2009
11.17

DIY – 12 to 2 – Passive Analog Summing Panel.

Category: Hardware / Tag: , , ,  / Add Comment

Made myself a nice little passive summing panel for my home rig out of parts laying around the shop. I wanted to simplify/improve my final summing stage. Pleased with the results, Sounds great. Gives me piece of mind bypassing the channel IC’s & master section on my Sony MXP desk.

I used a simple XLR panel, Neutrik XLR mounts, Neve super conductive cable, & OHMITE 1% metal film resistors. Took all of 2 hours to wire up.

10.7k resistors on the inputs & a 220 shunt on the outputs.

After level calibration my noise-floor is at -80 & crosstalk @ -88.

FYI – this is with a John Hardy M1 transformer pre for the make up gain. Nothing better IMHO.

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3 comments so far

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  1. Reply
    Mark said: 2009.12.12 14:57

    Looks great – congrats! A question…. would the resistor values stay the same if you were to increase it to an 18×2 configuration? I’ve got a J. Hardy that I’d love to mate to a passive homebrew summer, too.

    • Reply
      rhythminmind said: 2009.12.21 15:40

      I would use 10kohm input resistors. & 220ohm output resistors for that configuration. Read below for more info..

  2. Reply
    rhythminmind said: 2009.12.21 15:22

    My design is based on the mixing network in NYD’s drawing.
    http://www.twin-x.com/groupdiy/albums/userpics/balancedmixnetwork.pdf

    All you have to do is figure out how many channels you need. Pick a input impedance value. Modern balanced gear has an high input impedance usually around 20k. I picked 21.4kohm, only because I had “military grade” resistors on hand that happened to be 10.7k.
    Resistors between 5k-15k should work well for most applications. It’s a balance between noise & crosstalk. The higher the resistor the better the crosstalk but with increased noise floor. 10k input resistors work well.

    Now for output impedance most mic pres want to see 150ohm signal.
    I recommend using a transformer based pre for the make up gain that accepts a wide range of impedance, like the hardy.
    You should pick your shunt resistors on the outputs to spit out around 150ohm. Mine is 200 something. But the hardy can easily handle anything from 60-600ohm

    The formula to figure out the output shunt resistor value for a 200ohm output with a input impedance of 20k =

    shunt ohm resistor = (20000/n)*200/((20000/n)-200)

    WHERE N = NUMBER OF INPUTS

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