Hi all,

I'd like to build a first order ambisonic 9V electret microphone, that I can travel with.

Balanced outputs is important since I will need cable lengths of 6m and longer.

What are the smallest form factor PCBs available for a project like this?
Are there any SMD based circuits openly available?

Thanks and I wish you all a lovely day!
M

See the attached ZIP (KiCAD 8 project with schematic and board layout, Gerbers, and a PDF of the schematic).? It might be suitable for you to use, either as-is or as a starting point for changes.?

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A couple of years ago I was gifted with a "long past its use-by date" Philips stereo microphone.? The old dynamic elements were (I decided) quite useless by modern standards, and the foam had turned to goo and powder, but the shielded enclosure and stand were nicely built and in great shape.? I decided to experiment, gut it, and rebuilt it as a full-field microphone.

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The guts now consist of four JLI-240A-T capsules, glued into a 3D-printed holder I built using OpenSCAD.? It's based on a basic pattern I found here in MicBuilders, with a graceful branching Bezier-curve support structure which fits into the screw-locked metal shaft in the microphone base.

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I decided I did want a proper phantom-powered balanced drive for it, so I implemented one of the standard two-transistor circuits using a small PC-board layout (.86" wide by 1.26" long).? This is small enough that I could place four of them around the support shaft, wire them to the mics and to four (Electronics Flea Market surplus) XLR cables, and fit everything comfortably inside the case and shield.? I laid out the boards to suit the parts I had on hand - all surface-mount except for the two 1 uF film capacitors, for which I used through-hole parts I had handy.? I'm sure the boards could be made smaller, using SMT caps and 0805 or 0603 resistors.? I used BC856 transistors because I had a bunch of them in a samples box;? there are many other types which should work fine.

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OshPark delivered 6 boards, in about two weeks, for under $11... hard to beat!

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The result - it works, and with the shield closed there's no detectable hum pickup.? I got distracted by other projects and haven't done anything more than confirm the mic's basic functionality, but considering that it's using a standard sort of circuit and decent capsules I don't expect that there are any horrible surprises waiting (he said, walking confidently towards a cliff :-) )

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On Wed, Feb 19, 2025 at 03:42 PM, David Platt wrote:

The guts now consist of four JLI-240A-T capsules

Do you mean JLI-140A-T capsules? I have a few of those. They're nice.

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Thanks for sharing your circuit.

Yes - sorry about the typo!? The ones I used are definitely the JLI-140A-T.