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> Lm386 And Radio Reception Questions
Mackhack
Posted: March 05, 2010 06:36 am
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Hi @ all,

I just have 3 simple question which I want to understand. I used a LM386 and tried out the 200 gain setup. Works perfect on the breadboard.

1) When I connect a source like my Blackberry or iPod and I don't play any music I hear a radio station. Is there a way to calculate the theoretical frequency this circuit receives?

2) Why does the reception of the radio station becomes better when I touch the 9V battery with my hand?

3) Why do I really get good reception when I pull the 2 wires apart which come from the circuit to the 3.5mm jack that goes into the source?

Thank you!


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kellys_eye
Posted: March 05, 2010 12:52 pm
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1. Not easily - too many variables. Try to identify the station being received. They should transmit an ID code (verbally or otherwise) at regular intervals.

2. 'You' act as an aerial making the reception clearer.

3. The loose wires are the input to the amplifier which is amplifying the radio signals because you haven't 'de-coupled' the RF signals at the input. All amplifiers 'amplify' a range of signals and, when designed properly, they are limited (by design) as to which frequencies they pass/amplify. Your amp is meant for audio frequencies but can actually amplify up to a few 100kHz (well into the longwave and possibly mediumwave bands) unless you put components on the input to bypass these signals to ground (short them out).



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Mackhack
Posted: March 05, 2010 06:30 pm
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Thanks for the answer.

I knew that I'm acting as like an antenna but why does that increase the quality especially when I hold the battery? I tried it on different other parts of the circuit but that didn't make a difference or only little.

And a additional question to 3.) Why does the quality change when I bend the 2 wires basically away from each other? It's funny b/c when I leave them nice and neatly together the quality is poor, as soon as I bend them away from each other the signal becomes clearer.


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tekwiz
Posted: March 05, 2010 06:37 pm
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Spreading the wires increases the RF pickup. More signal=better quality.
I'll bet it's an AM station you are receiving. wink.gif


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kellys_eye
Posted: March 05, 2010 08:14 pm
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each wire acts as it's own antenna but with them in parallel (running side-by-side) the signals the two wires pick up are self-cancelled - whereas having the wires at 90 or 180 degrees forms a 'dipole' antenna which effectively increases the sensitivity of the circuit.


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Mackhack
Posted: March 06, 2010 09:47 am
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QUOTE (kellys_eye @ March 05, 2010 12:14 pm)
each wire acts as it's own antenna but with them in parallel (running side-by-side) the signals the two wires pick up are self-cancelled - whereas having the wires at 90 or 180 degrees forms a 'dipole' antenna which effectively increases the sensitivity of the circuit.

That's exactly what it is... They are standing there in an angle between 90 and 180 degrees to each other.

OT: Wondering why I only receive sporadically an email notification on answers.


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Mackhack
Posted: March 06, 2010 09:48 am
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QUOTE (tekwiz @ March 05, 2010 10:37 am)
Spreading the wires increases the RF pickup. More signal=better quality.
I'll bet it's an AM station you are receiving. wink.gif

That's a good possibility since it's talk radio... Hi Tekwiz nice to read from you again smile.gif


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tekwiz
Posted: March 06, 2010 09:48 pm
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QUOTE (Mackhack @ March 06, 2010 12:48 am)
QUOTE (tekwiz @ March 05, 2010 10:37 am)
Spreading the wires increases the RF pickup. More signal=better quality.
I'll bet it's an AM station you are receiving. wink.gif

That's a good possibility since it's talk radio... Hi Tekwiz nice to read from you again smile.gif

I'm still here. smile.gif Glad to hear you are Ok, too. wink.gif

What is actually happening is that something in the the input of the 386 is acting as a rectifier & is detecting the RF signal that the wires pick up. This is quite common, not only with the 386, but with any high gain audio amp.
Proper use of decoupling capacitors on the input to the 386 will eliminate the problem. This usually takes the form of a 1nf capacitor from each input lead to ground. These cap(s) should be mounted as close to the amp chip as possible, & tiny SMT chip caps are ideal for this application.


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MacFromOK
Posted: March 06, 2010 10:50 pm
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QUOTE (tekwiz)
What is actually happening is that something in the the input of the 386 is acting as a rectifier & is detecting the RF signal that the wires pick up.

Yep. Think "crystal radio"... biggrin.gif


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