Building an AM receiver

[FadysKool]

Hello everybody,

I'm trying to build a simple AM receiver (FM is too complicated for me at this point) with no success so far. I've tried messing around with pretty much simple circuits,

**broken link removed**

Yet I still get no sound from the speaker.

I'm wondering what values do I need for the variable capacitor / inductor.

On another side, is it possible to use some other type of oscillator for the tuning circuit? RC or crystal maybe?

Any help would be greatly appreciated.

 

[audioguru]

You have a very simple "crystal radio" with the diode as the crystal. Look it up on Google for capacitor and inductor values.
The antenna must be very long, the ground must connect to a cold water pipe earth, the stations must be strong and local and the earphones must be high impedance. It doesn't have a power amplifier so it can't drive a speaker.

It is much too simple to have an oscillator. It is just a single tuned circuit (real radios have many) and an AM detector diode.
Oh yeah, use a fast diode, not a 1N400x rectifier that is too slow.

 

[rigdoctor99]

Crystal radio

OK try this one........

 

[FadysKool]

So I guess going for a superheterodyne receiver would be a better idea.

Thank you alot, audioguru.

 

[audioguru]

So I guess going for a superheterodyne receiver would be a better idea.

People made simple crystal radios 70 years ago when there were hardly any stations so interference between them wasn't a problem. My city today has about 20 AM stations and there are many more all around, so I think a simple crystal radio will pickup most of them at the same time.

A super-heterodyne radio has many tuned circuits for good selectivity. It also has amplification for good sensitivity and doesn't need a long antenna and earth connection. It also has a power amplifier to drive its speaker.

 

[Tarsil]

So I guess going for a superheterodyne receiver would be a better idea.

Thank you alot, audioguru.

A superhet is a bit complicated...why not use a specialised chip?
https://www.st.com/stonline/products/literature/ds/10574/tda7513t.htm
Won't learn as much but it's easier.

 

[audioguru]

The TDA7513 is a nice radio IC.
It has only 80 pins to connect.
It might take you a week to program it.
Its 59 pages of datasheet doesn't show a typical circuit.

 

[stevez]

Somewhere at home I have a magazine article that describes a vacuum tube crystal radio. It uses a common vacuum tube with a 6 volt AC supply for the filament and 10 or more of the low cost 9 volt batteries in series for the high voltage supply. The vacuum tube is used as an RF amplifier so that the voltage going to the diode (that serves as a detector) operates in a more linear range. The author said that this would provide some improvement in fidelity. The RF amp might help to over come some lack of signal strength but would do little for selectivty. The learning experience would seem valuable. Be sure to heed advice/information offered by audioguru as it is important.

Send me a note if you would like me to look for the article. I can scan it and send it to you via email.

 

[audioguru]

I don't even want to think about a lousy super-regen, but I remember a radio that has an RF amp then the signal is detected and the same amplifier stage is also used as an audio amplifier. What was it called?

 

[Nigel Goodwin]

I don't even want to think about a lousy super-regen, but I remember a radio that has an RF amp then the signal is detected and the same amplifier stage is also used as an audio amplifier. What was it called?

Wasn't it called a reflex receiver? - back when transistors were VERY expensive it made sense to use it twice!.

Essentially the tranistor first works as an RF amplifier, it then feeds a germanium diode as a detector, the resultant audio is then sent back through the first transistor using it as an audio amplifier. By using an RF choke and a few capacitors this works remarkably well.

 

[audioguru]

Thanks, Nigel. Yes it was called a REFLEX circuit.

 

[Tarsil]

OK. U cought me! I was mean and sarcastic. Had a bad day.

 

[audioguru]

OK. U cought me! I was mean and sarcastic. Had a bad day.

Thanks for sharing info about the very nice radio IC. You were not mean and sarcastic.

 

[Hero999]

If you want a simple cheap and chearful AM radio then use an MK484 IC. You can find plenty of circuits that use it from Google.

 

[FadysKool]

Thank you a real lot for your help !

I'm going thru this whole thing mainly to learn, but trying a radio IC sounds like a good idea. I hope I can find these here.

Somewhere at home I have a magazine article that describes a vacuum tube crystal radio. It uses a common vacuum tube with a 6 volt AC supply for the filament and 10 or more of the low cost 9 volt batteries in series for the high voltage supply. The vacuum tube is used as an RF amplifier so that the voltage going to the diode (that serves as a detector) operates in a more linear range. The author said that this would provide some improvement in fidelity. The RF amp might help to over come some lack of signal strength but would do little for selectivty. The learning experience would seem valuable. Be sure to heed advice/information offered by audioguru as it is important.

Well I got this book from a friend which has several detailed receiver circuits (including superheterodyne, along with the required parts/values, etc). It sure looks complicated but I'm gonna give it a shot ..

OK try this one........

Thank you alot rigdoctor99. I'm just wondering why many circuits require low impedence headphones .. Couldn't some AF amp help out?

I try my best at not being a newb. But sometimes you just have to.

 

[Nigel Goodwin]

Thank you alot rigdoctor99. I'm just wondering why many circuits require low impedence headphones .. Couldn't some AF amp help out?

I think you mean HIGH impedance headphones?, it's simply because it avoids the need for an audio amplifier - historically 2000 ohm headphones were quite common, and you can use piezo ear pieces as well.

 

[Hero999]

Normal piezo transducers will work, make your own high impedance headphone.