These are like the nerves in your body, transmitting signals to and from the various components. In addition to volume, bass, etc., the head unit may contain other systems within its casing. That's because we mentioned them above. You may have a home system like that yourself. This depends upon a number of factors, such as heat, space, cosmetic design and other concerns, and every car is different.
Signal amplification is actually a two-stage process, handled by two separate components.
These are the preamplifier and the power amplifier . Whether it's reproducing a cassette signal, a CD signal, or an FM broadcast, the preamp puts out a signal inaudible to the human ear. This is where the power amplifier takes over. It's the power amp's job to take the signal from the preamp and boost it into the audible range. We'll cover this in more depth in our Amplifier column in this series.