4sum Portable Audio Mixer Now Available

SyntherJack has introduced the 4sum – a portable audio mixer project, designed to be a companion for mini synths like the Korg Monotrons and Volcas.


  • 4 mono inputs / 1 stereo (double mono) output with individual volume controls
  • Long runtime (average 100 h)
  • Small factor
  • Beginner-friendly build.

SynthJack notes that “It’s a great companion for mini synths, like Monotrons and Volcas, and also makes a excellent first audio-related DIY project.”

Pricing and Availability

Details of the 4sum design are available on the SynthJack site. Full kits are available via Patch Audio for 210.00z? (about US $56).

7 thoughts on “4sum Portable Audio Mixer Now Available

  1. Seems like a pretty reasonable price for the kit– I mean it’s not outrageous. Are those 3.5mm mono jacks for the inputs and a stereo 3.5mm for output?

    I wonder how this deals with signals of different impedance like guitars, vs line level. (I don’t really know what the Volca’s outputs are like).

  2. A six or eight channel version of this would be great! I’m still on the search for an ideal mixer for my Pocket Operators and a slightly longer 6 channel version that could also distribute a sync out signal would be perfect. Mounting two of these cases: https://dichstudios.com/collections/po-gear/products/3po-case
    together with a 6 channel mixer on the “hinge” would make for a nice portable Pocket Operator setup.

  3. The Volcas are stereo output and behave poorly when you short to mono (audio bleeds through sync cables). I would buy if the device could handle summing each input to mono.

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