Grampian Carbon Microphone

[rob aylestone]

ebay listing
I am always looking for old collectable microphones, but I saw this listing and smiled.
Back when about 15, I got one of these given to me - and it was a carbon granule microphone. The exactly model is missing from the listing but I'm 99% certain this is the mic because of the grill. Needs a polarising voltage and totally useless to anybody with modern gear. It sounded like an old telephone. Anybody else old enough to remember these?
I've grabbed the image because the ebay listing will go soon.

 

[TalismanRich]

When I got my dad's 1950s ear Wilcox-Gay tape deck, it had a nice brass microphone. I tried plugging it into an amp to see if it still worked. Nothing!

I thought maybe there was a broken wire or something, so I unscrewed the case and found that it was a salt crystal microphone and after 50 years, the salt crystal had completely disintegrated. I thought about trying to replace the capsule, but the elements are pretty expensive now.

 

[Papanate]

ebay listing
I am always looking for old collectable microphones, but I saw this listing and smiled.
Back when about 15, I got one of these given to me - and it was a carbon granule microphone. The exactly model is missing from the listing but I'm 99% certain this is the mic because of the grill. Needs a polarising voltage and totally useless to anybody with modern gear. It sounded like an old telephone. Anybody else old enough to remember these?
I've grabbed the image because the ebay listing will go soon.

I’ve never seen one of those - How does it work? And why won’t it work with modern gear?

 

[rob aylestone]

The thing is two plates separate by carbon granules. It has a voltage across the mic and the resistance varies as one of the plates squishes the tightly packed carbon. telephone technology, used up until the sixties, I believe. Oddly, one of the last uses was in aircraft, where the standard radio interface always had the voltage component.

There's a good article on the Shure site for the history
here.

I'm a bit uncertain this one is carbon. My research says 1935 dynamic, but memory clearly says the black powder falling over my desk was like coal dust. Maybe a different model from these pics, but the front chrome grill sticks in my head? Could be age!

 

[Slouching Raymond]

I had a job interview just after I first got into electronics, and they asked me how a 'granule microphone worked'.
I said I'd heard of it but never encountered one so didn't know. Then they told me to guess how it works.
My guess was wrong, they said, but gave me points for an imaginative guess.

 

[Papanate]

The thing is two plates separate by carbon granules. It has a voltage across the mic and the resistance varies as one of the plates squishes the tightly packed carbon. telephone technology, used up until the sixties, I believe. Oddly, one of the last uses was in aircraft, where the standard radio interface always had the voltage component.

Or so they a Telephone technology - I have seen them before - but not really as a recording mic.

There's a good article on the Shure site for the history
here.

I'm a bit uncertain this one is carbon. My research says 1935 dynamic, but memory clearly says the black powder falling over my desk was like coal dust. Maybe a different model from these pics, but the front chrome grill sticks in my head? Could be age!

I wonder how you would about restoring the mic?

 

[rob aylestone]

I decided that i wasn't going to buy it in case it was the earlier one. I can even remember how it came apart! undo the screws and the casing front pops off, and the housing was just screwed together - more like a watch than a mic. My dad brought it home from work when they had a new 'sound system' - Grampian DP4 microphones for the stage - a little revue venue this must have been around 1970. A holiday camp - people stayed in little shingle covered chalets. Lots of sports during the day, music, dancing and shows at night - food and drink. Totally off topic, but if US folk cannot understand this very British thing - Go to YouTube and search hi-de-hi. you will find a BBC series you will find quant and oddly British - but this series is EXACTLY how it was. Even the characters are so real.

Can't remember the brand of amp, but rotary controls and 4 mics plus a record player input. The connectors for the mics were 3 circuit balanced for the DP4 mics, and the jacks were locking - like the ones we now use on Sennheiser radio packs in 3.5mm, but these were ¼"

The old carbon mics were useless with the new technology. Oddly, I don't remember them sounding bad when sung through? Looking back now, I guess they had the same issues we had today? when you need a new system, everything had to match properly. The mics to amp, and then the 100V out to column speakers!

I suppose the carbon powder would still be available, and there are no copper coils to rewind - so cleaning up the end plates and insulation should be quite possible. There were two individual sockets on the back for little one pin plugs for the cable, a red and a black one. This is 55 years ago at least - amazing what stuck in my mind as a kid!