Hidden mini omni gem.

[k brown]

I know there's not a whole lot of interest 'round these parts in SDC omnis (this is kind of a multi-pattern LDC playground), but I thought I'd share my discovery of these beautifully-made mini capsules from inside an Audio Technica AT841R boundary mic.

Separated from their stock power modules with tiny tunes transformers, and hooked up to a quality transformerless circuit, these sound amazingly good; and shockingly quiet for such a teeny thing: 7.5mm outside diameter, with a built-in 2SK660 FET (and NOT SMD) and RF caps. With the top tamed about 3dB, these hold up extremely well against my Sound Room cherry-picked Oktava MC012 omnis.

The later iteration of these (in the U841R) have just a simple, small hole in front of the diaphragm, instead of these precision-machined slots. Body is not stamped aluminum, but machined brass. The actual capsule unscrews from the tiny back-end that holds the little PCB with the FET on it.

Though the published response curves are quite similar, they sound way less bright than my Shure Microflex omnis (which I've never been able to find a use for, except inside really dull pianos).

The top end is less hot hooked up to an 8533 power module, compared to it's stock module; maybe a boost circuit in it?

 

[cyrano]

I use several AT lavaliers and love them for outdoors use. Same 8533 modules. Balanced cable for a non-balanced connection. Some of these are "hanging" choir mics and come with 9 meters unbalanced cable between electret and power module.

 

[k brown]

Here's the first complete build with these little beauties. Tucked into the back-end of a TA3F connector, using it's strain relief to secure the cable.

For power modules, I'm using the boards from AT8533 modules, with the transformers replaced by impedance-balanced outs and phantom-feed resistors.

Still amazed how quiet these things are for their size. At least half the noise was from the stock power modules, which appeared to be very much like the 8531 modules, but with SMD parts and some different values. 8533 is a more sophisticated biasing circuit.

 

[cyrano]

Nice.

Mine are AT853 with 8533 in the XLR. I used four of them for my first ambisonic experiments. And some others. I like the entire range, cause they're small and you can still order anything in parts from AT. Even for stuff that's over 20 years old.

 

[k brown]

Turns out these are actually just 'stubbier-bodied' versions of the ESE-O capsules for the ES943 series mics.

 

[k brown]

Second build with these little guys - much more robust, and look much less silly up on a stand! Also allows mic clip/shock mount to be placed further from capsule.

Carbon fibre tube in heavy duty 1/4" plug barrel.

 

[k brown]

"Poor-man's DPA 4060".

Though the ES943/O lavs are only half the price of the DPAs, (and give them a serious run for their money) one can get the same thing for even less by getting the AT841 boundary mics (which can be had quite cheaply on ebay) and hot-gluing a mini cable directly to the little FET housing/capsule mount.

They require a slight top EQ (-3dB@8kHz) to match the response of the DPAs, but the noise spec is similar, and they're only slightly larger. Plus you can swap the capsule out for a cardioid or a hypercardioid (even shotgun)!

Here's a clip of an orch recorded with just a pair of these little guys:
https://www.idrive.com/idrive/sh/sh?k=d4b3o8e0e9
And a bit of piano concerto, again with only the two mini-mics:
https://www.idrive.com/idrive/sh/sh?k=z4e2y2r6d4

 

[capacitorless]

Just popped the top off of a cardioid AT871 boundary mic, and it has a similarly sized capsule, albeit with a black finish. A little research says that the AT851 might be a cardioid version of the AT841; the 871's body is a little wider, but shaped similarly to the 851.

I'm wondering if this also a useful capsule, and maybe a card version of the one in the AT841...

The circuit board looks intriguing. SMD parts and what might be a flat transformer. Maybe it's the insides of a 8531 power module scratch that as the AT8531 is battery powered.

 

[k brown]

871 and 851 had the earlier, larger 853 capsules; the 841 boundary mic, ES943 lav, and other 900 series mics have the smaller 7.5mm capsules.

Difference between 851 and 871 is that 851 had external power module with simpler circuitry. The 871 was the only AT boundary mic that had a more sophisticated push-pull output circuit, similar to that in the AT8532 power module for the ATM35 clip-on instrument mic.

Interestingly, these smaller capsules are quieter than the 853s.

 

[k brown]

Just popped the top off of a cardioid AT871 boundary mic, and it has a similarly sized capsule, albeit with a black finish. A little research says that the AT851 might be a cardioid version of the AT841; the 871's body is a little wider, but shaped similarly to the 851.

I'm wondering if this also a useful capsule, and maybe a card version of the one in the AT841...

The circuit board looks intriguing. SMD parts and what might be a flat transformer. Maybe it's the insides of a 8531 power module scratch that as the AT8531 is battery powered.

The original 871 circuit board was larger and had all through-hole parts; they're getting hard to find now. That's why the SMD boards are so wide, with lots of empty space - so they can be mounted to the plate's mounting holes, without requiring a new plate to be made.

 

[Icantthinkofaname]

Interesting, I wonder how the ES925 gooseneck capsules would be for DIY.

 

[k brown]

Those are the current version of the 853-series capsules. Larger than the ones I posted about.

I got a couple of the newer omnis a few years ago, and they're much brighter than the original 853s. So much so, I've found them unusable. Brighter even than the chart would suggest. Don't know about the other patterns.

The original 853 cardioids are what I used in this figure 8 project:
https://groupdiy.com/threads/diy-fig-8-minis.82600/#post-1072151

 

[Icantthinkofaname]

Those are the current version of the 853-series capsules. Larger than the ones I posted about.

I got a couple of the newer omnis a few years ago, and they're much brighter than the original 853s. So much so, I've found them unusable. Brighter even than the chart would suggest. Don't know about the other patterns.

The hypercardioid is supposed to be quite flat. The cardioid is pretty bright though.

 

[k brown]

That looks pretty impressive.

I certainly wouldn't call that a hypercardioid pattern, though. Classic supercardioid.

 

[TromNek]

I've been wanting to build some pairs of boundary mics. And keeping my eyes out for those cheaper omni sdc mics to use.
I picked up a couple cheap Sennheiser MKE 10 with preamps to start with.
If they work out, I'll start experimenting with other cheaper lavalier omnis.

Any suggestions or ideas are welcome.

 

[Icantthinkofaname]

That looks pretty impressive.

I certainly wouldn't call that a hypercardioid pattern, though. Classic supercardioid.

If most of the noise is just from the preamp I might grab a couple from eBay if I see some.

 

[k brown]

It's hard to do a whole lot better than the Primo EM273 for building boundary mics.
https://micbooster.com/microphone-capsules/209-primo-em273-microphone-capsule.html