I have old crystal radio high impedance headphones, and I would like to be able to use them to listen to music on my music player as well as to use them on my PC.
What kind of circuit do I have to assemble to make it work with low power DC, if that is even possible?
Have you tried connecting them straight to the headphone port? Depending on the sensitivity of the headphones, they may actually deliver acceptable volume.
If not, you'll need something to raise the voltage. You can do this in so many different ways. One method is to use something with a pro audio level line output - like a mixer. A balanced pro audio line out delivers a signal up to 60Vp-p which should be enough for the headphones.
Another method is to use a small audio transformer. The turns ratio of the transformer depends on the sensitivity of the headphones. You want something with a low impedance primary (8 to 32 ohms) and a high impedance secondary (several kohms or tens of kohms). Normally, good quality audio transformers are expensive. But considering the fidelity of crystal headphones won't be very high, you can just use a cheap transformer.
A third method is to connect the headphones to the output of a normal audio power amplifier. You may be able to connect them to the headphone port, but certainly they will work on the speaker terminals. The more powerful the amplifier the better, because you need a decent voltage. You're not actually delivering any significant power to the headphones, but you do need the voltage that a high power amplifier delivers.
The fourth method is to build your own amplifier around an opamp (5532 or TL072 are good choices). You will need to use a suitable power supply, probably +/- 12V or more. A PC power supply isn't a good choice because they're very low quality and noisy. You'll probably need to build your own supply. You might be able to use two or three 9V batteries. The gain required of the amplifier depends on how high the signal level from your audio devices is, and the sensitivity of the headphones. You might use a pot to adjust the gain, as a starting point I'd suggest you'd need an amplification factor of around 6 times (36dB gain).
They ought to work with no electrical modification (amplifiers or what have you.)
Attach them to a standard headphone plug and try it out. You should be able to rewire them for stereo pretty easily.
Don't expect any kind of overwhelming sound quality.
They were built to the requirements of the equipment in use way back when. Working with almost no power was a must. Hifi audio wasn't.
Considerations:
I believe this is a copy of another question, or perhaps - the answer to another question also answers this one. See the following question answered by @wbeaty: stack exchange
Answer:
Essentially @wbeaty suggests using a transformer in series with the headphone to alter the voltage and current to match newer devices. The idea here is that just as a transformer can be used to transform voltage and current, it can be used to "emulate" impedance such that a circuit see's a different impedance than actually is present (note this is done in transmission and distribution as well).
Calculations:
You would look into the impedance of your current head phones and then compare it to that required by an 8mm headphone jack and size the transformer accordingly.
$$ Zs = Zp * ({{N2} \over {N1}})^2 $$ $$ Zp = {{Zs} \over ({{N2} \over {N1}})^2} $$
Where N2 and N1 is the secondary and primary transformer winding turns. Zs and Zp are the secondary and primary impedances. However you choose to pick your transformer and primary or secondary, you want to make sure that the side connected to the new jacks is the smaller impedance side.
Keep your old headphones .They are valuable now .Using a step up transformer for each ear is fine ,others have stated this .Your old phones are much much more efficient than modern phones .When acoustic transducers are designed there are compromises between efficiency and bandwidth.New phones are supposed to do the full audio range .These old phones are designed for clear speech when receiving noisy signals .I found in 1975 that 1 volt of audio ballparked on an Analog meter from a simple ZN414 circuit with a basic BC107 single ended amp provided more than enough volume for my 13 year old ears.It was said then that 1mW was enough to drive the old phones .There were many simple single ended transistor circuits running on low battery voltages that would drive such phones but would never drive a speaker .If your phones are anything like what I remember in junk boxes in the 1970s then a direct connection should work .
