DeccaTree stereo downmixer

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.

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.