extremely sensitive speaker for xtal radio?

A crystal radio puts out virtually no power - the whole point of the exercise is that all the power is obtained from the 'source' via the antenna without any local amplification or power source. A matching transformer can be used to optimize impedances.

Usually it's necessary to use earphones to get enough sensitivity. But then only one person can enjoy it. Size is an issue, a crystal radio and it's speaker should ideally sit on a desktop. But bass response can be sacrificed.

I've read of people using 'cheap' PA horn speakers. Is this the best that can be done ?
 
Go buy a plastic funnel & stick it on the earpiece.......simple. OR...take a scope & measure the output at the earpiece.......Take your DVOM & measure the RE of your earpiece. Find a suitable high sensitivity driver( Probably a full-range) , & build an appropriate enclosure.

_____________________________________________________Rick.......
 
Once you measure Re of earphone (probably high Z) buy budget full range 16 ohm 3 in Aura blowout drivers ($2 ea from Madisound) and wire in series to get equivalent Z. Efficiency of drivers increase as more are added, but helps to start with one in at least 85 dB range. Mount drivers in basic open baffle as tight together as possible and build basic front horn for whole assembly out of foam core (square 45 deg) to boost efficiency of array. I am guessing you will need like 320 ohms and a 2:1 transformer will get you to 160 ohms so wire 10 of these drivers in series to get 160 ohms. The efficiency of 10 drivers in series will be in the high 90 dB/W/m. If you need more efficiency, skip the transformer and use 20 drivers in series ($40).

Edit: I think you need 2000 ohms in which case you need an 8:1 transformer and run 16 drivers in series. Alternatively, buy high impedance piezo headphones and mount drivers in a horn.
 
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Crystal sets have traditionally used a crystal earpiece or 4000 ohm headphones.
You'd be better off with a single high-efficiency driver in a properly designed horn and use a tube amp output transformer to match the impedances. It needn't be an expensive transformer, one out of an old tube radio would be fine.
 
One thing we may be missing is the order of magnitude of sensitivity difference here. Earphones are rated in dB/mw and speakers are dB/W a factor of 1000. I don't think you will have enough power even with 115 dB/W speakers. I have in ear balanced armature driver earphones that are very sensitive and they are 115 dB/mw rated. Sorry to burst everyone's bubble but I overlooked this before. Has anyone actually got sound from a xtal radio with a horn and CD? The power carried in the air can't be more than 100 microwatts RF received and put to the voicecoil.
 
IIRC you need the high impedance because the earphone is directly across the the coil, it affects the Q, which affects the reception bandwidth (selectivity).

A bunch of speakers in series sounds good, maybe, but you're going to need more than 320 ohms. I think you need ~20 kOhms.

I believe you can make a detector with black strap iron, but I couldn't get mine to work. I even tried oxidising my own iron. A germanium diode helps.
 
Impedance is critical, you can't afford to load down the tuned circuit - some kind of impedance matching transformer is preferred.

Crystal Radio and Unamplified Speaker - YouTube

How about a crystal earpiece in front of a horn ? - how sensitive are they ?

I found one method but it could be considered a cheat. It's rather clever, I will have find the link again sometime. You use a low power op-amp to driver a small sensitive speaker. The opamp needs power. But an xtal radio is 'not allowed' to have a power source other than the antenna. So what happens is you tune in to a strong station and switch the output from the diode detector to a Supercapacitor. You let it charge overnight and it gives power to the opamp for a limited time during the day. It seems to fall within the spirit of the goal for an xtal radio_Of course, you could wait til it's lightning outside and get a real boost :eek:
 
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...
I believe you can make a detector with black strap iron, but I couldn't get mine to work. I even tried oxidising my own iron. A germanium diode helps.

Old fashioned blue razor blade and a safety pin... both screwed to a block of wood, with the pin bent so that the point rested on the blade. Spend hours moving the point around the blade until you found a spot that worked. Then the slightest vibration and it would stop...
 
One thing we may be missing is the order of magnitude of sensitivity difference here. Earphones are rated in dB/mw and speakers are dB/W a factor of 1000. I don't think you will have enough power even with 115 dB/W speakers. I have in ear balanced armature driver earphones that are very sensitive and they are 115 dB/mw rated. Sorry to burst everyone's bubble but I overlooked this before. Has anyone actually got sound from a xtal radio with a horn and CD? The power carried in the air can't be more than 100 microwatts RF received and put to the voicecoil.

0.1mW into a [email protected] driver?

I make that 75dB.

Seems plenty enough to be audible.
 
Btw, I also recall that the horn speaker crystal receiver had an antenna that was a bit larger than average which I would imagine probably helped improve signal recovery - I recall it was a circular solenoid style coil perhaps 8" diameter and maybe a foot long? This was from about 40 years ago so forgive my lack of exactness.


How about FM crystal receivers?

FM Crystal Radio
 
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