DeccaTree stereo downmixer

Connect phantom-power mics with balanced XLRs to a simple stereo recorder with fixed mixer settings for 3 or 5 channel Decca Tree recording

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I like making audio recordings and I want to experiment with a Decca Tree mic set-up but my Sony PCM-D50 only has 2 channels & I can't justify replacing it yet. It also only has "plug-in-power" so I use a 48V phantom power injector box with my good mics.
The Decca Tree recording set-up uses 3 or 5 mics & I can't find a single phantom power injector with 5 channels without going to rack-size kit. I recently bought a decent but very used mixer to test with & proved the concept nicely but with a bewildering array of buttons & knobs to get just-right.
The combination of mixer & 1 or 2 phantom power injection boxes, with all of the interconnecting cables is far from field friendly. So I have designed here a single, unobtrusive box to do it all. Mic leads plug straight in & output is via jack lead to my recorder. Nothing to fiddle with or get wrong.
Any adjustment is by mic positioning - just like the original folks did.

The history of this mic technique is full of experimentation but sometimes too many variables is a bad thing. Having a fixed mixer should allow me more consistent results while still leaving some room for fine tuning by mic positioning.

From ... 

"... we would pan the left and right tree half left and half right, and the outrigger mics we would pan hard left and right ..."

With this in mind, I have labelled (& positioned) the XLR inputs in the following order (when looking at the rear):

  • L/1 at full signal strength into left channel. For L mic of 3 or Left outrigger mic of 5.
  • L/2 at half signal strength into left channel. For L mic of 5 or not used.
  • C/1 at full signal strength into both left & right channel.
  • R/2 at half signal strength into right channel. For R mic of 5 or not used.
  • R/1 at full signal strength into right channel. For R mic of 3 or Right outrigger mic of 5.

This naming convention is used for the XLR socket, the P48 jumper & the GND/float jumper of each input.

This makes sense whether using 3 or 5 channels but is also to make use of KiCAD's enforcement of having component IDs ending with a number.

  • Disappointing first run at 19:21 0 comments

    With caution in mind, I ordered a couple of cheap mics partially to avoid harming my good mics if I've got the phantom power wrong but also because I'll need another pair for outriggers in the 5-mic set-up.

    Then a couple of hours later, overcome with impatience, I tried it with one of my good mics. No smoke. But no sound either. Power to the mic & opamp seem fine. With headphones on the monitoring side of my recorder, I can hear a little mains hum when I touch either of the output wires. But nothing when I touch any of the input wires. Maybe the 100k resistors are too high?

    The way I've built the alpha prototype means I'll need to desolder the input/mix array from the opamp board in order to test if the opamp side is OK. I will also explore with an oscilloscope tomorrow. Meanwhile I'll add a pic of the alpha prototype in all it gory.

  • Minimal viable product at 21:38 0 comments

    This first revision has been done in a bit of a hurry inspired by the opamp challenge. It's the minimum viable product & uses a couple of small DC-DC converter modules. But even this will be a very welcome downsizing from what I've been using before it.

    For the second version, I am planning to integrate ... 

    • my own DC-DC converters & USB LiPo charger etc.
    • a headphone amplifier
    • individual switches for XLR shield ground/float
    • individual switches for phantom power: 48V, 24V, 12V, 5V, none.
    • individual switches for muting an input channel
    • switches on the /2 inputs to bring the level back up to the same as the /1 inputs*

    The last item would allow it to also be used as a 2x stereo (+1 mono) mixer.

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