A couple of months ago, I tried making a Cortado-based bridge pickup for the octave mandolin. It worked okay – better than the ad-hoc clamp arrangement I’d bodged together for last January’s Conflikt show, but not what I’d hoped. It was a lot more stable, but still needed lots of equalisation help.

I’ve built a new one, with a new design! It’s much better. Here’s an mp3 of the previous design alternated with the new design, on octave mandolin – no eq of any kind, no effects, just raw output from the old design vs. the new. (Old design is first.)

While working with the previous attempts, I’d figured out what really improved things was the right kind of pressure on the piezo disc itself. My thumb was pretty optimal, but you can’t exactly do that and play at the same time. The clamp wasn’t bad, but it was slippery and awkward and actually came off on me during rehearsal, so I didn’t trust it. Most piezo-style pickups live under the bridge of an instrument, but you can’t do that the usual way with this one, it’d be destroyed by the pressure.

So I went about trying to fix that.

First was to take the bridge plate and add a wide, flat channel – one wide enough specifically to contain the entirety of the Cortado piezo element. I made it by wrapping sandpaper around a flat piece of metal, and scrubbing back and forth to excavate out the wood I needed removed.

This is actually a new bridge plate.
But it’s made of the same material, so no real diff.
You need to sand away enough wood to make room for the piezo and all the tape wrappings, and some extra. But you do not need to sand away enough for the wires soldered to the disc – you want to avoid those entirely.

Keep sanding away wood until the bridge slides freely over both the new channel and the piezo, like so:

What this makes is basically a wide clamping chamber around the pickup element itself. It doesn’t do any clamping yet; it just creates a space for it. At this stage, in fact, if you hook it up and try it, there’ll no change in sound from the previous version.

(In fact, the “old design” recording I used in the sample is actually this version at this point in the process. I verified that it sounded exactly the same as the previous version, as predicted, which means I’d re-established the old baseline. Important for science!)

But now, of course, I have a clamping chamber! We just need something to apply pressure.

So what’s our clamp? Pieces of paper. Post-it notes, to be specific, just because they were handy. The right number of sheets in this exact case turned out to be four.

Five also worked, and did not feel like too much pressure inserting the papers under the bridge. But it did sound like a bit much compression, tonally, so I went back to four.

(Here’s that sample track again, alternating old design and new, old first.)

The beauty of this is that since it takes several thicknesses of paper, and since that paper be changed out without taking apart the pickup, you can use any number you like. You could even adjust the tone on the fly.

Interestingly, the pickup didn’t even get quieter with more paper. I’d worried about that, but didn’t need to. In fact, adding more sheets made it louder, meaning that the pressure is not so much “damping down treble” as it is pulling up bass. Which, in turn, makes me wonder if it’s not so much “resonating better” as moving the zero/no-vibration point of the crystals’ charge state from all-electrons-in or all-electrons out (doesn’t matter which) to a more middle-range position, which…

…hm. Actually, that’s interesting. No, that’s really interesting. That would explain why the pickup got louder with more clamping, rather than muffled or…

…huh. This is an hypothesis. If I’m right, I can make my next crystal mic substantially more modern sounding, by enclosing the piezo in a small clamping chamber, which is, like this, attached to the resonating disc of the microphone, and possibly…

…possibly I should take my SRMD meds now or I’m going to be up until 5am next Thursday playing with crystals and possibly taking over the moon again, aren’t I? Yeah. I am. Okay. BRB.

So. Yeah! I’m super-pleased with this result. I’m also thinking that maybe this could be used on other items that have flat surfaces which need pickups – like, a piano, maybe – and instead of the bridge, as here, you use a weighted flat bar of some sort across the pickup plate to create the clamping chamber. Then you’re off with tonal control via paper again. I have no need for this functionality at the moment, but it strikes me as legitimate nonetheless.

And most importantly for me, I now have a much more conventional DIY pickup for the octave mandolin. Here y’go, doc – just plug ‘er in, and we’re off.

Much better.

I have some mics that react badly to phantom power, so I made a phantom power blocking took. I had an old Smarties block handy, so I made it out of that.

It seemed appropriate. ^_^

I also went at improving an effects box I built a while ago called the Trash-o-Matic 68000. You would have heard it on Daleks Behaving Badly (Dalek Boy), a joke track that really needs a shorter edit, because it takes way too long to build up.

