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

[forrest]

I actually sourced everything from lcsc and had jlcpcb assemble the boards for me, its so inexpensive these days and saves some hassle, pretty sure the stencil layers are in that gerber so ya can probably just upload the bom during that process.

Edit: oh it looks like I didn’t upload the placement csv, thats on another computer, i’ll upload it in a day or two then you can just have them build it, easy peasy.

Edit 2: updated the repo with the placement csv

[ramphands]

Finally built up the tiny version. Have to get some suitable wire to finish them off. I’ll be interested to see if there’s any noticeable audio difference between this version and the original. In theory, due to the shorter distance between the mic and the circuitry, it should be less prone to interference, although I didn’t really notice any with the original.

[Rodrigo]

I got to thinking about a project where I have a mic mounted to each finger in a glove for really dense explorations of the mic/friction stuff (like this) and I remembered this project. Specifically the Tiny Mic Pre variant.

I could conceivably have 4 (or 5) of them on little finger mounts (banjo/finger picks?) and have all of that going to a breakout box with a battery (presumably you can power all of them off a single battery?), then have that go onto the rest of the system from there.

Does anyone have spare PCBs for this in the UK/EU?

[monopole]

@Rodrigo I have a few spare stereo preamps designed by @forrest up here, based on the same design.

[Rodrigo]

Hmm, that does look good, but those mics are definitely on the big side, though it would be interesting to consider just having the capsules themselves on their own, and having whatever else in a breakout box handling the power. I guess you risk more noise, but given what I’m doing, that’s not a huge deal.

Lemme have a think on it and I’ll drop you a PM if it seems like it would work well (wouldn’t want them just sitting here unused if it’s not).

[mlinton]

I’ve got most of the parts to start building mine, do you mind sharing what wire you’d recommend to use for the tiny version? What gauge, 2 or 4 conductor, shielding preference?

Thanks!

[ramphands]

Hey that’s great!
Which version are you building? Once I get home I’ll check exactly which wires I used. With the tiny version I used three conductors and a shield (4 connections total). The non-tiny version just uses a single conductor and a shield as you don’t need to care about the battery.

Edit:
I’ve just checked and I used:
Mogami 2790 for the tiny version (Mogami 2879 would also work but perhaps too thin)
Mogami 2330 for the standard version (Mogami 2943 would be fine and thinner/more flexible)

[mlinton]

Thanks, yeah I’m actually going to build two of the tiny’s and two of the non-tiny.

So for the 2790 / tiny / three conductor version just run the power and ground alongside the signal cable?

[ramphands]

yep - I use the shield and the…green? wires for the signal and the black and red wires for the battery.
Wiring the tiny one is a bit of an effort in figuring out how to break out the battery wires. I ended up cutting a 30cm piece of the wire off, trimming back all the ends about 15mm then resoldering all the connections while leading the battery wires outside of the resultant mess. I then soldered the battery wires and heatshrunk the resulting mess, finally hotgluing the heatshrunk connection to the side of the battery box for security.
By contrast, the standard Mic Pre is a breeze to wire up. The only issue is the run from the board to the microphone shouldn’t be too long otherwise it may induce oscillations. It’s mentioned somewhere upstream how long I recommended.
Hope that helps - any more questions, please fire away!

[ploubus]

Mine looks like this:
https://arvidkammler.com/nextcloud/index.php/apps/files_sharing/publicpreview/4WyMmyr5pDzTEyj?file=/&fileId=3840&x=1080&y=2247&a=true

It’s a bit weird to handle with the battery attached but worked out fine so far.

[Hlp]

Could somebody explain what J2 on the tiny board is?
I need designators for the bom and J2 is unclear.

If iam bot mistaken its simply the output solder holes?

Edit

Also does anything (impedance etc) speaks against using the tiny version with sth like a micbooster em258 mic, leaving the pui audio capsule head of and adding a trs instead?

@forrest are the value changes transferable to the 2 designs from @ramphands ?

[joshhh]

@ramphands - any reason you can think of that a USB → 9V adapter like this wouldn’t work well for these preamps?

[jaseknighter]

@joshhh, did you ever try this usb power solution? i just ordered a similar cable for suuuuper cheap here and will report back on whether it works.

[joshhh]

I haven’t used it for this specific mic pre, but I have been having overall good experiences with those USB 9V adapters (and a different one for USB-C PD → 15V for my modular)

[mlinton]

Finally got mine wired up, and it is VERY quiet. I need to crank the gain on my pre to get any level at all, normal? what should I check to make sure everything is soldered correctly?

[ramphands]

It should be fairly loud. All of mine just plug straight into Norns and give a decent signal as is.
Which version did you build? How’s the soldering? Some macro shots of the pcb would help me check if anything is out of place.

[mlinton]

Reflowed the components and replaced the op-amp just to be sure. Not the greatest pictures, I’ll see if I can get better ones.

And these are the op-amps I’m using (the part numbers match the spec).

[ploubus]

What kind of connector is that and how are you connecting the power source?

[mlinton]

So I have two channels in the overall design, the signal from this mic goes into the 3.5mm TTRS connector and then gets routed to the left signal output of a TRS connector that goes to my mixer. This should let me use standard 3.5mm audio cables of varying lengths to connect the mics.

Same thing for the right channel. I realized afterwards that I could likely use a TRS connector and just combine the grounds into the sleeve instead of running two wires from the board to the jack.

Ignore the ugly prototyping soldering job on the protoboard, I’ve checked it all for continuity.

[Durdee]

Just got a tx-6 and with a usb stick, the mic pre turns it into an extremely serviceable field recorder! With the onboard gain these mics get shockingly sensitive with very little noise!

I tried to build a box with two stacked inside, separated by kapton tape and it worked, but with the gain up a bit I got some really strange complex noise. I’m wondering if it was the proximity of the boards or that they were sharing a battery? Anyway, hardly a bother, gonna order a couple more of those excellent enclosures and separate the boards and see if that helps.