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.
Circuit diagram:
Audio Level Adapter Circuit Diagram
