What Happened to Carbon Microphones? The Granules Inside American Telephones for Decades

Comments · 33

• UT

  @utah133 · 11 days ago

  When I was a kid I did a lot of tinkering. I once made a carbon mic from scratch using broken up pencil leads.

  5

• DI

  @DishNetworkDealerNEO · 11 days ago

  As a young boy, I learned that to get a telephone with the carbon microphone to work better, to take the receiver off the hook, use my other hand  to keep the telephone hung up, and moderately rap the microphone end on the table to unpack the carbon granules and make the microphone more sensitive to pickup your voice!

  3

• TI

  @timothystockman7533 · 13 days ago

  The electret microphone actually required not only the electret backplate, but the field effect transistor. Normal transistors had far too low of input resistance and would completely swamp out the feeble signal from the electret's diaphragm. Electret microphone capsules have an FET built in, and its extremely high gate impedance draws a negligable current from the electret diaphragm, effectively converting the audio to a much lower impedance which can be processed easily. In the late 1970s electret transmitters became widely available, which were drop-in replacements for the carbon transmitters, which could be easily changed in most existing telephones.

  5

• SA

  @samsonperera9872 · 6 days ago

  Thank you.  I used to play arround with them a long time ago.  I still have one removed from an old telephone.

  1

• AM

  @Ampex196 · 12 days ago

  Carbon mics were used in the 1980s and beyond. For speech-quality audio, they were perfect!

  5

• BI

  @bill-2018 · 8 days ago

  I bought a candlestick type carbon microphone in the early 1970&apos;s. It has several patent dates stamped on it, 1901, 1907 and 1908. A metal base and body and bakelite for the mouthpiece. Marked as a T-48.<br>I remember that because we had gone to the new decimal money and it was priced at both 10/- and 50p. It measures something like 90 Ohms on a multimeter.<br>I also have two crystal MIC 45 crystal microphones.<br>G4GHB.

  2

• AR

  @armandoruiz4385 · 10 days ago

  I still have a land line, all the phones in my house are from between 1936 and 1956 and they all work fine, I imagine they have carbon microphones. I also have a VOIP system on my computer (just in case the phone company decides to pull my land line) with a single rotary phone from the 1940s, it also works very well.

  3

• DI

  @DishNetworkDealerNEO · 11 days ago

  The telephone voltage when on hook was between 48-52 Volts. Off hook, between 5 to 7 volt depending on how close the telephone was to the central office equipment.

  2

• MA

  @Mark3ABE · 10 days ago

  The carbon microphone is remarkably efficient at turning speech into electrical signals. However, even with more and more refinements, it finally achieved its maximum efficiency. For longer distances, some improvement was necessary. In Britain, the GPO thought “outside the box”. Since it was impossible to improve the performance of the carbon microphone any further, why not improve the performance of the receiver instead? So, the primitive device invented by Bell all those years ago and used ever since was replaced by a far more refined “balanced armature” receiver. The diaphragm was much thinner and more responsive to tiny signals. It was moved by two coils operating in reverse alignment - as one was pushing, the other was pulling. This remarkable invention enabled the carbon microphone to continue to be used, without amplification, for many years on longer distance lines. In fact, the balanced armature unit was so well made that the first electronic telephones used the same unit both as the microphone and receiver. &nbsp;Two of these units at either end of a long wire would imitate the early Bell system. A telephone &nbsp;which was completely voice powered. Eventually, of course, electronic telephones took over and the life of the carbon microphone came to an end. The GPO provided electronic transmitters which were drop in replacements for a carbon microphone. They were not fitted as a matter of course - only if an Engineer was visiting for some other purpose. There were carbon microphones still in regular use right up to 2000.

  4

• GA

  @gailsmith9835 · 11 days ago

  Carbon microphones are most rugged, simple,and not affected by humidity

  1

• JI

  @jimmyday9536 · 8 days ago

  Born in 1958, I grew up with carbon microphones in the Bell telephones. &nbsp;I never heard of anyone having to tap it, and I sure don&apos;t remember any crackling or hiss.

  1

• TI

  @timothystockman7533 · 13 days ago (edited)

  At least one of the broadcasting pictures you show features the distictive diamond case profile of another of the microphone technologies which replaced carbon: the ribbon microphone. The RCA 44BX may be the most recognizable ribbon microphone.

  4