Gain Control for Elektor DRM Receiver

Adding automatic gain control (AGC) to the Elektor Electronics DRM Receiver (March 2004) is a useful undertaking. The original receiver was designed to have fixed gain for the strongest signals, with some headroom afforded by the large dynamic range of the sound card. Admittedly weaker signals we would like to boost a little, if only that were possible.
A gain control add-on would be desirable, provided its addition does not compromise the linearity of the receiver. The extension proposed here employs a home-made optocoupler consisting of a superbright red LED and an LDR.