mic pre - a tiny line-level electret microphone preamp for Norns/modular/field recording and whatnot

Hello.
I made a low-cost (about $10 in parts) microphone preamp for use with electret microphones. It’s basically the Texas Instruments reference design here


It’s extremely clean and high fidelity, running on a 9v battery for minimum noise. I designed it to fit on the side of such a battery so it’s as tiny as possible.

The specified electret microphones are 6mm with a 68dB s/n ratio and a -35dB sensitivity, which is rather great for the low price.

Using 1.5m Mogami 2330 wire, I connected up a couple of units and hooked them up to my oscilloscope. I then played a sine wave out of my studio monitors and checked the output with the mics at different distances and angles:





So yeah, it works pretty nicely! Especially good for doing some binaural stuff into Norns, which was my primary reason for doing this.

Here’s the github page with all the files:

And here’s the Mouser cart (I think…)
https://www.mouser.com/ProjectManager/ProjectDetail.aspx?AccessID=81b8966b79

Notes:
The microphone-side cable should be as low capacitance as possible, and ideally extremely short. Longer (>1m) cables that are not low capacitance may cause instability in the amplifier. The cable from the output can be fairly long without issue.

Updates:
Added the “Tiny Mic Pre” version, found in the subfolder with the same name. In theory, this will sound better than the previous version as there is no distance between the microphone capsule and the circuitry. The microphone cable used should be have relatively heavy-gauge conductors in order to minimise the effects of having the battery far from the circuitry. 26 or 24AWG should be fine, two or three conductors and a shield for the ground.
It’s designed to fit inside a 6mm inside diameter bit of brass/steel tubing.

I’m adding the zip file containing 3D-printable .stl/.obj files for a really elegant housing for the board and battery, as designed by the wonderful @infinitedigits
https://llllllll.co/uploads/short-url/sTYmwfYB7siqERfN3NrHpef2ENr.zip
Definitely beats my blutak system anyway!

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Wow, very nice. Thank you for sharing!

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This is truly fantastic! Thanks for sharing!

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That is very cool!

If anyone is planning on getting some PCBs made (in the EU/UK), I’d totally be down for piggybacking on the order.

Couple semi-tangential questions as I’ve been wanting to make some things with electret mics for a bit:

  • does something like this play nice with any/most electrets or is the impedance/circuit specific to that mic?
  • can you run a bunch of these off a single battery in parallel (with less battery life obviously)?
  • has anyone made/seen something similar for straight up 48V/pre for condenser mics?
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1 - it is specific to the mic but mics with similar values should work okay. The formulas in the TI paper allow you to tweak the values for other mics (see sections 2.1 and 2.2 of the paper)
2 - I believe so, although I haven’t tested it. Certainly wouldn’t cause any catastrophic issues if you did try it.
3 - There’s this: Espresso Portable Phantom Power Supply - Zeppelin Design Labs but I can’t comment on its quality
edit - there’s also this P48 Phantom power from 9V d.c. – Stompville which seems extremely similar to the above.

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Is there no pot for setting the gain?

No, its gain is fixed to output line level at the nominal SPL of the microphone.
If you really wanted a pot, you would have to replace the 75k resistor with a 100k pot but the 20pF capacitor may need to be replaced too.

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I’m not sure of this is what you’re looking for, and (unfortunately) the electret it uses is out of production, but I’ve built a pair of 48v phantom powered mics using this schematic:

From this site: Microphones

I think they sound great, and have been using them as a AB stereo pair with a Zoom H5 with results I’m very happy with.
As I said, the Panasonic WM-61a is no longer made, but you should be able to find similar schematics for other capsules still in production.

