The Doepfer A-138D is a crossfader and effect insert module for the Eurorack A-100 system. When used as a crossfader, the signals are connected to the module via the in1 and in2 sockets, and the crossfader control knobs defines the blend of the two signals that will appear at the mix out socket. The A-138D can also be used as an effects send/return with t a wet/dry control.

Typical applications of the Doepfer A-138D include:

• Crossfader for two different A-100 signals
• Crossfade between original and processed audio signal (dry/wet control). For this the incoming audio signal is patched to input 1 of the A-138d and passed on to the audio input of the sound processing module in question (e.g. filter, phaser, flanger, frequency shifter, wave multiplier, wave shaper ...). The audio output of the sound processing module is patched to input 2 of the A-138d.
• Integrating external effect units into A-100
• Dry/wet control for the external effect