The Doepfer A-138p is a 4 channel Stereo Mixer with Mutes, Panning, and Aux Send.  The A-138p MUST be paired with the A-138o as it provides the outputs for the mixer, but multiple A-138p's can be chained to the 138o in order to create a larger stereo performance mixer!  Per Doepfer:

Modules A-138p/o are used to built a simple performance mixer (instead of an external stand-alone mixer). Module A-138p is the 4-fold input module, A-138o is the output module that can be combined with one or more A-138p.

A-138p features:

• 4 channels
• controls and inputs for each channelinternal connection to the output module and to other A-138p modules
  • input (3.5 mm mono socket)
  • signal LED (as a coarse monitor of the incoming level, the level is measured after the gain control)
  • mute switch
  • gain (control to adapt different audio levels)
  • Level (the main volume control, linear response curve)
  • Aux
  • Pan
  • internal jumper for each channel to choose between Aux pre/post main Level control
• A-138p requires A-138o (no stand-alone use)

A-138o features:

• level controls for Left, Right and Aux
• output sockets for Left, Right and Aux (3.5 mm mono sockets)
• A-138o requires at least one A-138p (no stand-alone use)
• Application example for Aux Send and Aux Return: looping in another module for effects (e.g. DSP module A-187-1, BBD module A-188-1, Spring Reverb A-199)
• Application example for Aux Send only: pre-listening in combination with the headphone amplifier module A-139-2

Specifications:

4hp

40mm Depth

20mA @ +12v

20mA @-12v