RCA pre-outputs are sockets on the back of a head unit that send a clean, low-level audio signal to an external amplifier. They're how you add more power, a subwoofer or a better speaker system later, while the head unit still controls the volume, the source and the sound settings.
What RCA pre-outputs are
A head unit has two kinds of audio output. The speaker outputs carry the signal from the head unit's built-in amplifier, strong enough to drive speakers directly. The pre-outputs carry the signal before it's amplified, hence the name, at a low level meant for an amplifier's input.
They use RCA connectors, the round plugs also called phono plugs, with a centre pin and an outer ring. They're colour coded: red for the right channel, and white or black for the left. A head unit may have several pairs:
- Front: left and right front channels.
- Rear: left and right rear channels.
- Subwoofer: a bass-only output, sometimes a single socket and sometimes a pair.
The pre-outputs follow the head unit's volume, fader, equaliser and DSP settings. On the subwoofer output, the head unit's low-pass filter and subwoofer level control the bass sent to the amplifier.
Why use pre-outputs instead of the speaker wires
You can feed an amplifier from the speaker wires through its high-level inputs, and some amplifiers are designed for that. Pre-outputs are usually the better choice when the head unit has them.
- Less noise and distortion, because the signal hasn't been through the head unit's own amplifier.
- Separate control of front, rear and subwoofer channels, including the head unit's crossovers.
- Easier gain setting on the amplifier, which helps avoid hiss.
The pre-output voltage affects how the amplifier's gain should be set. If you're matching an amplifier, check the head unit's specifications for the pre-out level.
Adding an external amplifier or subwoofer
Subwoofer only
This is the simplest upgrade. Run an RCA lead from the head unit's subwoofer output to an amplifier that powers the subwoofer, or to a powered subwoofer with its own amplifier built in. The door speakers stay on the head unit's own amplifier.
Full system
A four-channel amplifier fed from the front and rear pre-outputs powers the door and rear speakers. A separate amplifier, or a five-channel amplifier, powers the subwoofer from the subwoofer output.
What else you need
- RCA leads long enough to reach the amplifier, usually in the boot or under a seat. Use good-quality, shielded leads.
- A remote turn-on wire from the head unit's amplifier remote output to the amplifier, so the amplifier switches on and off with the head unit.
- Power and earth cables for the amplifier, with a fuse close to the battery.
Route the RCA leads down the opposite side of the car from the amplifier's power cable to reduce interference.
How many channels you need
Count what you want to drive.
- Front speakers and a subwoofer: the front and subwoofer pre-outputs. Common where the rear speakers stay on the head unit or aren't used much.
- Front, rear and subwoofer: front, rear and subwoofer pre-outputs, for a fully amplified four-speaker system with a subwoofer.
- Active front speakers: some systems give tweeters and mid-bass speakers their own amplifier channels, controlled by a DSP. That needs more outputs than a typical head unit has and is usually handled by a separate DSP or a DSP amplifier.
If you only plan to add a subwoofer, a head unit with a subwoofer output is enough. If you might build a full system later, front, rear and subwoofer pre-outputs give you the most choice.
Common problems and fixes
A whine that rises and falls with engine speed
This is usually an earth problem. Make sure the amplifier's earth is short and fixed to bare metal, and route the RCA leads away from power cables. A ground loop isolator on the RCA leads can help as a last resort.
No sound from the amplifier
Check the remote turn-on wire is connected and that the amplifier's power light comes on. Check the subwoofer output is switched on in the head unit's settings.
Hiss at low volume
Turn the amplifier's gain down and use more of the head unit's volume range. A gain set too high is the usual cause.
Left and right are swapped, or the bass is thin
Check the red and white plugs are the right way round at both ends. Thin bass from a subwoofer can mean it's out of phase with the other speakers; swap its speaker wires or use the phase setting.