾, and gains at 0. You slowly turn up the gains until you begin to hear distortion; and then dial them back a little bit.  It’s very imprecise since it’s completely subjective to what you perceive as distortion.

The preferred method for most people will be described below. You will need 2 things:

- An AC Voltmeter. These are pretty easy to find. (Radioshack, ebay, parts express, home depot, etc)

- A Test Tone CD. You can get a complete test CD from Mr. Arve Bersvendsen’s blog website (http://virtuelvis.com/)   the file to download as well as the explanation and instructions on how to create the CD can be found here: http://virtuelvis.com/archives/2004/09/audio-test-cd

Before going into the car, you have to sit down and do some math. Remember, the goal of setting the adjustment, is that the MAX input voltage will be able to drive the Amplifier to its maximum output voltage:

If you’re calculating for your own system, substitute the appropriate numbers into the equation, and use your common sense. Do the same calculation twice, because a mistake here could be costly. If you end up with a negative number or a very large number, you did something wrong.

Now that we have our target voltage, we head to the car with our voltmeter and test cd.

• Disconnect all the speakers from the amplifier and set the gains to 0.

• Remove all sound enhancements at the head unit (absolutely everything). Load the appropriate test frequency for the equipment you’ll be testing. For a subwoofer amplifier, use a 50~ 60Hz tone, for a speaker amplifier use a 600~ 1200Hz tone. Set the volume to ¾ of  the MAX and keep the source unit playing on a loop of the test track while you do your testing.

• Connect the AC voltmeter to the amplifier outputs, and start increasing the sensitivity. You will see that the measured voltage begins to increase as well. One you reach the target voltage we calculated previously you’re done!

That wasn’t so hard now was it?
Reattach everything and enjoy your properly calibrated system!

Multiple amplifier systems

If you have multiple amplifiers, follow the same procedure for all of them. You may notice that some will play louder than others.

In this case you want to lower the gains on the louder amps so that sound output matches that of the softest playing amp. We do it this way, because increasing the softer amp could drive it over its limit, which is what we wanted to avoid all along!