Line preamp acting as a tuner – how is it possibile ?

ginetto61 · 12:45 PM

Hello to everyone !

I have a really strong curiosity and I am posting here in the hope to get and explanation.

A friend of mine is building a line preamp prototype on a breadboard
Of course there are a lot of flying wires, and I suppose that they behave like small aerials
He is testing this preamp and it constanly hears in the background a radio local station

In other words his line preamp is acting like a “bad” tuner

How can it be possible I really like to understand

I mean, radio transmissions are in MHz region but he is hearing them in the AF band.
Which components could be the culprit ? bjts ? capacitors ?
Thank you sincerely to all who will take care to explain this “mistery” to me

Kind regards,

gino

jaycee · 1st November 2009, 06:46 PM

The junctions of transistors work as diode rectifiers in much the same way as the first AM radios did, and there is capacitance by board tracks and semiconductor junctions, and inductance from traces and wires. All those form a resonant circuit that can be modulated by AM transmission, and pick up audio.

File:Circuit diagram of a crystal radio receiver.svg - Wikipedia, the free encyclopedia

You could perhaps see from this diagram, how a crude AM reciever could be formed by parts of the preamp.

ginetto61 · 1st November 2009, 07:39 PM

Quote:

Originally Posted by jaycee

The junctions of transistors work as diode rectifiers in much the same way as the first AM radios did, and there is capacitance by board tracks and semiconductor junctions, and inductance from traces and wires.
All those form a resonant circuit that can be modulated by AM transmission, and pick up audio.
File:Circuit diagram of a crystal radio receiver.svg - Wikipedia, the free encyclopedia

You could perhaps see from this diagram, how a crude AM reciever could be formed by parts of the preamp.

Thank you very much for your kind and very hepful reply
Do you think that the use of a breadboard is a bad thing in general ?
Moreover, putting all in a metallic case can tame this effect ?
Thanks a lot again and kind regards,
gino

1st November 2009, 12:41 PM	#1
ginetto61 diyAudio Member Join Date: Sep 2009 Location: Near Milan	Line preamp acting as a tuner – how is it possibile ? Hello to everyone ! I have a really strong curiosity and I am posting here in the hope to get and explanation. A friend of mine is building a line preamp prototype on a breadboard Of course there are a lot of flying wires, and I suppose that they behave like small aerials He is testing this preamp and it constanly hears in the background a radio local station In other words his line preamp is acting like a “bad” tuner How can it be possible I really like to understand I mean, radio transmissions are in MHz region but he is hearing them in the AF band. Which components could be the culprit ? bjts ? capacitors ? Thank you sincerely to all who will take care to explain this “mistery” to me Kind regards, gino Last edited by ginetto61; 1st November 2009 at 12:45 PM.

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1st November 2009, 06:46 PM	#2
jaycee diyAudio Member Join Date: Apr 2004 Location: Norwich, UK	The junctions of transistors work as diode rectifiers in much the same way as the first AM radios did, and there is capacitance by board tracks and semiconductor junctions, and inductance from traces and wires. All those form a resonant circuit that can be modulated by AM transmission, and pick up audio. File:Circuit diagram of a crystal radio receiver.svg - Wikipedia, the free encyclopedia You could perhaps see from this diagram, how a crude AM reciever could be formed by parts of the preamp.

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