Edit: here’s one : Locus Sonus - Electret mics

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Supposedly the Primo EM258 is pretty much a clone of the Panasonic: Cheap Microphones for Ultrasound – Zach Poff

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Orrrrrrr…
(just an idea)

I’m thinking a 6mm inside diameter stainless steel tube with this stuck inside. Board is 5.6mm wide, so plenty of space behind the capsule.
Wire would have to be 22AWG and ideally maximum 1m to minimise voltage drop. The benefits would be reduced microphonic noise/capacitance and the fact that the other end can simply be split off to a battery with the other end going straight to Norns/whatever. Kind of pointless in a way, but also a bit more elegant due to the lack of exposed electronics.

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I want to order as well. I would order trough JLCPCB, the only problem will be that the shipping to UK from my adress (Belgium), will be more expensive than the complete order (+ shipping) at jlcpcb :smiley:

Maybe try https://aisler.net/ ? They’re sort of OSHPark of Germany - more expensive than eg. JLCPCB, but you can get a small number of prototypes and somewhat more locally (and potentially faster) than ordering from China.

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I’m actually in Portugal these days, but go back and forth a bit still, hence the “EU/UK”.

thanks so much for designing this board @ramphands. it was such a joy seeing your process and learning from your designs and getting acquainted with board manufacturing and component sourcing. also I had never soldered smd before and I was sure I messed up multiple times but the three boards I made so far just worked despite my soldering!

well…one did have a capacitor explode but I think it was because the battery shorted something when I pressed the component side of the board to the side of the battery (I replaced the capacitor and its working great, no explosions since).

I’m getting a little bit of a high pitched whine but I think its because my cables are long and are picking up some radio frequencies - I have a dual mono to stereo cable stretched across a marimba so that I can record “in stereo” (one mic on the left and one on the right). if I jostle them in the right way it can sometimes find a position with almost no noise.

as for the levels into a norns - they are excellent. I was playing pretty close to them and I got a little bit of clipping, but that’s my fault for placing them there. the levels seem perfect going into the norns if I just play a little farther away.

here’s my first play with them through the blndr script on norns.

I have a few extra boards if anyone in the US wants to try - just pm me.

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Oh wow amazing job, especially if you’ve not done SMD soldering before! To be honest, the parts selection and layout is pretty beginner-unfriendly, especially the tiny opamp. Huge props for getting them working with only one mishap. Tantalum capacitors are famously horrible with regard to exploding if they get shorted, but they’re the least microphonic SMD cap that isn’t a huge electrolytic, so they’re a necessary evil.

Yeah the mics do clip if the proximity to a loud sound source is over a certain threshold. I forget the SPL range of them, but they’re pretty good for the most part.

I’m so so so so so happy that you took the time to write up your experience though! I think you might be able to alleviate the whine if you use cables that are low capacitance, ideally with a shield around the conductor - use the shield for the negative and conductor for the positive.

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These look pretty good. If you and @claesbert haven’t already ordered I would definitely join a pcb run.

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Here’s a little example of a couple of the mics running through @infinitedigits’ OOOOOO script.
I actually recorded a video but Da Vinci Resolve is not playing ball with syncing up the video, so I switched to a photo and a soundcloud link. You can at least get to hear the sound quality a bit before it gets chewed up in the loops.

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@ramphands here’s a little microphone box I 3d-printed (stl and obj files):
microphone_holder_v1.zip (11.8 KB)

its very basic, its literally the first thing I tried making on a 3d printer. but it fits all pretty snug without a lid and keeps the board away from the metal on the 9v, which is all I needed.

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Thank you so much for this!
I don’t have either a 3d printer or the know-how to put together such a design, so this is really wonderful.

I’ve updated the original post with a link to the file, as well as a couple of updates on the Github for the alternative “tinier” version which is designed to be housed in a bit of brass/steel/alu tubing. The battery is then located at the device (Norns et al) end of the cable which is… I dunno. Less ergonomic? More? Both!

Anyway, I do recommend getting some metal tubing for either version of the mic anyway - it gives a bit of shielding and rigidity, and makes it easier to stick a bit of foam on for some shielding.


I went a bit fancy and used a brass/alu combo with a bit of 40 gauge mesh at the end

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Whoa! Tiny Mic Pre is TINY

(Or my hands are enormous)

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