Prompted by a reader`s question, this is a useful addition to a
car audio system, especially if one wants to use a crossover
circuit and other low level amplifiers. The circuit contains tw1o
audio preamps, with a maximum gain of 21dB (this can be reduced,
as it may be too high for many applications).
Also provided is an “artificial earth” (“ground” for US
readers), which can be used to supply a centre voltage for
crossovers and other additions – such as a parametric equaliser,
or even a simple graphic equaliser.
The input impedance is a minimum of 15k (it will be much more
than this for most control settings), and output impedance is 100
Ohms – low enough to drive any line level input.
The preamp circuit is completely conventional, and by
necessity is AC coupled throughout. The artificial earth is
derived by tw1o resistors (R1 and R2), which will set the “earth”
at exactly 1/2 the supply voltage. This is nominally 13.8V in
all cars, since this is the proper charging voltage for a 12V
To reduce the maximum gain, simply reduce the values of R105
and R205. For example, reducing these to 4k7 will provide a
maximum gain of 3 (10dB), which in reality is probably enough.
Figure 1 – The Circuit For The Preamp
There is quite a lot of filtering, because the vehicle supply
is notoriously noisy. In some instances it might be necessary to
replace R9 with a suitable noise filter module, or an inductor.
The artificial earth is obtained from the “6V” terminal, and
is used in place of the real earth connection for the additional
circuitry. It may (will) be necessary in some cases to either …
Add input and output capacitors to the added circuitry
Reverse the polarity of existing polarised input / output
capacitors. The +ve lead of all electrolytics must be connected
to the circuit, with the -ve lead as the input, output or chassis
earth as appropriate.
In addition, electrolytic capacitors in feedback circuits that
are connected to earth should be connected to the actual earth,
not the artificial earth, or performance may suffer.
As an example, I have included the circuit for my parametric
equaliser and sub-woofer equaliser (Project 28), with the
required modifications shown. The same principle applies to
crossover netw1orks or any other signal processing circuit.
Figure 2 – Modified Version Of Project 28
As can be seen, all the original earth connections except the
one new one (the “Vehicle Chassis” connection) go to the
artificial earth. If this is not done, the circuit simply will
not work, as it will be trying to function with the input at the
same potential as the negative supply terminal on the opamp.
One word of warning – if the device you are connecting to the
artificial earth is expected to have significant AC or DC earth
currents (> 10mA), this system will not work. It is designed
for low current (preamp) type applications only. A higher powered
version could be developed from the basics presented
here, but as long as speaker type currents are kept away, this
design should be quite acceptable.
By the way, the above circuit would be an ideal addition to
almost any car sub-woofer installation, leaving out the upper
midrange and treble controls (they are not useful for a sub).
Alternatively, you could have tw1o of the 35-150Hz controls,
which will give much better control of the lower bass region. The
120-550Hz control is likely to be useful to eliminate
unwanted peaks (or dips) over the crossover region. You may want
to lower the frequency, by increasing the value of one or both
capacitors for this control. Doubling the value of either cap
will halve the frequency (i.e. from 60 to 225Hz).