The problem that this
circuit is designed to solve appeared when the author was installing a
new radio in his Audi A3. The new radio had four outputs for
loudspeakers and a line-level output for a subwoofer. However, the A3 as
delivered from the factory already has an amplifier for the rear
loudspeakers, as well as the pre-installed subwoofer, in the boot space.
The original Audi radio therefore has only line-level outputs for the
rear loudspeakers. So, to replace the original radio without making
other changes to the installed amplification system, he needed to
convert the outputs of the new radio corresponding to the rear
loudspeakers into line level outputs.
Most of the commercially-available adapters to do this job contain
small transformers for galvanic isolation. These introduce phase shifts
and create a certain amount of distortion, which the author was keen to
minimize. The result is this simple adapter circuit that does not employ
a transformer. The outputs of most radios available today have a
differential (bridge-type) push-pull output stage. There is thus no
ground output, just two outputs per channel with a 180 ° phase
difference between them. If the outputs are each connected to a common
point via a 100 Ω resistor, that point becomes a virtual ground.
The ground is relatively stable as (in the stereo case) it has an
impedance of 25 Ω. Each output driver is seeing a 200 Ω load: if the
amplifier is rated for 50 W output into a 4 Ω load this means that each
resistor will dissipate less than 0.5 W. Hence 1 W rated resistors will
be more than adequate, especially in view of the fact that typical music
has a crest factor of at least five. Even a small DC offset from the
virtual ground is not a problem, as most modern amplifiers feature
differential inputs or at least allow the ground connection of an input
to float. To reduce the signals to line level, each has to be connected
to a potential divider: a multi-turn preset potentiometer is ideal.
The author used a linear 10 kΩ trimmer to reduce the output voltage
level from up to about 12 Vpp to around 2 V to 3 V. This latter level is
suitable for the input to a power amplifier. An appropriate trimmer
setting can be found by ear, adjusting the volume of the rear speakers
for the desired balance. There is no need for a printed circuit board
for this project. The 1 W resistors can be soldered directly to the
connections of the multi-turn presets, and so the whole thing can be
assembled ‘in the air’ and shrouded in heat-shrink tubing. The circuit
can then be tucked away in the space behind the radio itself.
Author: Jörg Ehrig (Germany) – Copyright: Elektor Electronics 2011