when last we left our intrepid crystals
- June 13th, 2016
- Posted in diy . studio
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Okay, up front: I have built a working crystal microphone! It wants a fair amount of equalisation added to it, but that’s okay because I can do it in the digital audio workstation. (Normally there’d be some circuitry in the microphone to do that, but in this case there’s not. Reasons.) And, happily, it’s picking up the kind of range you’d expect out of one of these old mics. It may be boosting the midrange pretty hard – harder, I suspect, than traditionally – but it’s picking up a good chunk of the spectrum.
It also really should be used with a pop filter (I didn’t), because it has the biggest damn mic diaphragm you’ve ever seen:
70mm. YES 70mm I HAVE BUILT A LARGE-DIAPHRAGM CRYSTAL MICROPHONE.
So enjoy some old-school crystal microphone test recordings, made with a mic built from cardboard tubing, wires, piezo cristals, and old styrofoam cups(!), and I’ll tell you how everything I did last time got thrown out before I eventually got this to come together.
It’s 1944 Forever Faux BBC Radio: NO equalisation
Faux BBC Radio: WITH equalisation
Constant Sorrow: NO equalisation
Constant Sorrow: WITH equalisation
Okay, so, right. When last we left our intrepid crystals, I had a nice little circuit in a nice little modular box, so I could test about 90 kinds of resonating bodies without having to solder everything to everything, and maybe I could keep the most interesting ones and use them in different situations.
But it had a lot of noise – I mean, like goofy amounts – and wasn’t boosting signal the way I thought it should be. I just chalked all that up to being in a test harnesses, and all that.
WRONG.
I still don’t understand what was going on. I thought I’d built the circuit wrong, but taking a known good one and putting it into the modular box made it misbehave as well. Removing the plugs and soldering directly didn’t help either – just as much noise, just as little signal.
Eventually I figured out that if I had the circuit in the modular box, it would be full of noise and lacking amplification. But it could be the box, she said, desperately clinging to sanity, that doesn’t make sense! Besides, I’d taken the circuit out of the modular box, and set it nearby, and that didn’t help.
Then for unrelated reasons I moved the circuit further from the box. A lot further – like, up to head level.
And suddenly everything started working. NEAR THE BOX BAD. FAR FROM THE BOX GOOD. I AM NOT EVEN MAKING THIS UP. I DO NOT KNOW HOW A PLASTIC BOX LINED WITH METAL CAN DO THIS. THIS IS PUREST STUPID ACTION AT A DISTANCE AND I DON’T KNOW WHY.
Given that behaviour also improved when I shortened the cable leading to the piezo pickup, I suspect there is Something about My Cable Stock, and for now, I’m just going to leave it at that. But really, I don’t know.
That case is now Gone. It can be Someone Else’s Problem Forever.
I had also mentioned in comments a couple of places that I had a Really Cool Idea for a suspension harness to hold up the resonating element, which I’d chosen to be the base of a styrofoam cup, as it tested best overall. I was so pleased with this idea that when it utterly failed I was a whole ‘nother layer of So Very Angry.
Anwyay, the idea: take some nylon mesh, the kind used for pop filters. Stretch it across the resonating body. Adhere it to the styrofoam’s outer ring using – hm, This to That says hot glue. OKAY!
Then take that same foam and nylon assembly, and stretch the outer nylon across the microphone case’s front opening. Hold the nylon in place on the outside of the can with a rubber band. It’s perfect! The nylon is acoustically transparent, so will have no effect on sound, and being so lightweight, it won’t dampen responsiveness! It’s GENIUS!
We’ll cut away that middle mesh as soon as the glue is stable
Another can! This one make of shipping tube and aluminium tape.
ABSOLUTELY NONE OF THAT TURNED OUT TO BE TRUE! NONE OF IT!
Well, okay, it was pretty transparent in terms of frequency blocking, I guess that part was true, but even at nearly slack, the amount of response damping just… okay, when I was testing this, I was still testing it with the EVIL CASE OF EVIL, so that was probably part of it, but the amount of signal reduction just depressed me. SOUND SOUND WHAT IS SOUND NONE FOR YOU.
