I’ve received a very old pair of mono bakelite headphones that I would like to re-use for demonstration purposes. Just for voice audio and morse code tones.

The writings on the side of each headphone:

SB & CO 

This leads me to believe it’s a C-type unit (whatever that means) with Low Resistance (Impedance) from the year 1939. The DC resistance I measured is around 62 Ohms for each headphone and there are no shorts or open circuits. The two earpieces are connected in series and each earpiece has two coils below a thin metal disc.

inside closeup of headphone coils

Image 1: inside closeup of headphone coils (disc removed)


Image 2: wiring

What would be the ideal or maximum signal levels can I drive these headphones with, and what sort of pre-amplifier would I need to make these headphones produce the best possible (probably still awful) sound when driven from a laptop or radio's headphone socket?

Is this where I need to dig up and replicate an old, valve radio's audio output stage? I am hoping to do this with slightly more modern components..

  • \$\begingroup\$ These headphones should be compatible with any device that can drive ear-buds, or drive a loudspeaker. Except that hard bakelite against your ears can be painful after awhile. Be aware that the small hole in the bakelite cap forms an audio resonant cavity that probably has a peak below 1 kHz. Not ideal for music, but quite acceptable for voice. \$\endgroup\$ – glen_geek Nov 13 '17 at 16:57
  • \$\begingroup\$ I had no idea that these headphones would be so sensitive. I will hook up some leads and a connector and try it out. It's definitely not for long-term listening but the headphones look in a reasonably good state and I'm just very intrigued as to how they might sound almost 80 years later.. \$\endgroup\$ – captcha Nov 13 '17 at 21:32
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    \$\begingroup\$ As a side note, they can function as a mic too. Also as a sensitive magnetic pickup by removing the steel diaphragm and placing them against a magnetic surface and amplifying the output. That reduces most feedback, allowing very high amplification. I used a pair long ago on an old metal fire safe. \$\endgroup\$ – Optionparty Nov 14 '17 at 3:36
  • \$\begingroup\$ Hehe, the possibilities are endless! Thanks everyone for their excellent feedback. I've accepted Bimpelrekkie's answer, who not only provided a lot of additional information but also responded to my question with 10 minutes of my asking. SE is awesome. \$\endgroup\$ – captcha Nov 14 '17 at 9:11

Modern headphones are often 30 ohms per channel, some high-end ones can be 600 ohms per channel. So this 62 ohms falls within that range so I do not see why you could not drive them with almost any headphone output.

The maximum signal will very likely be determined by the distortion the headphones introduce. Having said that, most portable devices with a battery supply cannot deliver much power in a 124 ohm (62 ohm + 62 ohm in series) load since that requires a higher voltage.

If that is problematic you could improve this by connecting the two drivers in parallel instead of in series like they are now.

I would not put too much effort in an amplifier for these headphones until you've tested if they're worth it with decent equipment. So I would simply test them with a standard headphone socket output from a decent "Hi-Fi" amplifier. Amplifiers that can drive proper speakers are capable of generating high enough voltages to drive this headphone beyond what it can handle without too much distortion.

If you really need to build something then I suggest one of the headphone amplifier modules you can get on eBay, here's an example.

  • \$\begingroup\$ That is very encouraging to hear. If the headphones are still producing good audio it would make demonstrating them on laptops and radios also that much easier. I'll try it out later today. \$\endgroup\$ – captcha Nov 13 '17 at 21:49
  • \$\begingroup\$ Wow, that worked beautifully! I tried them in series as per the original wiring and got very good sound. Even the drive levels of a small mp3 player (one channel to both headphones) was more than sufficient for normal playback. Maybe not as loud, and definitely hollow-sounding, but quite amazing for something that old. Listening to CW on 40m was quite the experience.. \$\endgroup\$ – captcha Nov 14 '17 at 9:19

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