audio amp crystal radio

Audioguru

Joined Dec 20, 2007
11,251
I think the circuit will work only if the transistor used has a high hFE so that it is saturated with no signal. When there is an AM radio station received then when loud the negative excursions of the high amplitude radio waves reduces the base bias which causes the collector to use less current in the 10k resistor producing a positive output swing.

Crystal radios were used a long time ago when there was only one or two radio stations around you. Not much interference.Today it would pickup maybe 10 or 20 AM radio stations at the same time because it has only a single LC tuned circuit. A real AM radio has six or more LC tuned circuits so that it rejects interference from other station frequencies.
 

danadak

Joined Mar 10, 2018
3,712
Still a lot of interest in Crystal Radios and circuit variants. Google "crystal radio groups",
a lot of activity.


Regards, Dana.
 

Thread Starter

Zeeus

Joined Apr 17, 2019
481
Made this, it works!! (copper clad)..think understand a little on it now..made transistor amplifier for it

C3 is not needed it seems..what's not needed? R7 variable 1k and C2 variable

Same as the books' but added Q3 and Q4, Q4 to amplify the signal more before sending it into the distortion transistor...since Q4 then no need for Q2?

Also, don't really understand how the power detector works (but it has to distort the signal : clip at some point ) : this is mixer? or unmixer?

..it clips the signal but the RF signal is still available..Normal crystal radio with diode has capacitor later to extract the audio (correct?)..what is extracting the audio in this case? what is c10 for?

Also, any way to "test" on spice? what to test?
Thanks
 

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BobTPH

Joined Jun 5, 2013
2,136
Today it would pickup maybe 10 or 20 AM radio stations at the same time because it has only a single LC tuned circuit.
I have a “Red Sox Radio” I built to pick up only a single station, the one that broadcasts Red Sox games, AM 1440. It has one tuned circuit made by the ferrite rod antenna plus a fixed capacitor and trimmer to fine adjust the tuning. The tank circuit is input to an MK414 AM radio chip, which has an untuned RF amp, AGC and detector. It works fine, discriminating the channel quite well in in area with plenty if AM stations.

Bob
 

Thread Starter

Zeeus

Joined Apr 17, 2019
481
Hello,

Here is a website dedicated to crystal receivers:
http://www.crystalradio.net/

Bertus
Thanks for this one : https://www.qsl.net/va3iul/Homebrew_RF_Circuit_Design_Ideas/Homebrew_RF_Circuit_Design_Ideas.htm#RF Power Amplifiers

are the circuits compiled into a book?

watching the signal on scope from antenna, there is 60hz signal (noise)...what causes it? and can use diff amp to remove it?? : can't attach image unfortunately

Miss this site :( need to get back to study

Please assist TS
 

danadak

Joined Mar 10, 2018
3,712
You can remove 60 Hz with a notch filter, differential subtraction, high pass filter,
a number of approaches to experiment with.

Huge store of documents at https://archive.org/index.php.

Huge electronic magazine store at https://www.americanradiohistory.com/index.htm


Notch filters -

http://www.ti.com/lit/an/sbfa012/sbfa012.pdf

http://www.seas.upenn.edu/~belab/LabProjects/1998/BE309F98M1R01.doc


Or a twin T notch filter (passive filter) -

http://sim.okawa-denshi.jp/en/TwinTCRkeisan.htm

Using 1 uF caps (graph attached) -



Regards, Dana.
 

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