Panasonic WM-61A Mic
I needed a simple (and cheap) microphone with flat frequency response and decent noise level to record acoustic guitar. Projects built around Panasonic WM-61A electret (datasheet) capsules are on the top of the results when you search for cheap but decent mics. WM-61A capsules are praised for their super flat response and good signal-to-noise ratio. Unfortunately, like with some other components that are loved by the DIY community (J201 FETS come to mind), they are discontinued by Panasonic. You can still find some on the eBay (together with a bunch of Chinese clones advertised as the real thing), but there are also good alternatives, such as Primo capsules. WM-61A based microphone can be built for less than $20, so it was a great introductory project into the world of mics for me.
Linkwitz Capsule Mod
WM-61A capsules have an integrated FET that has its source terminal hard-wired to the capsule casing, which means that it will be grounded when wired with two leads as intended. In other words, the FET will operate in common source mode. According to Linkwitz, much better performance can be obtained from this capsule if we break this connection, wire the capsule with three leads and bias the FET ourselves. That involves some precision surgery on the capsule and cutting a thin copper trace that connects one of the two terminals with the capsule casing. Then we can solder two leads to the terminals and the third lead to the capsule casing.
In theory, this sounds pretty easy, right? Well, it’s not that easy 🙂 . The first problem is that the capsule itself is tiny. It may look big on photos, but in reality 6mm is very small. They are hard handle, hard to cut, hard to solder and easy to lose. The second problem is that the casing is made of aluminum and it’s difficult, almost impossible to solder to. Make sure you get more than one capsule, as it’s easy to screw up and destroy a capsule in the process.
Let’s go step by step. You’ll need a sharp blade, xacto knife or something similar. On the photo above you can see a modded capsule next to the stock capsules. Red lines show where to cut. Cutting the trace is the easier part of the job. Try to remove as much of the trace as possible to prevent it from bridging back together once you start to solder the leads. After cutting, make sure there’s no continuity between the casing and the solder pad.
Now onto the next step – making it possible to solder to the casing. As you can see on the photo above, there is a tiny circular PCB that is encased in the casing. We can’t solder to the casing, but there is a copper trace on the PCB itself that goes around the edge of the board and is normally covered by the lip of the casing. We can uncover part of the casing and expose some copper trace that we can use to solder the ground lead. To do that, we need to cut through the aluminum casing and peel it off. The way I did it was to place the blade against the casing and lightly tap the knife with the palm of my hand until it cut into the lip of the casing. Now repeat the same for the second cut and you should be able to lift the casing lip, exposing the trace. Just make sure that you don’t cut too deep and damage the trace. It’s sufficient to go halfway through and thin aluminum will give in once you try to peel it off the trace.
That’s it! If you did everything right, your capsule should look something like the capsule from the photo on the left.
XLR Phantom Powered Preamp
There’s a bunch of different circuits and most involve either a pair of transistor or an op-amp. The one I decided to go with comes from good guys at Direct Approach website and it’s based upon Behringer’s ECM8000 mic. They offer several different circuits, this particular one is titled “Best So Far” at the bottom of the page.
Since the circuit is relatively simple and cheap, I wanted a housing solution that would match the simplistic theme and not bust the bank. I thought about two ideas: house the mic inside a used shotgun shell (got the idea from 12GaugeMicrophones.com) or go even more compact and try to squeeze everything into a male XLR connector body. A fellow AcousticGuitarForum.com sent a couple of used 12 gauge shells and I decided to use one of those. The good thing about them is that the primer is also 6mm in diameter, so after taking it out, the hole can be used to fit the capsule. WM-61A is also 6mm and it fits like a glove, doesn’t need any adhesive to keep it in place. For the XLR connector part I bought a cheap XLR male connector and just took the part that holds the 3 prongs.
Below is the PCB layout I drew that should fit inside an XLR connector or a shotgun shell, as the board is only 0.5×0.75″ (13x19mm) big. My cheapo XLR connector has internal diameter of around 15mm (0.6″) and depth of around 20mm (0.8″), so the board should fit as long as components are mounted flush to the board to make it low profile.
PDF Layout File
If you are too lazy to etch the board, pretty much the same layout can be made on a perfboard. I, however, wanted to try something different and do a 3D point-to-point wiring directly on the XLR pins without a circuit board. That kind of wiring can look impressive if done neatly and this seemed like a perfect opportunity to try it. This circuit is relatively simple and beautifully symmetrical. However, this kind of layout can hardly be more compact than a well done PCB.
My 3D layout didn’t turn out as beautiful as some others I’ve seen and couldn’t fit in the XLR connector but it fits just nicely inside a 12 gauge shell.