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An Accidental radio?

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catcat

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I was breadboard prototyping a reliable amp for my fm radio project. When I made some modifications next day, I heard music from the speaker! I disconnected it from the audio source (which by the way was off!) with the other end in the amp and SIILL I HEARD MUSIC!!! I replugged it and I heard fm distortion. 3rd time nothing. Tried again by touching long wire and telescopic antanna it buzzed with fm distorton upon contact with only 1 terminal. BUT STILL WHAT WAS THE REASON FOR THE MUSIC???!!!
 

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Could I tune it? What type of reciever would it be? (i'm guessing superregen)
 
But there was only 1 station coming thru and I have alot of stations.
 
What you heard was an AM station.
The signal from the station is picked up by the unscreened wiring of your circuit.
Non-linearity in the transistors act like diodes and detect (rectify) the AM signal, resulting in the music which you heard.

This is quite a common effect, try leaving a mobile phone near some cheap computer speakers, every so often you will hear the phone communicating with the base station, whether you are using the phone or not.

If you want to classify such a setup as some type of receiver, I would call it an untuned crystal set.

JimB
 
That's a possible theory, heres what I thought, and also the music was very clear and static free, the music sounded kind of like one of the local fm's.
 

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catcat said:
That's a possible theory, heres what I thought, and also the music was very clear and static free, the music sounded kind of like one of the local fm's.

It's not a regen, it's not a radio, because it does not have a sintonizing (read oscillator) stage.

This effect is very common, you can note that the station that your project picked up is a strong signal radio station.

You body can act as a antenna, once I was working with an amplifier, and every time I touched the amplifier input I could hear some music, very low volume.
 
Hayato said:
It's not a regen, it's not a radio, because it does not have a sintonizing (read oscillator) stage.

Whats a sintonizing stage and is is possible to add it?
 
catcat said:
Whats a sintonizing stage and is is possible to add it?

Well, a sintonizing stage is the tank oscillator circuit (tuning circuit), that resonates in the same frequency of the radio station, producing a strong-enough signal.
The simplest radio is the crystal set. You can receive AM stations. I uploaded a schematic. This is the simplest pratical receiver.
 

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A Google search for "sintonize" brings up a lot of Portuguese results. Hayato, where are you located? Does "sintonize" mean "synchronize" (or "synchronise") in English?
 
Hayato

That tuned circuit is NOT an oscillator, it is just a tuned circuit. There is a big difference.

JimB
 
Ron H said:
A Google search for "sintonize" brings up a lot of Portuguese results. Hayato, where are you located? Does "sintonize" mean "synchronize" (or "synchronise") in English?

Hello,
Yes, we are having some language trouble here. :D
"Sintonizar" in portuguese means to tune. Like when I turn the radio dial to listen to some radio station.

JimB,
I believe that the tune circuit is some sort of primitive oscillator. You have a inductor and a capacitor, they form a LC oscillator, similar to Tesla Coils.

I agree that they do not oscillate by themselves, but if a resonant frequency is "applied"/"supplied" to them, they will start to oscillate, increasing the signal power.

I might be wrong, but...

Sorry for my poor English.
 
Hayato said:
Sorry for my poor English.

No problem with your English (my Portuguese is non-existent), but please fill your location in, then we know you are in Portugal, and presumably don't speak English as your first language.

Anyway, as for the amplifier picking up radio stations, it's a common problem with strong local stations, basically down to poor design and poor construction - there are many components you can add to help prevent this, and layout should be designed to minimise the problem.

As usual, it's a good idea to consult either the ARRL or RSGB handbooks - everyone should have at least one of them!.
 
I decided I want to reinforce that phenomenon and make it a usable radio.
 
catcat said:
Im talking aboum fm superregen here!
A super-regen "radio" detects AM and static, not FM. If it is tuned to the side of an FM station then it detects the FM by "slope detection" because at the side of its tuning the amplitude of the FM changes as its frequency changes. The result is very high distortion and noise.

A super-regen "radio" uses positive feedback to oscillate. Your "amplifier" doesn't have positive feedback.

Your amplifier won't work as drawn. The pic shows a transformer driving the speaker but in the schematic the transformer is missing and the speaker is shorting the battery.
Why does it have two diodes at the battery? Only one is needed to protect the circuit if the battery is accidently connected backwards.

Before making a radio, try making an amplifier that works well. Connect a CD player to your "amplifier" to hear its poor performance then make a real amplifier.

You might as well put the speaker in an enclosure to prevent bass from its rear from cancelling the bass from its front.
 
Hayato said:
JimB,
I believe that the tune circuit is some sort of primitive oscillator. You have a inductor and a capacitor, they form a LC oscillator, similar to Tesla Coils.

I agree that they do not oscillate by themselves, but if a resonant frequency is "applied"/"supplied" to them, they will start to oscillate, increasing the signal power.

I might be wrong, but...

Yes you are TOTALLY wrong!

An inductor and a capacitor can be connected either in series or in parallel to form a tuned circuit.
Suitably connected to an amplifier with feedback, they can form part of an oscillator.


If a coil and capacitor have an AC signal applied to them, at some frequency, where Xl = Xc, the will resonate. They will not increase the signal power, (signal voltage or signal current will be increased, but NOT power).

JimB
 
The transistor is non-linear and when an AM signal is applied to it causes detection, this is the same principle as an crystal radio. I bet you can't recieve and FM stations on it!
 
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