Car audio gain architecture: the better you understand it, the better you can use it.
What is Gain?
The answer to that simple question will shed a lot of light on how your audio system works and how all the individual components interact. In the terms of a signal, which is what we are concerned with, gain is simply an increase in the voltage of the signal. The entire purpose of an audio reproduction system is to take a very low voltage (a few milli-volts) source signal and convert it to a mechanical signal that can be converted back to an electrical signal by your ears. The speakers do the actual energy form conversion, but they require a much higher voltage to operate than is present directly from the source converter. This article will be concerned with what occurs between the source converter and the speaker: Gain.
What gives Gain?
We now know what gain is but where do we get it? Every active component in the audio signal chain is a gain device of some sort. They all incorporate an amplifier of some sort and an amplifier is nothing more than a gain stage by another name. In the simplest of terms an amplifier takes a small voltage signal and uses it to modulate a constant voltage source to duplicate the source signal at a voltage different from the source. It does this with a silicon device known as a transistor. A transistor is nothing more than an analog switch. When you think of a switch, the picture that normally comes to mind is a digital switch. It has two positions: fully on or fully off. An analog switch differs from this in that in addition to all on and all off, there are an infinite number of partially on positions in between. Since an audio signal is by definition an analog signal, this is a requirement for a device that can accurately reproduce the signal it is given.
Every signal must have a source. In the case of you car audio system, the source could be the D/A converter in your CD player, your radio tuner, your satellite tuner, etc… In every case, this source signal is initially of a very low voltage. The first thing it encounters is a fixed gain stage of some sort to bring the signal up to a level usable by the next portion of the signal chain. The quality of the source signal and the quality of this initial gain stage are of utmost importance because any noise or distortion at this point is going to be further amplified by every subsequent gain stage in the system.
In the most basic of setups, the next stage in the signal chain is the source unit preamp. The preamp is little more than a gain stage with a variable gain. The variability gives you control of the volume. Without it you would have one volume setting on your system and you final signal voltage going to you speakers would be a fixed multiple of the voltage coming from the source. This wouldn’t be terribly useful so some form of variable gain needs to be readily accessible. Enter the preamp volume control.
In a perfect world, every preamp component would have the same range of output voltage and every power amp would have the same range of voltages needed to get full power out of the amp. The world is far from perfect, as you well know though, so something has to be done to make various pieces of equipment compatible with each other. For just this reason automotive power amps consist of two separate gain stages. The main section is the fixed gain block that takes the signal that it is given and increases it by a fixed amount up to a certain max value. The second is a level matching preamp stage that takes the signal from the previous component and matches it to the range of input voltages that will keep the fixed gain stage within its limits.