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Archive for December, 2020

T47 – An Inexpensive Dynamic Mic

Posted by Tom Benedict on 01/12/2020

A while back someone pointed me toward a really good sounding dynamic capsule made by BTM called a T47. They had put it in a mic shootout with a bunch of other vocal mics and it actually scored quite high, despite it being a bare dynamic capsule with an XLR plug.

Based on the shoot-out I picked up two of them and then… failed to do anything with them for a couple of months. I call this the “reduce heat and bring to a simmer” phase of fishing for ideas. I had a vague notion that I wanted to make some mics for high SPL sources (e.g. bottom mic for djembe, intake and engine bay mics for cars, etc.) but I didn’t really know how I wanted to do it. I hemmed and I hawed, but eventually ideas started coming together and I wound up with the design for a mic body that I could 3D print.

If you haven’t noticed, 3D printing factors into a lot of my mic builds. My background is really in subtractive machining – what they call “take off tools” – but once I got into 3D printing I really fell in love with the idea of being able to design and fabricate shapes that would be difficult to impossible to produce any other way.

This was not one of those projects. I think I could’ve knocked these out in the shop in a couple of hours out of Delrin, aluminum, whatever. But… I’d drawn them up in CAD already, and with COVID restrictions in place I couldn’t guarantee the shop time necessary to turn these out. So I sent them off to Shapeways to print. Here’s what I designed:

And here’s what I wound up with:

I love it when a design comes together!

It’s pretty straightforward. There’s a Switchcraft B3M connector on the bottom and the T47 capsule on the top. In the original design for the mic body, it was threaded on the bottom to receive the XLR jack. But as I found when I received the first rev bodies from Shapeways, I had cut my tolerances too tight. The jack didn’t screw in!

Rather than re-make these, I ran a tap through the body so the jack would thread in. (13/16″-20 NEF for anyone who’s interested.) The second rev of the body has a clear hole and a hexagonal recess inside it to take the jam nut that comes with the B3M connector. Way easier and probably more secure in the end.

The bodies are a little chunky, but at just under 36mm diameter they still fit in the larger universal clip mounts. (Because of my use case, mine are more likely to be mounted with gaff tape, so I didn’t put too much thought into how to mount them.)

Anyway, if you’re interested in building one of these, you just need three things: the T47 capsule, a Switchcraft B3M jack, and the mic body. Here are sources for all three:

https://www.aliexpress.com/item/32866241871.html

https://www.mouser.com/ProductDetail/Switchcraft/B3M?qs=IF7nv7MIRJ3ZKk2bybYqmw%3D%3D

https://www.shapeways.com/product/B3SF4W4AB/t-47-mic-body?optionId=181791683&li=shop-inventory

P.S. … (Or should I call this Part II)

After building these I took them out to the highway and recorded cars passing by. I think I had about 53dB of gain dialed in. That was a lot for someone who’s used to dealing with condensers, but not killer for a dynamic. I did get a bit of background hum, though, and soon realized I was standing under some high tension power lines. Noise removal in Reaper took care of it (I used iZotope RX Spectral De-Noise), but I wondered if it was RF pickup. So I did some additional testing.

Any time you get noise on an input, it’s worth plugging in a 150-ohm dummy plug and working outward to find out where the noise is coming from. As far as I can tell, I was running into the noise floor of my recorder. It’s not bad, but with the gain that high it was noticeable. Just in case, though, I came up with a backup plan:

One of the big problems with 3D printing parts for electronics is that plastic can’t act like an RF shield. Sure, I could’ve printed these out of brass or steel or something, but that would’ve been prohibitively expensive. Given the design, I’d just machine them at that point. But last night I finally found something I’d been searching for for ages: conductive paint.

Conductive paint isn’t new, but back when I first searched for it I could only find it in aerosol cans or as high-VOC oil-based products. I live on an island. I can’t even get most vendors to ship batteries out here. Conformal coating? No luck. Spray paint? You have to be kidding me!

But the stuff I found, on Amazon no less, is water based, low-VOC, and relatively inexpensive:

I’m going to run my dummy plug tests again tonight with the cable and mic right next to an AC cord to see if I can get a worst case noise measurement out of the thing. If it sounds like I’m having RF shielding issues, my plan is to disassemble the mics, paint the insides of the bodies with the conductive paint, and reassemble them with a solid connection between the painted surface and pin 1.

Meanwhile, I’m happy to have two nice dynamic mics and a means to add RF shielding to any 3D printed parts in the future.

Tom

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