Mixing In External Sound

I need to mix pre-recorded sounds with the output from my laptop. That allows me to do sound layering and smooth transitions during laptop dead air. Running a SanDisk mp3 player into a Behringer FCV100 pedal gets me foot control of the mix-in. Running both sources into a standard mixer works fine, but makes the rig more cumbersome. My first attempt to simplify was a basic summing cable, but that got too clunky when I needed a mic level mix output for recording. My next attempt was a proper passive mixer, but the mic level attenuation wound up too quiet for the recording device I was using, so here it is with a variable pad.

Things To Be Aware Of

  1. The 3.5mm headphone jack on the computer is TRRS not TRS. For a Mac, the TRRS is Left, Right, Ground, Mic. You don't want the mic connection involved in the mixing at all, since amplitude fluctuations between mic and ground are interpreted as control signals, which can cause mix cutouts as the computer audio driver reacts. The patch from the Mac needs to be TRRS, going into a TRRS socket with the mic disconnected.
  2. The 3.5mm headphone out from the SanDisk may also be a TRRS connector. If it is, it's most likely the standard (non-Apple) TRRS configuration which is Left, Right, Mic, Ground. Again, safest to use a TRRS jack with the mic disconnected.
  3. The mix and pad outputs on the passive mixer should be standard TRS to avoid any possible confusion.
  4. Mount the resistors on a board and solder the connections so things are visible and trustworthy.

Build Info

Here's a pic showing the active contacts for the pot. The center pin is the wiper variable output. This happens to be a Taiss 1K Ohm 6 Pins Split Shaft Rotary Linear Dual Taper Audio B Type Potentiometer, but I can't recommend it because it makes an audible woosh when you turn it. Good enough for a trim knob I'll adjust once at soundcheck. A 1 kΩ logarithmic would probably be better but couldn't find one.

This is the (kinda too small) project box with the inputs and pot mounted with lead wires.

The wiring diagram for the mixer describes the connections:

Board pin map (7x8):

BlkIn1 --- BlkIn2 --- RedIn1 --- RedIn2
ΩIn ΩIn --- --- --- ΩIn ΩIn
--- --- --- GndIn1 --- --- ---
--- --- --- GndIn2 --- --- ---
ΩOut ΩOut --- GndMix --- ΩOut ΩOut
BlkMixOut BlkPotIn --- GndPot1 --- RedMixOut RedPotIn
--- --- --- GndPot2 --- --- ---
BlkPotOut BlkPadOut --- GndPad --- RedPotOut RedPadOut

Finished wiring. Getting the project box closed was a trick but it works!