Install guide

6 – Fitting an external amplifier
If you really want to get the best sound possible from your car audio you will need to add an external
power amplifier. Your head unit has a built-in power amplifier, but given the size of the head unit it is
physically impossible to squeeze a high quality power amplifier in there. So, even though the power
amplifiers in head units are getting better all the time, they don’t even begin to compare to the
quality of a dedicated off-board power amplifier. Any decent head unit will have at least one 4V pre-
amp output, with higher quality units offering two or three dedicated outputs. Try to get a head unit
with three outputs, each output will be dedicated for a particular signal, in this case they will be the
front speakers, the rear speakers and the sub-woofer. A head unit with three 4V pre-amp outputs will
allow you to control everything from the head unit, so you can adjust the sound levels, crossover
frequency and subwoofer level all from the drivers seat. Of course you could use a single amplifier for
everything, but that is a compromise that I recommend you avoid.
External power amplifiers come in different flavours, typically you’ll find single channel, 2 channel
and 4 channel units. A single channel amplifier is typically designed to be used with a sub woofer. A
2-channel amplifier will power just two speakers, so you could use two of them, one for the front
speakers and one for the rear speakers. However, it is easier to opt for a 4 channel amplifier that will
power all four speakers inside the car.
Again, try to listen to the amplifier before you buy and take your time, choose very carefully. At the
moment the biggest names in car amplifiers are Rockford-Fosgate and JL Audio. There are some other
really great amplifiers out there from other manufacturers, so listen to as many as you can. JL Audio
are universally acknowledged as the best but they are not cheap. Stick to Rockford-Fosgate amps if
you want to stay in budget, they sound fantastic and they represent excellent value for money.
The ultimate configuration is a 4-channel amplifier for the internal speakers and a separate single
channel amplifier connected to a subwoofer. If you are going to add a subwoofer (details in chapter
7) then you should plan ahead at this stage and ensure that you have a suitable amplifier ready to
drive it.
Make sure that the amplifier or amplifiers you purchase will work well with the speakers you have
fitted. First of all, it is important to understand how the power handling relates to the power output
of your amplifier. Your speakers will be rated with a maximum power input, something like 100W or
200W. That is the maximum peak power you can drive into the speaker without damaging it.
Meanwhile your amplifier will have a power rating, something like 50W or 100W per channel. Now
this is the RMS value, not the peak value – What’s the difference? Okay, the peak power is the
absolute peak power reached over a given listening period whilst the RMS value is the “Root-Mean-
Square” value, essentially the average power output over a given listening period. Without getting
into to too much depth on this issue I always suggest that you use one simple rule of thumb, take the
RMS output value (or Watts per channel value) of your amplifier and double it to work out the peak
value required for you speakers. So if your amp outputs 50W per channel, then ensure that your
speakers will handle 100W max input. Okay, it’s not very scientific, but it’s easy to remember and it’s
a good enough rule to ensure that you won’t blow your speakers.
Furthermore, you should check that the resistance of your speakers match your amplifier
configuration. You may well find that your amplifier can be switched between 2 Ohms or 4 Ohms.
Typically your internal speakers will be 4 Ohm units, however, just to make things really complicated,