I have a problem wit selecting headphones for foxhole radio. i cannot find any crystal headphones near any store. How can I use diaphragm type speaker headphone?

 

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You placed you post in this thread - http://forum.allaboutcircuits.com/showthread.php?t=52766&page=2

That can be confusing, so now you have your own.

 

Hi, vijayram. Welcome to AAC
In order to use a diaphragm type headphone, you would need a pre-amp and an amplifier to increase the power of the signal. However, I have also heard that the old-style telephone receiver is sensitive enough by itself. Not sure if this is something you'll want to explore, but it would save you a lot of additional circuitry.
Good luck!
Der Strom

 

At the time when crystal sets were first used, sensitive and relatively high-impedance (2kΩ upwards) diaphragm-type headphones were made which were entirely suitable for the purpose. These things are sometimes available second-hand, although of course they are subject to deterioration due to their age, and may not work. A set in good working order may be costly, as what you are buying is approaching antique status. Here is a link to a website offering old headphones for sale. Please note that I make no recommendation of this vendor, it is just an example. http://oldheadphones.com/crystal/phones/phones.htm

If you want the last word in sensitivity, the type of receiver described as "sound powered", utilising a "balanced armature" system is better. These however generally have a poorer frequency response, and are not so often available with the ideal high impedance: a matching transformer may be used, but the loss will somewhat offset the advantage.

An receiver from an old telephone is a possible way forward, but the impedance will be too low and will need matching with a transformer. The frequency response may be pretty disappointing too, as generally telephones only need to respond between about 300Hz and 3.6kHz.

Some good high-impedance moving coil headphones may work quite well, at least if you have a strong signal. Not the little things sold with MP3 players etc, which are generally very low impedance and not very sensitive, but something like the Sennheiser HD414 headphones I had when I could still hear.

If you are desperate, get hold of the best low-impedance moving coil phones you can find. A pair with relatively large diameter drivers may be somewhat more sensitive. They will probably be about 32 ohms impedance, so you will need obtain a matching transformer of about 8:1 ratio (NB impedance goes as ratio squared). This is likely to require a really good signal for usable results.

Finally, you need to be sure that your crystal radio is actually working before blaming lack of success on not having some "magical" earphones. Try the set with a sensitive audio amplifier, but remember to load it into a resistor of say 2.2kΩ in place of the headphones, because otherwise the DC from the diode may have nowhere to go, which would stop it working. A bypass capacitor of about 100nF across he headphone terminals is also a must, as the amp. may not appreciate being presented with a lot of residual RF. Good luck!

 

I built a few crystal radios and had good results with the ceramic, high sensitivity type earpieces.
Unfortunately you usually have to buy them online or on ebay.
As derstrom pointed out, the old sound powered headphones, also work, I got ahold of some from an old submarine off of ebay.

 

I have a problem wit selecting headphones for foxhole radio. i cannot find any crystal headphones near any store. How can I use diaphragm type speaker headphone?

Here ya go!
http://www.midnightscience.com/catalog5.html#part9


I had a great chat with the Xtal society folks at the Dayton Hamvention. Crystal radio construction is getting BIG again! Cool!

Eric