extremely sensitive speaker for xtal radio? - diyAudio
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Old 16th March 2013, 02:36 AM   #1
Bigun is offline Bigun  Canada
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Default 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 ?
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Old 16th March 2013, 03:24 AM   #2
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The usual procedure is to put the earphone (crystal, you can still get them) in a bowl.
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Old 16th March 2013, 05:28 AM   #3
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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.

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Old 16th March 2013, 09:30 AM   #4
xrk971 is offline xrk971  United States
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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.

Last edited by xrk971; 16th March 2013 at 09:50 AM.
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Old 16th March 2013, 10:13 AM   #5
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I used to use an old telephone magnetic earpiece on mine back in 1958. I was living 4miles from Crystal Palace in London at the time and with my mum's clothes line, as an aerial, got good results. The ear piece is about 300Ohms impedance.
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Old 16th March 2013, 10:18 AM   #6
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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.
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Old 16th March 2013, 11:05 AM   #7
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How about using a compression driver and matching horn? 118 dB per watt sensitive enough? -> Overview
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Old 17th March 2013, 01:39 PM   #8
Bigun is offline Bigun  Canada
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Thanks guys ! --- seems that a horn is still tops when it comes to sensitivity.
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Old 17th March 2013, 09:11 PM   #9
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Subsbribing to this thread! A crystal set that was audible with a speaker would be great! A nice efficient midrange CD? Its never going to be hifi, but thats hardly the point of the exercise. I cant beieve I never tried to use a loudspeaker!
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Old 17th March 2013, 09:35 PM   #10
xrk971 is offline xrk971  United States
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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.
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