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.