The best part of this box is the Berthold Ray effect circuit that I legit invented. That must have happened in a pretty heavy Science-Related Memetic Disorder attack (or spark hyperfocus, if you’re a Girl Genius reader), because I was trying to figure out how the hell it worked and I am here to tell you that this is some serious-business Oscillation Overthruster bullshit right here.

It’s basically a multi-store self-reducing sampler feedback effect with frequency shift that’s using the device’s amplifier as a delay loop and sending the amplified samples back to the input via a combination of the internal system ground and negative phase of a balanced signal lead. Both matter. I… don’t entirely know why.

Anyway, the whole thing is noisy as hell, and much of that is the platform I was building onto, and I was hoping to fix that. I was able to reduce noise levels somewhat – no, that’s unfair, meaningfully, it’ll be easier to gate out noise now – but it’s still buzzy as hell.

I’m kind of interested in seeing if I can re-implement the Berthold Rays in a less trashy environment. Sure, it’s fun in this mess of noise and grind and crunch, but it’d be nice to have in a cleaner box as well. Maybe I’d use it more then.

I’ve never used it for music before, but here’s a little bit of noodling I call “Broken Music Box, Found After a Fire,” played on Irish Bouzouki and run through Trash-O-Matic voice 6, with full Berthold Ray attack:

I guess it’s obvious in retrospect, but Monday I learned that a drum machine is basically just an overgrown sequencer. I kind of wonder if the original progression was the opposite way.

This is new to me because I’ve never used one before – all those drum tracks on previous work is microphone on live drums. Timing edited, sure, but live. But now I need a drum kit for a thing and I’m not a kit drummer and don’t have a drum kit, so OFF WE GO.

I’m using Hydrogen. It’s pretty cool so far. And really easy to use and comes with a lot of sampled drums. I wish it had arbitrary label marks so you could make notes on where you are in a song other than by measure count. But maybe you can and I just haven’t found it yet.

aaaaaait has a mixer i just found the mixer i was trying to figure out how i would do this later in ardour and i don’t have to i can do it here 😀 \o/ \o\ /o/

I love discovering new useful tools. Have you discovered any lately?

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.
 
 

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This is part of a collection of posts on building microphones and microphone-related kit, such as mic pre-amps.

A really weird thing is happening with the crystal mic. I do NOT understand this.

The same circuit board taken from another box and put in this box is much noisier. Like, 15-20db noiser. Both boxes are metal or metal-lined, and I’ve checked – repeatedly -the metal lining on this one is grounding.

This is true even with no crystal element attached.

Also, any board put in this box is quieter – less signal. This makes even less sense.

I’m so confused.
 
 

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This is part of a collection of posts on building microphones and microphone-related kit, such as mic pre-amps.

Apparently, I’m really into old-fashioned microphone technologies at the moment, and really, I’m just fine with that. I’ve had this boundary-microphone idea in my head for a while – I even ordered a bunch of parts to build it – and that idea and crystal microphone technologies go together!

Okay, first, crystal microphones were an actual thing. Popular from the mid-1930s through the early 1950s, they were used on-air and in music recording. They’re still in use in certain applications, much like carbon microphones are, but more widely – if you’ve heard of a “piezo buzzer” or “piezo instrument pickup”, that’s exactly the same technology, only applied to a different goal.

The underlying physics: there are crystals which, when flexed, will produce electricity. The charge is positive or negative, depending upon how the flexing is done. Sound waves are enough to do it, which means bing! Microphone technology! This is Neat. And, yes, I have a sample mp3 below.

Hey, that 60s and 70s Skiffy fascination with crystals had to come from somewhere.
(Speak clearly into the pinky ring, Zed.)

It works the other direction, too – current one way will flex the crystal one way, current the other way will flex it back. You can make speakers out of this, and that’s been done. This is also how piezo buzzers work – cycling AC power through a piezo-effect crystal.

I’ve built a couple of piezo-based pickups before, using the Cortado kits, so that seemed like a good place to start. I’m not bothering with a second board-construction write-up; the first one is here, if you’re curious.

But building the carbon microphone driver circuit as an external box made me realise that I should build this mic using an external driver circuit as well, so I can experiment without taking apart the box every time. So I used the housing from a dead laptop power supply I’d recycled a while ago.

The best part was that the AC mains connector slot was almost exactly the right size to hold the XLR connector. I just had to file away a bit at the narrowest points. And, of course, I had to line the whole thing in conductive metal tape, for RF shielding, and ground it.

