Undertone Audio Pyra-Sum


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There are two faceplate options, standard and instrument (custom).

Eric Valentine's latest design, this passive summing mixer, addresses the need for more busses than just a stereo mix! Multiple busses as well as the ability to process in parallel, there isn't anything else out there like it. If you've enjoyed working on a console or want to experience a hybrid setup closer to that capability, then Pyra-Sum is for you.

The Pyra-Sum is a 1RU rack mount device that is 100% passive. It does not plug into an outlet and generates no heat. Feel free to mount other equipment immediately above or below. No ventilation space is needed.

The Pyra-Sum is a passive summing mixer that has what other summing mixers don't: multiple cascading busses. This bussing architecture allows you to truly have the sound and workflow of a large format console but with even more sonic flexibility and in a much smaller format. We've left out any active circuitry, which means that you have total control of the coloration that's added in the makeup gain stages—it also means you'll need to use this with eight of your own preamps. Want a different sound? Swap the preamps. You could use Neve's or API's for that 70s sound.  Tube RCA preamps for that 50s sound.  UTA preamps for a size and musicality that has never existed before.  Or, since the preamps are inserted after each stage of summing, you can mix and match— API's on the drums, UTA preamps on the music, Neve's on the vocals, etc. The sonic possibilities are endless.


  • Four discrete stereo summing busses
  • Recreate the bussing architecture of a large format console
  • Incorporate the sound and color of any mic preamps you choose
  • All passive circuitry
  • Make better use of all your favorite outboard gear
  • Make Pyra-Sum the heart of your ultimate hybrid mixing setup


  • INPUTS 1-8, 9-16, 17-24, 25-32 - These DB25 connectors should be connected to the line level outputs
  • of your DAW, ideally, via a patch bay so you can insert outboard effects before they arrive at the Pyra-Sum
  • mixer inputs.
  • TO PREAMPS 1-8 - This DB25 connector should be connected to the inputs of the mic preamps being
  • used to make up the 35dB of gain that is lost in the passive summing network.
  • FROM PREAMPS 1-8 - This DB25 connector should be connected to the outputs of the mic preamps
  • being used to make up the 35dB of gain that is lost in the passive summing network.
  • MIX OUT L/R - These XLR connectors are included as a matter of convenience so you can easily plug the
  • final mix outputs into the device being used for capturing the mix (digital recorder or tape machine). The
  • signal coming out of these outputs is the exact same signal coming from the mic preamps used to boost
  • the level on the “MIX” buss.
  • *All DB-25 connectors follow the “TASCAM” pinout standard.


  • OUTPUT LEVEL(To Preamps) -35dB
  • TRIM RANGE +/-2dB

See the manual for configurations and routing information to plan your setup.

Our ONE WITH EVERYTHING ZEN WARRANTY extends the standard manufacturer warranty by one year, see our ZEN WARRANTY page for details.