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Howdy folks. I've been really interested in building a simple AM radio for a long time, but I've always wanted to understand it rather than just put together something that somebody else designed. I'm particularly interested in eventually making one which can use a battery, and not your typical crystal set which requires a long antenna and earth ground. Problem is, things I find on the internet tend to go from simplistic crystal sets to complicated amplified ones (regenerative, super regenerative, super heterodyne, etc) without much explanation on what does what. I'm no electronics guru by any respect, so I get lost very quickly. Maybe one day I'll be able to look at these things and they'll make total sense, but not today.

So far, I have an understanding of envelope detection, and I recently grasped basically how a diode detector would work (smoothing out the peaks of the carrier to form a smooth audio wave). But I can't seem to find out exactly how one comes to the conclusion of what values of capacitor and resistor one should use in the detector. I believe I read that you choose a capacitor based on the maximum amplitude of the waveform, but how would I go about finding out that value, even? And then with that, calculating the right cap/resistor?

Wikipedia has some formulas for calculating something, but it's all greek to me. If I knew what the symbols referenced, I might have some chance of figuring it out, at least.

Anyway, I would appreciate any info or reading that anyone can offer to point a newbie like me in the right direction!

• posted

=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D Think about sending a tone... Say 1khz. We know that the carrier must be modulated (multiplied) by a modulating signal that goes from o% to

100%... so make the carrier 1mhz and make the modulation a 1khz square wave. This would look like 500usec of 1mhz on, 500usec of nothing, Match this to something that will drive an antenna. Should be able to hear it on an AM receiver,
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Yeah but . . . That's what you get when you try to go over 100% modulation and it causes adjacent channel interference.

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You don't really state a goal here. Is your aim to learn how to build a superhet or learn some of the intermediate versions of AM detection first.

A single tube/transistor regen or super regen or reflex receiver is only a step up from a crystal set and isn't way more complicated like you seem to indicate.

If you wanted simple you go from crystal detector to amplified crystal detector, then to tuned radio frequency (amplify the RF with 1-3 tuned stages before you go into the detector) then on to regen, reflex, superhet (which you can get on a single chip these days with minimal external parts).

Check out:

Here's a simple single transistor regen (leave out everything east of the 4.7 UF cap and connect earphones from the cap to ground - or add the amp and drive a loudspeaker).

Not significantly more complicated (if you leave out the audio amp stage - again). And some good explanations.

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Oops - back in the day, we had earphones with a mile or two of very fine wire in them with a 2K impedance - today's 8-40 ohm "ear buds" are probably not going to cut it - a crystal earphone may still be available, and work however.

I used to use tube matching (plate) transformers to drive low impedance earphones and speakers but even those are scarce today. Closest thing in production today are public address line matching transformers (attached to the public address speakers throughout a building to lower the 25 or 70 volt signal to something an 8 ohm speaker can deal with)

Bite the bullet and build an audio amp (or buy one at Radio Shack) - then experiment with "front ends" would be my recommendation.

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Yes. I dearly remember those headsets. I actually built one that kind of worked okay, too. But even Radio Shack had the old style earphones. These days, I suspect that they set you back an arm and a leg when you find them.

Jon

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I know what you mean. A lot of neat stuff goes obsolete.

Found on ebay:

Old dual coil Cannon earphones. Looks older than the one I had. Mine had chromed end caps with stamped manufacturer's data on them.

Only \$12. uses the old pin terminals for fahnestock clips

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The transformer from an old mobile phone charger or wall wart 240v to

3.4v makes a reasonable audio transformer.
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Best Regards:
Baron.```
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Never occurred to me. I guess at the levels we are talking about there's not much chance of ever saturating. Audio transformers had all the E's on one side I's on the other with a paper spacer - mains transformers use interleaved lamination stacks.

• posted

The phrase "Well stocked junk box" springs to mind !

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Best Regards:
Baron.```

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