The Now-Modular Cortado. Piezo lead on the right, XLR to board on the left.

Looks almost professional!

TRS: Tip is outer disc, ring is crystal disc, sleeve is shield ground

Standard balanced XLR mic-level output

This lets me plug in anything crystal or crystal-signal-level-like and use this amplifier on it, just as with the carbon microphone, but for carbon-technology elements. In this case, I’ll be plugging in a piezo disc. But since that’s just the crystal, the real question becomes, what resonates it? What vibrates in the presense of the sound, causing the crystal to flex?

My initial idea for materials involved a lightweight, rigid plastic. I’d also thought briefly about metal, but decided that would be too heavy, and I was right about that. The bad news is, that also turned out to be true for the plastic – it takes too much energy to make it move, so it doesn’t move very much just from soundwaves, and the signal levels were really low.

This is the best I got, using the lightest of the “heavy” plastics. That recording was made talking into a small, clear rigid plastic sheet – I think it’s some sort of acrylic, but I don’t know. It came from Boeing! But does not fly.

I love distant-shortwave-sound of this recording, but that hiss isn’t an added effect – it’s amplifier noise from boosting the signal high enough to hear properly. So, obviously, that won’t work as planned – unless I need exactly this effect, of course.

Still, I’m thinking I could put it in front of a guitar amp or something else VERY LOUD. It’s modular, so there’s no huge reason not to keep it, and I have like 50 of these piezo discs. It also works as a gigantic contact microphone/pickup.

So I started working my way down material weights until I found something too lightweight.

THE HEAVY HEAVY DELRIN SOUND

The thin and tinny base of a styrofoam cup

FAILURES, ALL OF YOU! GET OUT OF MY SIGHT!

More and larger pictures on Flickr, as usual.

The lighter I got, the more response to sound I got, and the more signal – to a point, of course.

It turns out that the best weight is way closer to the styrofoam cup bottom than to any of the plastics I’d hoped would work out. A pair of thin foam dinner plates did actually rather well – I’d thought it was just one thicker plate, but no, it was two plates! – and I’ll try that again with a better (by which I mean actually shielded) test harness pickup, and plates that don’t have divided food sections.

And also, that styrofoam plate with the last 15mm or so of the “cup” still attached worked pretty darned well, without the echoy effect of a “cup” microphone. Some people want that; I am not one of those people. (But again, modular! And I have 50 of these piezo discs, I could make one anyway.)

This gets closer to the original construction materials used in the original crystal microphones, so really, I have no business being surprised here. I was just hoping that with improvements in crystal technologies that a heavier plate would work. But it’s just not generating enough signal output.

And that’s really kind of putting the kibosh on my whole boundary-microphone idea – at least, using this technology. Nothing strong enough to deal with the requirements of a boundary microphone – they’re quite large – is going to react enough to sound to give a decent amount of signal. Unless there’s some unexpectedly light and strong foam.

At least, not with these discs.

Maybe NASA has something I could, you know, appropriate. And I wonder if I can find that crystal material in, oh, one big giant sheet, and stick that to something strong enough. It has to come from somewhere…
 
 

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This is part of a collection of posts on building microphones and microphone-related kit, such as mic pre-amps.

Building microphones is fun and seems to be of interest to readers, so here’s a collection post for posts about that! These posts discuss building both microphones, and, when applicable, their matching microphone driver circuits and/or pre-amplifiers.

Building a Carbon Microphone:

• A Microphone of Constant Sorrow, Part I – I just wanna be able to sing into a can, just like them fellers down at the radio station (19 May 2016)
• A Waveform Sung Into a Can, Part II – test of a driver circuit: version 2 (v. 1 didn’t work – 20 May 2016)
• Building the “Can,” or the a carbon element mount, Part III – wherein I build a shielded containing structure for the carbon pickup element. (23 May 2016)
• Once Upon a Time in a Catalogue, or A Microphone of Constant Sorrow, Part IV – putting the driver circuit into a case, as well as additions to that circuit, and the interface design (25 May 2016)
• Not Just a Display Piece, Part V: a final photo and ideas for use (27 May 2016)

Related posts:

• Understanding Something Better Now – wherein I discuss how building this microphone has made me understand magical technology in 1930s-1960s science fiction.

