Wood And Carbon Rods Used For This Handsome And Effective Microphone

Anyone who was active in the phreaking scene or was even the least bit curious about the phone system back in the Ma Bell days no doubt remembers the carbon capsule microphone in the mouthpiece of many telephone handsets. With carbon granules sandwiched between a diaphragm and a metal plate, they were essentially sound-driven variable resistors, and they worked well enough to be the standard microphone for telephony for decades.

In an attempt to reduce complicated practices to their fundamentals, [Simplifier] has undertaken this surprisingly high-fidelity carbon microphone build that hearkens back to the early days of the telephone. It builds on previous work that was more proof of concept but still impressive. In both builds, the diaphragm of the microphone is a thin piece of wood, at first carved from a single block of softwood, then later improved by attaching a thin piece of pine to a red oak frame. The electrical side of the mic has four carbon rods running from the frame to the center of the diaphragm, where they articulate in a carbon block with small divots dug into it. As the diaphragm vibrates, the block exerts more or less pressure on the rods, varying the current across the mic and reproducing the sound. It works quite well, judging by the video after the break.

Congratulations to [Simplifier] for another great build and top-notch craftsmanship. We’ve seen homebrew vacuum tubes, conductive glass, and solar cells from him before, which sort of makes him the high-tech version of Primitive Technology. We’re looking forward to whatever comes next.

20 thoughts on “Wood And Carbon Rods Used For This Handsome And Effective Microphone

    1. It is purely resistive circuit, so it will require amplifier of some sort. Recording requires preamplifier, eg. one in mixing console or audio interface. What a lovely retro sound!

    2. A carbon capsule microphone is typically connected in series with an impedance matching transformer and a DC current is run through the microphone and the transformer primary. The variations in the current caused by the microphone are transmitted to the secondary of the transformer as an AC signal that can be connected directly into an efficient high-impedance loudspeaker.

      The power you get at the speaker depends on the amount of DC current you pass through the microphone, so it doesn’t necessarily need any amplifiers. There is a limit however in how much DC you can pass through the transformer before its core gets magnetically saturated and stops working.

      The transformer changes the low voltage high current signal into a high voltage low current signal that can travel long distances in a wire. This is how the early telephone systems operated.

    3. Needs a smooth DC supply (a battery is best!) and sometimes an audio transformer to match impedance and then an audio amplifier or speaker with a built-in amp, as normal. Have to say, I sometimes wonder why people make these working decorative retro items, which really have very little practical use – not that I don’t appreciate the craftsmanship! :-)

  1. Back when I was a lad in the 1970’s and early 80’s, the grade school I went to had an old book from the 50’s or 60’s. It was one of those “experiments for boys” sort of books.

    One project was building a telephone system from scratch, including the speaker and microphone, from scrounged parts. The microphone involved carbon rods salvaged from old dry cell batteries, the big cylindrical ones cased in cardboard. What I recall of it was part of the rod had to be sharpened to a fine point on both ends and the ends were held in divots carved into the ends of two short pieces of the rod. It had to be just the right tightness of fit so that sound could vibrate the rod and vary the resistance. Too loose and there’d be no conductance. Too tight and the resistance couldn’t vary.

  2. Carbon powder microphones from old telephones are great for feedback oscillators. Gas density measurement and distance sensing. They seem to have some advantages over the carbon rod type and are also relatively simple to make.

    1. So stress on the carbon changes it’s resistance? How do you make a carbon powder microphone? Stress imparted to the carbon granules as they vibrate changes their resistance?

      1. Exactley. Old sewing machines also used piles of carbon to control the speed, the foot pedal compressd the carbon together and lowered the resistance and sped up the motor. If you ever saw anybody with an old machine it was common practive to step on the foot pedal a bit and spin the crankshaft by hand to get it started. The low voltage did not have much in the way of starting torque so it required a bit of help.

        Carbon mics used to be used for everything back in the day. When you see an old movie and there is a small mic in a big square or circle suspended by springs, it is probably a carbon mic.

      2. Stress is responsible for some change, but the main idea of carbon granule microphones is the increase in contact surface area between the grains. The granules are rough in shape, and forcing them together locks them together, while the opposite stroke of the wave pulls them apart. Occasionally you have to tap the capsule to get them loose again, so the microphone can work.

        The point contact carbon microphone works in a similar way: when the point is pressed in, the contact area increases. It ends up making and breaking contact as the carbon rods are vibrated by the sound.

  3. The way carbon capsule microphones traditionally work, you pass a DC current through a transformer in series with the microphone, and the changes in the DC current are reflected as AC signals out the secondary of the transformer.

    No amplifier is necessarily needed: the driving DC current defines how much power you’ll get out, and a high-impedance loudspeaker can be driven directly off of the AC output. This is the basis of early telephone systems.

      1. Most earphones will have an impedance somewhere between 8 and 32 ohms.

        An impedance of this magnitude is expected by a lot of chipsets which take care of TRRS sockets for audio out on laptops and phones, and is a source of frustration when trying to use 600:600 audio isolation transformers for digital modes and other audio capture/signalling; also, the microphone is typically only detected if it has an impedance in excess of about 1k

  4. I am interested in rebuilding a non-working carbon microphone element. Two questions:
    1) Which activated carbon is more effective, small granules or finely ground carbon dust?
    2) Is voltage from a computer USB port adequate? (About 5 VDC)

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