I don’t have any pictures of that setup, which is again because SO ANGRY. So there y’are.
After that, things started turning around. That’s about when I realised how light the styrofoam disc resonating body was. It’s made of sides of two styro cups, flattened a bit and adhesed together with very permanent double-sided tape and cut into a circle, and weighs practically nothing. It is, in fact, so light, that…
…the wire connecting it to the circuit board could maybe be used to hold it up. As long as we can hold the wire in place, that’s worth a try, right? And I’m taking everything apart anyway, so let’s try it:
Piezo crystal is on back of that foam
Circuits just kind of hanging out the back, lol
Foam pushed into the can. Giant resonating disc in front. Hit it.
And it worked. FINALLY SOMETHING ON THIS PROJECT WORKED it was such a relief – on Friday, with things just exploding everywhere, I was pretty damn crazy because seriously it was one of those escalating-personal-chaos-field days, and physics just took a holiday or something and it took a few days to hammer it back towards reality.
So then it was time to make a more proper kit. First, of course, cut some foam more precisely, so the resonator would stay held firmly in place, making sure you leave enough room for all the circuitry bits.
I used some of the leftover delrin plastic to make a back cap for the microphone can. This let me use a standard XLR connector, which I really wanted to do – wire nuts and twisting may’ve been okay in 1938, but with the amount of RF flying around the Lair (and off me!) I really can’t do that. It has to be shielded, too – more copper tape solved that problem just fine.
I still need to build a proper hanging system, so it can hang the way these are supposed to. It’s not as cool looking as the carbon microphone, I’ll just acknowledge that up front. But it’s nice and compact – relatively speaking – and it works.
Another variant will be to replace the styro with that clear Boeing plastic. When I was running tests, the signal level on that was… not real high. But neither was the signal level for anything else, and in the breakthrough moment when I figured out that somehow the plastic case was A Problem, I was using the clear Boeing resonator. And of all the things I tested, that had the best sound. So I think that’s worth another go, and I do have a spare circuit.
(I think. I think I have a spare circuit. I also have more and larger pictures, like usual, over here on Flickr.)
But even if that works, and if I prefer it, I’ll keep this one. It does have a very old-time-radio sound – newer than the carbon mic, but still… old-time.
Plus, the damn thing functions. After this past weekend, that counts for a lot.
2 comments on ello.
What did that microphone start out life as? Without looking it up, I think Rochelle salts are used for crystal microphones, as well as those piezo igniters.
uh – parts?
The silver can is part of a heavy-duty cardboard shipping tube, wrapped in conductive aluminium tape. The mesh is aluminium window-screening material. The resonance plate is the sides of two styrofoam hot-temperature cups, cut out, industrial-duty double-sided adhesive taped together, and cut down to a large circle. The crystal is piezo, connected by that black bit of cable to the circuit board, which I built from a kit. The foam was originally… packing material of some sort. I don’t remember what came in it. I cut it down with a $2 electric carving knife from a thrift store, like you do, and did some fine-tuning with scissors. The black back plate is made from leftover Delrin from the stage monitor rebuild, and the conductive copper tape is just something I keep on hand. The XLR socket is also just supplies-on-hand.
Did you cast the crystal part of the microphone yourself? Or did the crystal part start out as something else, like one of those Sonalert type piezo noisemakers, or an igniter? About 40 years ago, I saw Rochelle salts cast onto a small cylinder during a physics class, as part of a piezoelectricity demo. Somewhere, I have a crystal earphone from the ’70s….
A frozen food knife makes a great plastic cutter..which is more or less the same blade on an electric carving knife.
Two jobs ago, I saw a guy make speaker housings from heavy styrofoam containers( the kind dry-iced vaccines come in). He brushed on a coat of Elmers Glue All over the interior, claiming it improved the sound quality. Would that help the foam part of your microphone?