Building a Crystal Microphone:

• Crystal Gems would use crystal microphones, right? Sure. Yes, I’m building old-technology microphones again (9 June 2016)
• Confused by Background Radiation, wherein nothing makes sense and everything is terrible (11 June 2016)
• It’s 1944 forever: the crystal microphone, part II, wherein it works, and that was not a fait accompli, things got real weird (13 June 2016)

Building a Ribbon Microphone:

• Ribbon Microphone Buildout – it’s surprising now simple a build this is (9 December 2013)
• Calculating the Terminal Velocity of Falling Microphone Ribbon – ruminations on how incredibly thin this material has to be to work (5 December 2013)
• Microphone Preamp Build Report – build the necessary preamp. Also works with dynamic microphones, but cannot receive or send phantom power (2 December 2013)
• Should Have Seen This Coming – a short note about debugging said preamp (4 December 2013)

Building a Large-Diaphragm Condenser Micrphone:

• Let’s build a Micparts RK-47/990B kit microphone! – the complete build report (24 March 2017)
• A Bit of Errata (and something I forgot) – turns out the “complete build report” was only mostly complete; I neglected to mention an interesting build option. Also, I guessed wrong about the design’s “omni” mode behaviour (26 March 2017)
• Let’s listen to a Micparts RK-47/990B kit microphone! – with test recordings and comparisons (27 March 2017)

Other microphone and preamp customisation/modification posts:

• The ART TUBE MP microphone preamp, and Let’s Try That Again with Russian Aid, which is to say, a Russian-made tube of more interesting qualities (March 2016)
• Mod Report: re-componenting an Oktava MC-012/MK-012, wherein Oktava mics have great designs but sometimes iffy components (November 2016)

Laying down timing tracks for the new album, finally. I did four and a half tracks yesterday – this is a quick process. None of these recordings will make it onto the final, but it’s a helpful step.

For the moment, the new microphone is living as a display piece in the corner of the room, on the shelf with all the other microphones. Every time I see it, I consider more places I might actually use it on this album. XD

This whole thing got started by looking up how to fake bullhorn vocals, like I needed for the new single, Pee Police (on Bandcamp, YouTube, and Soundcloud). Universally, people said the best way to do it was use a real bullhorn. I mean yes, I’d wanted to build a carbon microphone for a long time. But in terms of actually doing it, this was the prompt.

If you want a specific sound made by a specific thing, the best way to do it is have that thing. $19 in parts later, I have it!

“Starship on Fire” is so far the most likely track to get some carbon mic vocals in the mix. Since it’s told – sung? – from two different viewpoints at once (past character, present character implied), one of those viewpoints having this kind of effect makes storytelling sense. I will at very least try it, and see how it sounds.

I’m going to use the Korra On the Air icon for all the LJ and Dreamwidth crossposts that mention this microphone forever, aren’t I? Yes. Yes, I am.

It’s a long weekend for a lot of people reading this, so – yay! Go have some fun.

The last parts for the carbon microphone arrived yesterday! And once I got back to the Lair, I set about adding them to the circuit and building out the final version. Here’s a quick sample recording I’ll talk about more later.

I’m starting with the previously-discussed circuit, now taken out of the test harness and reassembled on one of the small breadboards. The new isolation transformer on the left – basically the outputs of the original circuit are attached to one side, and new outputs picked up off the other. This serves two purposes: first, it eliminates some kinds of hum noise if they start to crop up, and second, combined with 2K of resistance on the other side, brings the line-level(ish) output down to microphone level.

(These are standards which matter in a studio and … not many other places. XD )

The zig-zag in the resistors doesn’t serve any function purpose other than fitting into a smaller space – ideally, I guess I wouldn’t’ve been making needed values out of collections of other values? But I had what I had.

Signal flow is right to left in this photo. Underneath, at the top and bottom of the board, I’ve built wire rails to connect the components. That probably means I’m not really using the breadboard entirely as intended? I don’t even know. It holds everything in place and that’s definitely what I intended. XD

To house all this, I’m using an “experimenter’s case,” which is basically old-radio-speak for a metal box. XD It has soft metal on two sides, easily drillable and workable, and a hard case. I’m using one side for input, the power lamp, and the on/off switch; the other side is for output, or, as it turned out, outputs.

The carbon element (in the can) is connected to the driver/amplifier circuit via a 1/4″ TRS phone jack – like an old large headphone jack – with the two leads to the carbon element being on tip and ring, and the shielding ground being on the sleeve. (Tip, Ring, Sleeve: T R S.) That socket is on the left in the above photo; the middle component is a small LED, to indicate power on/off, and the right is a BIG CHUNKY POWER SWITCH. I love big chunky power switches. CHONK

For output, I quickly realised that I could have both balanced XLR output at microphone level, and line-level output on a phone plug, if I could find a way to isolate the chassis ground from the phone socket’s sleeve connector.

