My carbon microphone is actually in a can now! I’m super happy with it, and yes, it works, even if most of the circuit is still in a test harness. The can itself does not contain anything important in the way of circuitry, though it does contain quite a bit of grounded shielding which is very important, and that shielding is carried forward out the cable, which is a TRS phone plug.
I made the decision to put the support circuitry in a separate box for a couple of reasons. First, the signal level is pretty high, so a decently-shielded cable should prevent most issues from being actual issues. Second, the support circuitry for these things tended to be external in the originals, so it’s period-accurate. Third, opening the can to replace a battery every 15 hours of use or so was going to be annoying; much easier to do it an an external experimenter’s cabinet.
And yeah, one of the downsides of this mic is the high power consumption. I thought about trying to tap phantom power, but from what I can tell reading up on it, the amount of power actually supplied – voltage aside – is dramatically lower than a battery’s supply and is not well defined, by which I mean is not defined, so… yeah. Batteries it is.
Anyway! I decided to make it look like a vintage carbon-element microphone of its era, only using elastic instead of springs. Springs look cool, but they’re also noisy as hell and create other issues, and it’s easy to tell that they would’ve used elastic if they’d had it just by how quickly they changed to using elastics as soon as they had elastics to use.
BUT SOLARBIRD, HOW DID YOU DO IT?
Well, first, let’s ask the eternal question: ah, junk shop, is there any build problem you can’t solve?
No. No, there is not.
If you’re wondering, that’s a vaguely-20s-looking kind of drink can holder in the middle, and I believe a hand towel holder designed to hang from the top of a door.
My only worry was whether the towel ring assembly was just screwed into the plate, or bolted to it; most excellently, it turned out to have been attached with a screw. So I just separated the ring from the existing base, sanded the attachment point down a bit to make it nice and flat, and attached it to a small metal L, using a rubber plumbing washer to make up the extra space.
Attaching to a standard microphone stand attachment is just as easy; unscrew one of those infinite number of cheap mic clips that are floating around every studio ever, toss the bit that holds the mic, drill out the hole in the L metal to be large enough, and pad with more plumbing washers.
Now the ring is ready to attach to any standard microphone stand.
Now, the can itself was a little more of a trick. This is all mechanical construction, not circuitry, since all that will be in an external box. But! There is some electrics, because given my particular… affinity… with RF (and radiating it, hi, it’s solarbird for a reason), I wanted a good heavily-shielded pickup enclosure.
Did you know you can buy adhesive tape made of copper? With conductive adhesive? It’s made exactly for this purpose. I love it. I started by lining the back, and peeling excess up the sides a bit intentionally. You want a good amount of overlap with this tape.
Lining the sides involves another ring of copper metal tape, with – again – overlapping tabs made of the excess height. Getting the height right is really simple – just put the tape in and cut inwards, using any common scissors – this may be metal tape, but it’s pretty thin, and no special tools are required.
After I took that photo, I realised I needed smaller tabs, so I went through and made another set of cuts, halfway between each existing cut.
Once you’ve fiddled around with it a bit, you’ll end up with something that looks like this:
Make sure the copper is well rubbed down against each other, so the conductive adhesive can really carry current without adding any resistance. Again, no special tools, a fingernail is fine – but make sure it’s well stuck down.
You might also notice in that photo, a small black line – I broke the tape, and fixed it with just a small piece to cover the gap. As long as you have well-connected metal throughout, you’ll be fine. We’re talking very low power with RF noise, in most circumstances, so you don’t have to worry about carrying power or anything like that.
Unless you have a tesla coil, maybe. That’s different.
Now, I also needed a grill for the microphone, and – importantly – it had to be a conductive grill, because I need that RF blocking all around the carbon element. I know, I know, some of you are going, “it’s a carbon element how are you doing anything to it?!” and all I can say is I have recordings and I have to ground myself with a wrist strap if I’m using AKG microphones, and again, supervillain.
Fortunately, a material that serves this purpose quite well is common and cheap: aluminium window screening! I’m kind of annoyed with myself, because I threw away a bunch two weeks ago – used but still clean and good – because I had no thoughts I’d need it. What was wrong with me? I can’t even tell you. Moods. So I had to
stealbuy some. Fortunately, it’s pretty much dirt cheap.
I got a ring of heavy flexible rubberised foam to make a structural ring, and measured the right size just by pushing the screening into the bottom of the can until I had good edges. Since it is window screening, it has a lot of room to compress, and that helped. You want that excess screening material, for reasons which should be obvious momentarily.
In the above photo, I’ve sized the screening material, and am getting ready to make a ring of copper tape to surround both the inner and outer layers of the support ring. This is partly structural – you can see that my support ring is not a single loop, but a bent straight piece – and partly to help make sure of good, solid contact between the metal screening, through copper tape, to the interior copper shielding of the can. Make this part a little too large, if anything, and you end up with a solid pressure-contact connection.
Holding the carbon element in place is also a job for foam. In this case, I have some high-density impact-absorbing foam left over from my case making projects earlier, so I just used that. It can be a very rough cut, as long as it’s just a tad bigger than the ring it’s going into. It’ll compress, and that provides a little more outward pressure to make the grounding contact between the grille and the interior shielding better.
You’ll also note inside the can, against the back, I’ve placed a spacer ring. This keeps the grille’s support ring from going too far into the can; it’s just a physical element, since you want the carbon element nice and forward, and not sinking into the can where sound would get echoey.
What you can’t see is an important step I … didn’t remember to photograph. Sorry! And that’s drilling a hole for the cable. The cable is three-conductor; two signal leads (which connect to the two contact points of the carbon element) and one shield ground. The shield ground gets soldered directly to the copper tape, which is why you use copper metal tape instead of some other metal. This is a little tricker than you might expect, mechanically; I had to use higher temperature on the soldering iron. I think it mostly has to do with soldering wire (physically complex, wicks well) to a flat surface (physically simple, does not wick well). Just take that part as read.
Some of you might be looking at this picture and going, “…wait. The carbon element is right up against the grill. It’s touching, and metal, and isn’t that a problem?”
Well spotted, you! It would indeed be a problem! I solved it by cutting out a couple more layers of plastic window screening – also left from another project – to provide an insulation barrier between the metal screening and the actual pickup element. If you don’t have any of that, hosary would do fine – the same material used in pop filters. Acoustic transparency is important here, of course, but to be frank – less so than usual.
“That’s not like you.” Yes. But think about it, I mean, what’s a telephone use? Oh yeah, THICK PLASTIC WITH SOME HOLES IN. Does it hurt the sound? NOT VERY MUCH, because it’s a carbon element with a range of around 300hz to 3500hz, maybe. So you can afford some loss in the high end, because what you don’t lose, the mic will lose for you. And this screen material is plenty acoustically transparent for these circumstances. So would be hosary, or the right foam, or all sorts of other things. Just, you know, use good judgement.
Anyway, the build stuck on hold – ON HOLD! HA! – for a couple of days as I wait for the 600ohm isolation transformers to arrive. They aren’t essential, necessarily, but do reduce RF noise and hum in some circumstances, so I’m going to use one. I’ll post more when I’ve got more done. I’m really pleased with how it’s coming along; this is fun.
Bigger pictures on Flickr, as usual.
ps: People were asking for a sound sample. This is from the test harness, before I built the ‘can.’ New recording sample next time.
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