a mic of constant sorrow
- May 19th, 2016
- Posted in diy . studio
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I’ve been trying to build a carbon microphone. Why? Well, partly, because I can, and partly, because if you want that “telephone” sort of sound, the best way to do it is to use a telephone element rather than fake it later (see also: BULLHORN), and partly because I want to be able to sing into a can, like them fellers at the radio station.
A Microphone of Constant Sorrow
And this should be – electrically, at least – very simple. Small power supply – battery is fine – resistor or two, capacitor. Done. Very simple circuit.
But it isn’t working, and I have absolutely no idea why, and I’m highly frustrated. I’m going to try a higher-voltage circuit, which I’ve been avoiding for no good reason other than it shouldn’t help, but I’m starting to think the carbon element I got new-old-stock off eBay isn’t up to snuff.
Anybody else built one of these monsters before? I have actually managed to get extremely-low-volume recordings out of it a couple of times – far too low level to be useful, I’m afraid – so I don’t think the element is actually dead. But honestly, I have no idea.
eta: GUESS WHO HAS A WAVEFORM 😀
A WAV of Constant Sorrow
2 comments on Dreamwidth, 5 comments on Tsu.
Check your email, I sent you a message about the mic element!
http://www.modernbluesharmonica.com/board/board_topic/5560960/465298.htm
Ahh, see, he says in comments he switched out for a Shure CM element (transmitter, in carbon terms) at 3:50, which – while still from what I can read elsewhere is a very good blues harmonica pickup – is a magnetic technology, similar to a dynamic microphone. That’s why he doesn’t have any power source – he doesn’t need one with that class of pickup.
Carbon transmitters, by contrast, are basically variable resistors. And as such, need a power supply – they are in general completely different beasts, technologically speaking.
GUESS WHO HAS A WAVEFORM? 😀
I see what you did there….
Old telephone carbon mikes need about 80V from memory
Oh no. Thankfully. That’s ringer voltage.
Line level transmitted from the station was standard at 48v for carrier operation. But being lossy as fuck (for a lot of reasons), the base of the telephone (with the amplification, pulse generators, all that) had to be able to function at single-digit voltages, and a variety of people have had _success_ with 1.5v power supplies if only using the handset.
The reason I’m thinking of moving up to 9v is that from some sources I’ve been reading, 1.5v is borderline on activating carbon mics. I’m not sure why that would be true as they are variable resistors more than anything else, _but_ since people are also reporting success with this 9v-driven circuit, I’m willing to give it a go.
(This is North America Bell System; other systems would use higher voltage, sometimes 60, sometimes 80, I don’t know but I saw those numbers being discussed for non-Bell POTS.)
http://www.instructables.com/id/Telephone-Handset-Microphone/ may help
Yes, that’s the circuit that isn’t working.
Which is so frustrating because it’s nice and simple!
Try a non-electrolytic capacitor just in case it helps. You should be able to get up to around 2.2uF pretty easily, although not as cheaply of course
GUESS WHO HAS A WAVEFORM
THAT’S RIGHT
THIS SUPERVILLAIN THAT’S WHO 😀
Woohoo! Now, is it the CORRECT waveform – I once thought I’d fixed a connection and accidentally created a short-wave radio pickup instead.
It is! It’s got a bit of hum to it, but that’s not real surprising in open air and also I touched the carbon mic’s housing and it doesn’t like that (I’ll isolate it of course later) and I’ll be building in an isolation transformer when that gets here (eBay) so that will also help.