http://www.mobileaudio.com/rac-faq/
copied from the Basic Car audio web site and from FXN
A power amplifier takes an input signal, usually a preamp level signal, which has both low current and low voltage characteristics, and produces an output which will have higher current and voltage levels. The power supply available to the audio output IC in a head unit is limited to the battery voltage of the vehicle. This means that the head unit can produce an audio signal with a limited (by the battery voltage) voltage swing, and therefore a limited power output to the speaker. Most amplifiers have a special circuit (switching power supply) to boost the available battery/charging system voltage to a higher voltage. The higher voltage developed in the amplifier's internal switching power supply will allow the audio output voltage swing to be greater. This allows the amplifier to produce more power into the speakers connected to the amplifier's output terminals.
Most amplifiers will have some sort of level or "gain" control. This control is used to match the output of the head unit to an amplifier. The maximum audio output voltage from different head units will vary. If there were no gain controls, some head units would not be able to drive the amplifier to its maximum power level. Other head units may drive the amplifier to full power at a fraction of its volume control's range.
Virtually all amplifiers have battery, ground and remote connections which must be connected for the amp to operate. The battery connection is the high current +B source that's connected to the battery via a properly fused wire. The size of the power wire is determined by the current the amplifier draws and the length of the wire (from the battery to the amplifier). The ground is another high current connection and is connected to the chassis (body/floor pan) of the vehicle. The ground wire is typically as large as the power wire. The remote connection is a low current control input that tells the power supply of the amplifier to power up.
The remote input current for amplifiers varies with the amplifier and the model. Some draw minimal current. Others draw a little more. The upper limit of a properly functioning amplifier is approximately 50ma (0.05 amps). If you're using/controlling more than 2 amplifiers, it is (in my opinion) much better to use a relay to control the amplifiers. Actually I really prefer having a relay in the remote circuit (no matter how many amplifiers I'm using) because it protects the head unit's remote output circuit in case of a short circuit.
The input circuit (sometimes called the 'front end') generally employs a noise cancelling circuit which compares the signal on the center conductor (the audio signal) to the signal on the RCA shield (which generally has little or no signal and is only used as a reference) and amplifies the difference between the two.
The input impedance is the impedance (that the signal source 'sees') from the center conductor to the shield on an unbalanced input circuit. A typical input impedance would be ~10,000 ohms but some amplifiers may have an input impedance of more than 50,000. If the input circuit uses a mini DIN type connector, the input impedance could be measured from one signal terminal to the other or from the signal terminals to the shield ground. Ideally, the impedance should remain constant throughout the audio band. More than a few amplifiers employ some sort of high frequency noise filter which will cause the input impedance to fall slightly at the upper end of the audio spectrum. These filters are designed to reject high frequency noise from the amplifier's switching power supply. It should also remain constant regardless of the position of the gain control. Some amplifiers (especially budget amplifiers) will have varying input impedance when the position of the gain control is changed. Head units (or equalizers, crossovers...) with low output impedance will handle these variations better than standard head units. Generally, a head unit with high output impedance will have reduced high frequency response if the amplifier's input impedance isn't consistant across the audio spectrum.
Unbalanced Input Circuit:
This type of circuit has a shield ground that's not directly connected to the chassis ground but may have only a few hundred ohms of impedance from the shield to ground. This type of circuit would be designed to accept a single ended signal (signal only on the center conductor).
Balanced Input Circuit:
Some Amplifiers have balanced inputs. This means that both the center conductor and the shield (if they're using RCA type connectors) can accept an audio signal. If the amplifier uses RCA type connectors and has balanced inputs, it likely uses the chassis ground as a reference (which is a testimony to the noise rejection abilities of a balanced input circuit). If the amp uses a mini DIN or some sort of professional audio connector, the connector will have provisions for two audio signals per channel and a dedicated ground (reference) connection.
- Steve