Every head unit has a built-in amplifier. It takes the music, radio or call audio and boosts it enough to drive the car's speakers directly. For most cars with factory speakers it's all that's needed, which means a new head unit can simply replace the old stereo without adding anything else.
What a built-in amplifier powers
A head unit's built-in amplifier usually has four channels: front left, front right, rear left and rear right. Each channel drives one speaker, or a pair wired together such as a door speaker and a small tweeter in the corner of the dashboard.
It powers:
- The door, dashboard and rear speakers in most cars.
- Factory or aftermarket speakers of normal sensitivity.
It doesn't usually power:
- A subwoofer, which needs much more power for deep bass. A subwoofer needs its own amplifier, fed from the head unit's subwoofer pre-output.
- Speakers in a car whose factory sound system has its own separate amplifier (see below).
What the power figures mean
Head units often quote a peak or maximum power figure for each channel. Peak figures look impressive but describe short bursts rather than steady output. The continuous (RMS) power is the figure that matters when comparing amplifiers, and for a head unit it's much lower than the peak figure. In practice, a head unit's built-in amplifier plays clearly at normal and fairly loud volumes in most cars, and starts to sound strained sooner when pushed hard.
Built-in or external amplifier
Built-in
- Simple: no extra wiring or space needed.
- Enough for most drivers, especially with factory or efficient aftermarket speakers.
- Limited headroom: at high volume or with bass-heavy music it can run out of power, and the sound gets harsh.
External
An external amplifier is a separate box, usually under a seat or in the boot, fed from the head unit's RCA pre-outputs.
- More clean power, so music stays clear at higher volumes.
- Better control of upgraded speakers, especially component speakers that want more power.
- Needed for a subwoofer in almost every case.
- More work to fit: it needs a fused power cable from the battery, an earth, RCA leads and a remote turn-on wire.
When an external amplifier drives all the speakers, the head unit's built-in amplifier isn't used for them. The head unit still controls the volume, sources and sound settings through the pre-outputs.
A middle path is to keep the built-in amplifier for the door speakers and add a separate amplifier just for a subwoofer.
Keeping a factory amplifier
Many cars with a premium or branded sound system have a separate factory amplifier, often in the boot or under a seat. The factory stereo sends a signal to it, and the factory amplifier powers the speakers.
When fitting a new head unit to one of these cars:
- The head unit must send a signal the factory amplifier can use. This is usually done through the car's wiring adaptor and, often, a CAN bus decoder that also switches the factory amplifier on.
- The head unit's built-in amplifier isn't used for the speakers, because the factory amplifier does that job.
- Some factory amplifiers need a specific interface, especially those connected by fibre optic or other digital links. Check what your car has before ordering.
- The factory amplifier may add its own tuning. Set the head unit's equaliser gently at first and listen before making big changes.
Many head units have a setting for cars with a factory amplifier; turn it on if your car has one.
Common problems and fixes
No sound from the speakers
On a car with a factory amplifier, check the amplifier is being switched on, by its turn-on wire or the CAN bus decoder, and that the head unit's factory amplifier setting is correct. On other cars, check the speaker wires are connected through the wiring adaptor.
The sound distorts at high volume
The built-in amplifier is running out of power. Turn the bass down a little, use the high-pass filter on small speakers, or consider an external amplifier.
The sound is very quiet even at full volume
This often means a factory amplifier isn't switched on, or isn't getting the signal level it expects. Check the factory amplifier setting and the wiring adaptor.
Hiss or whine through the speakers
Check the earth connections and route the wiring away from power cables. On a car with a factory amplifier, the wrong signal level into it can also cause hiss.
Only some speakers work
Check the fader and balance settings first. Then check the wiring adaptor's plugs are fully home, and that a speaker wire hasn't come loose behind the head unit.