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Author Topic: Low audio output from crystal set earphone, any suggestions?  (Read 4388 times)
KU4UV
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Posts: 376




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« on: May 29, 2011, 06:21:16 PM »

Hello everyone,

I have recently constructed a homebrew crystal radio set and am having difficulty getting enough volume out of the earpiece.  The earphone that I am using was ordered from Mouser.  It is a Kobitone earphone, part Number 25CR060.  The earphone has a rating of 20 Meg Ohms according to the data sheet on the web page.  I know that I have a good radio signal from the crystal set.  I am using an outside long wire about 100 feet long.  I have an external amplified Radio Shack speaker that I have hooked up to the crystal set, and I am able to receive one of our local stations with no problem.  When I disconnect the amplified speaker and hook up the Piezo earphone that I purchased from Mouser, the signal is almost unnoticable, i.e. very low audio.  I know a crystal set isn't going to have blaring volume coming out of the earphone, but I can barely hear any signal at all through the earphone.  I remember building crystal sets about 24 years ago when I was in 7th grade using an almost identical earphone, and the signals were quite loud, especially at night when our local 1,000 Watt AMer (and a former employer of mine Smiley ) would sign off at midnight and the shortwave stations would come booming in.  I can remember listenig to Radio Canada and the BBC on one of the Science Fair crystal radio kits that I built that Radio Shack used to sell.  Granted, my hearing was a lot better back in 1987 when I was 12 years old, but it just seems like the signal coming from the earphone should be MUCH louder than what I am hearing.  The signal is barely audible through the earphone right now.  I have tried adding the 47K Ohm resistor thinking that would help, but no luck.   I don't know if the earphones are now manufactured with different materials than in the past and maybe they are not as sensitivwe as they once were?  Can anyone that builds crystal sets offer any advice, tips or suggestions, or recommend  a good set of headphones or an earphone for crystal sets?  Thanks for the help!   

73,
Michael Brown KU4UV
ku4uv@arrl.net
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W5FYI
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« Reply #1 on: May 29, 2011, 07:03:40 PM »

Try paralleling the earpiece with a .01 to .1 uF disc capacitor and see if that helps. GL
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WB6BYU
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« Reply #2 on: May 29, 2011, 07:22:37 PM »

Did you put the resistor in series with the headphone or from the diode to ground?

The crystal earpieces don't pass DC the way a set of magnetic 'phones do.  You need
a DC return for the diode to work properly - hence connecting a resistor across the
headphones.  You might have to experiment with the value.

I have some similar Kobitone earphones, one of which I use with a crystal set for
121.5 MHz.  I don't think they are as sensitive as the ones I remember as a kid, but
it is difficult to compare given the difference in applications.
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KE3WD
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« Reply #3 on: May 30, 2011, 07:36:54 AM »

Test a high impedance crystal headphone by simply scraping its two wires together and listening to hear that in the headphone.  No battery needed.  If you hear the static of the wire scrape, suspect something else in the circuit. 

The resistance issue as mentioned above should be investigated further.

Also the bypassing of the headphone with cap may be the issue.


73
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W0BTU
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« Reply #4 on: May 30, 2011, 03:01:58 PM »

Do you have a schematic? I know a ham who's an expert on crystal radios. If you can link to a drawing somewhere, I'll ask him.

It is amazing listening to him talk about how different crystal circuits work. There's much more to it than this, but how you connect the earphone can make all the difference in the world.
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KU4UV
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« Reply #5 on: May 30, 2011, 03:14:48 PM »

The schematic I am using is simply one that I got off of the Internet.  It is a pretty basic circuit really, with just a 120 turn coil of enambled wire on a 1.5 inch PVC pipe.  I am able to hear "clicks" in the earphone whenever I rub the two lead wires together, however the audio is still almost undetectable, even after adding a .01 and .1uF disc capacitor and 47K Ohm resistor.  I am going to play around with the circuit a little tonight and see if the audio output on the earphone improves any.  I am open to any suggestions at this point.  I will stay in touch,  Thanks guys!
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WB6BYU
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« Reply #6 on: May 30, 2011, 04:18:17 PM »

I presume you are using a germanium diode rather than silicon?  A bit of forward bias might help
as well.
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W8AAZ
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« Reply #7 on: May 30, 2011, 05:42:41 PM »

Got a really good ground for that?  That can surely help. I use the antique type magnetic phones.  Baldwins, of which I have several sets.  BUt they are known to be about the most sensetive sets you can get easily.  I would think that crystal sets would be just about as good if hooked up correctly.  Diodes can make a diff like the man said.  Either Germanium or maybe some of the more exotic signal types for microwave work. 
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W0BTU
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« Reply #8 on: May 30, 2011, 06:52:40 PM »

The schematic I am using is simply one that I got off of the Internet.  It is a pretty basic circuit really, with just a 120 turn coil of enambled wire on a 1.5 inch PVC pipe.

Just a single coil? I'm not saying a single coil won't work, but the better crystal radio circuits have separate, coupled coils. Here's one example: http://www.w0btu.com/files/Crystal_Radio/AE0KU/

There's some more stuff at http://www.w0btu.com/files/Crystal_Radio/. I forget where I got it all, but maybe you'll get some ideas there. Click on the html files there.
« Last Edit: May 30, 2011, 07:03:21 PM by W0BTU » Logged

WB6BYU
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« Reply #9 on: May 30, 2011, 07:56:08 PM »

The Kobitone crystal earphone that I got wasn't as good as the Trimm Featherweight
units that I found at a hamfest (but which have since gone open circuit.)  They were
about the best that I had found of the old fashioned ones.  Probably the most
sensitive are the units out of the old military "sound-powered" telephone systems
(though I also had a pair of military crystal headphones that were so sensitive I
couldn't turn the gain DOWN far enough on my radio!)
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KU4UV
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« Reply #10 on: May 30, 2011, 09:02:25 PM »

Thanks guys, I will try some of the tips and suggestions!

73,
Mike KU4UV
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N0SYA
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« Reply #11 on: May 31, 2011, 09:02:42 AM »

Hi
Would it be sacrilege to use an audio amp?
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If you have a clumsy child, you make them wear a helmet. If you have death prone children, you keep a few clones of them in your lab.
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