Normally, both being grounds on the same circuit, they’re connected automatically. Finding one that isn’t already connected is actively difficult! But careful use of electric tape did the job; I drilled the mounting hole larger than it needed to be, and basically lined anyplace the case and the socket would touch. Isolation achieved!

If you look for the blue and white wires, you can see where the TS (mono) phone plug is tapping the raw (line-level) amplified mic signal, just before it’s fed into the isolation transformer.

The transformer is really pretty optional – powered carbon circuit signals are pretty high as microphone signals go, and as I mentioned above, we’re actually reducing that signal to create the balanced XLR output on the other side of the transformer. But it’s nice to have the option of using line level, since it already exists. That’s what built-in sound inputs like on your laptop want, too, so there’s a point to it.

And here’s the whole driver/amplifier circuit, with a battery holder made of velcro.

Is that cheating? Holding the battery down with velcro, I mean. totally cheating I’m hoping it works out – I didn’t have a 9v battery case and it seemed excessive to try ordering one.

That LED power indicator? It’s warm white, left over from another project. I was planning on putting in your typical red LED, but realised that if they’d had a power indicator on one of these in 1932 or whenever, it most certainly would’ve been a little incandescent bulb, and it may and may not have had a colour lens. So I went with warm white, because period accuracy! Sort of.

The neat thing about the way this circuit works – and all carbon microphone driver circuits work – is how it points you right at vacuum tubes, and from there transistors, conceptually. It really, really does.

See, in tubes and transistors – which are both signal amplifiers – the input signal is used to create an amplified copy by controlling how much raw input power is let through, from another source. That’s why tubes were called “valves” originally; it’s because they are valves, electrically controlled, and regulating the flow of electricity from an input, just like the valve on your faucet controls the flow of water from the plumbing.

In this case, exactly the same thing happens yet again. But the input signal is sound pressure (how loud the sound is), which is controlling how much electricity is let through from the battery. And those changes in sound pressure – and therefore electrical flow – make the electrical copy of the sound waves.

Neat, huh?

Anyway, that’s the inside. Let’s look at the case!

I really like how chunky and primitive it looks. This is an old experimenter’s case; I’ve had a box of random cases in which I can build things for a while, and I don’t even know where I got this one, or when. If you saw it on the set of a 1950s television SF show, nobody would give it a second glance.

Always document your builds! You never know what might confuse people later. And by people, I mean yourself, after you’ve come down from the science-related memetic disorder high. I want at least the theoretical possibility of using this amp with other carbon elements, so writing down how the interface works is pretty important!

Except for the glare from the power light, I think this would be the Radio Shack Catalogue photo from, say, 1975:

Good, Better, or Best? Probably “Good.” It is just carbon, after all!

Or maybe this is the catalogue shot? Not sure.

Finally, here’s a test recording I made, using both outputs (phone/line level and XLR/balanced mic level) at once, hooked up to two different inputs on my board. I put both recordings in the same mp3; one’s on the left channel, the other’s on the right. The two tracks should be pretty much identical – being the same signal picked up at two different places on the board – and I wanted to see if that actually happened. Fortunately, it did!

Well, eventually it did. This is actually the second time I tried this, because the first time, I discovered that I’d managed to wire the two outputs up as electrical inverses of each other. Playing the two tracks back at the same time resulted in massive waveform cancellation. Which was hilarious, but also a good indicator; they wouldn’t’ve cancelled so well if they weren’t really similar. 😀

EXTREME WAVEFORM CLOSEUP

So that’s about it for this project! I’ll most likely do something to the ring to control the elastic better. And I’ll probably build a case for the whole kit, like I’ve done before – mics should have cases for protection! – but that’s a separate project.

This has been such a fun build, you have no idea. If you have any interest in this kind of DIY audio, I totally recommend this as a fun, easy project. Particularly if you don’t have studio gear, because you can look up the line level part of the output to damn near anything (including a PA system, I might mention) and it’ll work.

As always, more and bigger pictures on my Flickr account. And if you’re out of work, that’s a great time to listen to the new (NSFW lyrics) single! It’s awesome.

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.

Result!

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.

Swaaaaank

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.

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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.