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|mgschwartz||Posted - 9 June 2013 12:12 |
Years ago a found a radio project book, probably published late 50's or 60's that had a crystal radio project that claimed to be powerful enough to power a speaker. If I remember correctly it may have been a Howard W Sams book, but I'm not sure. It had good drawings of the projects and was great for those with less technical knowledge. I would like to find the title of this book, I had read it at the University of Hawaii campus library but I doubt if it would still be in their archives.
Thank you all!
|Richard||Posted - 9 June 2013 20:0 |
You'll probably locate what you're looking for on this site.
|gzimmer||Posted - 9 June 2013 21:36 |
As Richard said, see http://www.crystalradio.net/crystalplans/
#118 "High Power Crystal Set"
#126 "How To Build A Loud Speaker Crystal Receiver"
Both of these are from Magazines. The only book I can think of (for the moment) is "Loud-Speaker Crystal Sets" by Bernard E. Jones 1925, but this one is mostly about using mechanical amplifiers (eg a carbon mike connected to an earpiece).
And be sure to read "Crystal Radio Engineering. Driving Speakers" by K.A. Kuhn
|_J_||Posted - 11 June 2013 14:53 |
I wonder if a "High power" crystal set is a gimmicy, almost a deceptive name. Getting power from a crystal set is mostly a matter of the set getting power from the antenna, using all the familiar things that we apply to any crystal set. Guess Im missing something??
|Richard||Posted - 11 June 2013 15:59 |
"Guess Im missing something??"
Of course you are, John. Don't forget Zero Point and Eloptic energy.
It's everywhere and FREE! ;)
|gzimmer||Posted - 11 June 2013 22:47 |
Because of the trade-off between Sensitivity and Selectivity, a Loudspeaker set needs to be optimised for the former.
So perhaps the term "High power" does make some sense.
Mind you, few of the designs which I have seen actually do this. The emphasis seems to be on double-tuned circuits and voltage-doublers.
A simple loudspeaker set wouldn't even need a tuned circuit, just some impedance matching. And a powerful BC TX close at hand of course.
Edited by - gzimmer on 6/17/2013 10:55:10 PM
|golfguru||Posted - 12 June 2013 0:31 |
One would think that double-tuned would be an automatic volume killer by a couple of dB.
|gzimmer||Posted - 12 June 2013 3:43 |
And thinking a bit more, there are possibly three ways to optimise a set:
- for Selectivity
The last two options are not necessarily the same thing. The least efficient stage is usually the diode detector, hence the need for high impedance to give higher voltage on weak signals.
But with a strong signal the diode will be switching cleanly, so optimum impedance matching will instead be the priority.
Edited by - gzimmer on 6/12/2013 3:59:27 AM
|_J_||Posted - 17 June 2013 12:14 |
Re: "The emphasis seems to be on double-tuned circuits and voltage-doublers."
I agree, but double tuned reduces power to the speaker because it adds losses, and voltage-doublers reduce power to the speaker because they add diodes which waste power and they simply change impedance (in the wrong direction).
As simple as crystal radio is supposed to be, it is unfortunate that many who publish these circuits don't really understand how they work (or fail to work). Still, I love to make them and try them, what can I say?
|homebrew||Posted - 18 June 2013 7:16 |
I also love the circuits and can not keep from trying new ones.
I can not agree with the losses statement at least with headphone circuits. The best so far has been with a 30% increase in voltage across the Benny. Not enough to hear a difference but enough to not be completely disappointed. One voltage doubler bridge set with Benny, run thru a no power amp (amplification depends on signal strength), matching xformer, and small speaker is good enough to impress most non technical friends.
|gzimmer||Posted - 18 June 2013 11:19 |
> The best so far has been with a 30% increase in voltage across the Benny.
Well, a voltage doubler merely changes the load impedance, so if an increase was noticed then that implies that the original set-up wasn't optimum.
Plus with two diodes there will be half the voltage across each, so again theory suggests that the diodes will be less efficient (on weak signals) due to square law effects.
You should able to get more power from a single diode as there will be lower losses.
|homebrew||Posted - 18 June 2013 15:22 |
I've read the in depth argument about available power being split between the two diodes. I can not find fault in the math but my visual image is with a single diode you are using only half of the signal available (half wave rectification)and on a dual rectification or bridge circuit you are using both halves. I can not comprehend visually where the same amount of signal is split between the two diodes when you are only using 1/2 of the signal with a single diode and all of the signal in a bridge detector.
|gzimmer||Posted - 18 June 2013 22:29 |
In a mains power-supply (full wave) the power available on each half-cycle is quite separate, so you can draw twice the current (assuming the transformer can supply it).
However in a crystal set the tuned circuit acts as a strong flywheel. If you draw power on one half cycle, there will be proportionally less power available on the other half cycle. You are limited to the total amount of power that the antenna can harvest.
To put it another way: The tuned circuit forces the resonant current to be a sine wave. You can't clip one half-cycle and still have a sine-wave. If you load one half cycle, the other is automatically loaded by the same amount. The tuned-circuit tightly couples the two half-cycles together.
Its not just a theory. Is easy to demonstrate on the bench with some test equipment.
Edited by - gzimmer on 6/19/2013 12:06:31 AM
|_J_||Posted - 19 June 2013 1:50 |
Very nice explanation, Zim! I will remember that.
And it is definitely not just theory either, it is real, but it takes quite an effort to prove it because it is hard to compare evenly. To do so requires optimal matching of both circuits. When the full wave is better matched, we draw the wrong conclusion.
|gzimmer||Posted - 19 June 2013 3:11 |
Also there are various different factors which confuse the issue:
- Weak signal v/s Strong signal (is it in Square Law mode or not?).
- Single diode v/s Full Wave rectifiers v/s Voltage Doublers or Multipliers...
In particular I would like to try the "Modified Dickson" charge pump
It can be driven single-ended or in push-pull.
You could easily compare the output available from each of the diode taps without having to change the circuit.
Presumably more stages = more voltage but less current available....
|gzimmer||Posted - 19 June 2013 3:25 |
Edited by - gzimmer on 6/19/2013 4:06:02 AM
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