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There are two types of live supply which concern us here.

Permanent Live
This connection provides 12 volts regardless of whether the ignition is switched on or off. A common example of this is the supply to the cigar lighter or the interior lights. It is selected wherever the car designer wishes an ancillary to work with the ignition off.

For our purpose, we use this supply to connect to the power input of the amplifier (and the sub) so that the amp comes on whenever the radio is turned on. And yes, the radio in the Scorpio switches on without the ignition, but turns itself off after an hour.

It is important to remember that because this wire is permanent live does not mean that the amplifier or the sub is permanently turned on. The current is available but the equipment does not turn itself on until it receives 12v from the switched live wire (See Below).

Switched Live
This is the supply which only provides 12 volts either when the ignition or an ancillary is turned on.

In this case, a wire emerging from the rear of the car radio will provide 12 volts only when the radio is turned on and originally would have been used to turn on an amplifier relay or raise an electric radio aerial. As soon as the radio unit is turned off the volts on this wire drops to 0v.

In the case of the Ford 7000 series head unit in the Scorpio Ultima  the wire is colour coded blue, with an orange stripe but this must be confirmed with a circuit tester before you continue.

We do not use this wire to provide the power for the amplifier because it does not provide enough amps to drive the wattage required. It is simply a signal wire intended to be used to switch a relay built into the amplifier (and the sub, if you fit one.)

Relay
A relay in it’s simplest form is simply an electrical switch. When a voltage is supplied across two of the terminals in the relay it closes a circuit across two other terminals. The relay is carrying out the function performed by your finger when you flick a switch – simply losing the circuit. (There are many other types of relay – open circuit, delayed timer, repeat timer, overload and so on.)

In this particular instance, relays are usually built into amplifiers and subs. There are two live connections; one the switched live and the other a permanent live. Although the permanent live is always connected to the amplifier, the amplifier does not turn itself on until it receives 12 volts on the switched live connection. In this way the act of turning the radio on also switches on the amplifier, although one is unaware of this (until you hear the sounds!) Why bother? Well, if an amp is permanently connected and did not have a relay it would flatten the battery even though it was not in use, but if it was not permanently connected it would not work when you turn the radio on with the ignition off.

RCA (Low Level) Signal
When a standard head unit takes the signal from the medium to which it is tuned, be it radio, tape or CD, it is a very low power signal which by itself cannot drive a loudspeaker. Most after-market head units have RCA outputs which is fed with this pure signal from the source.  It is intended that this connection is then fed to a separate amplifier to drive the speakers, and, since it is using the pure signal, it will provide the most distortion-free sound.

An example of inputs is shown here, on a Pioneer sub-woofer. The RCA inputs are clearly seen, awaiting phono connections for left and right. To the right are the connections actually in use – to the SP (speaker) inputs.

To the right of these are the switched live and two 10amp live supply wires, which are ballasted to smooth out noise from other electrical circuits.

Hi-level (speaker) Signal
The head unit will also comprise its own amplifier, which gives the signal enough power to drive a speaker. This output is the High Level, or Speaker output. In the case of the Ford 7000 series radio, which is not equipped with RCA connections, the only signal which is available is the Speaker or High Level output.