Using 2n3904 transistors built-in audio amplifier

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I just built in class a two stage audio amplifier using a 2N3904 transistor but I received a radio station. My circuit doesn't have any tank circuit for tuning but it picked a random radio station. How is that possible?

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Tessie Swift wrote:

I just built in class a two stage audio amplifier using a 2N3904 transistor but I received a radio station. My circuit doesn't have any tank circuit for tuning but it picked a random radio station. How is that possible?

 

A transistor is, at one level, a pair of diodes so can easily rectify an AM radio signal, usually across the B-E junction. And any capacitance, real or stray, can form a tuned RC circuit.

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Congratulations! As Brian suggests, you have successfully built your first Crystal Radio!

 

laugh

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Pre-amplifiers are capable of much more than that.
Imagine 1kW++, SSB, 3-element cubical-quad-antenna just 100m away from the church, and HAM activity was well heard during a sunday sermon.

Last Edited: Mon. Jul 19, 2021 - 09:46 AM
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Back when radio morse code was the only game in town, the guys about fell out of their chairs, when they started "hearing voices" in their headphones & probably drank a barrel of whiskey.  That was a new beginning. 

 

Your ship receives a signal in Morse Code, “Be prepared for something of great interest to follow.” You wait for the next series of beeps, but instead of a beep, you hear the first human voice transmitted from hundreds of miles away. You’re literally hearing history being made. Is this an angel speaking to you from above? “O Holy Night” begins its soft melody from a violin nowhere in sight. This isn’t an angel playing, it’s the Father of Radio at work, Reginald Fessenden!  https://www.autodesk.com/product...

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

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grohote wrote:
Imagine 1kW++, SSB, 3-element cubical-quad-antenna just 100m away from the church, and HAM activity was well heard during a sunday sermon.

 

blush

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Tessie Swift wrote:
I just built in class a two stage audio amplifier using a 2N3904 transistor but I received a radio station.

Post your schematic, please!

Amplifiers, Oscillators, seems every time I try to make one, I end up with the other!!!   Ain't Electronics fun!  devil

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Last Edited: Mon. Jul 19, 2021 - 12:53 PM
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It's a fundamental law: amplifiers pick up radio; oscillators amplify, and radio receivers oscle.

 

Neil (remembering fifty or so years back and trying to kill a 50Hz hum that was only there when the lid was off the box...)

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ki0bk wrote:
Post your schematic, please!

I should add: and the photo, please.

Interesting, it may be a mixture of stuff: HF and LF path, loop, positive reaction, amplifier on the brink of oscillation...

I should also ask for a photo, because the wiring also counts.

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Hello, Tessie -

 

Welcome to AVR Freaks. While we usually do AVR microcontrollers, here, there are enough folks who are skilled at electronics and might be able to help you.

 

Bipolar transistors such as the 2N3904 are excellent at detecting radio signals. Usually it will be a radio station that is close to you so that the strength of the signal is quite high. That is the usual reason why you don't hear many stations at the same time.

 

The biggest factor that determines whether a circuit will do this is HOW it is built. Is it on a "spring board" (like this: https://www.sparkfun.com/product... )? Or, is it on a circuit board (like this:  https://www.sparkfun.com/product... )? Or built some other way?

 

A photo would really help. A basic schematic would also be useful.

 

Jim  

 

Until Black Lives Matter, we do not have "All Lives Matter"!

 

 

Last Edited: Mon. Jul 19, 2021 - 05:12 PM
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barnacle wrote:
amplifiers pick up radio;
Some impedance in the correct current loop may be enough to attenuate out-of-band.

8.3.5 Input Differential Voltage | TLV333 datasheet | TI.com

...

This differential input voltage results in activation of parasitic diodes inside the front-end input chopping switches that combine with 10-kΩ electromagnetic interference (EMI) filter resistors to create the equivalent circuit shown in Figure 20.

...

"Dare to be naïve." - Buckminster Fuller

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I am unfortunately not skilled enough to tell you exactly how to determine if this will happen, or for that matter how to correct it, but I have built an intercom system that did the same thing. In that instance the cables between intercom units acted as a perfect antenna for radio stations! I also read somewhere many years ago about someone who had a tooth filled with a metal filling which became a radio in his mouth! laugh

Wayne

East London
South Africa

 

  • No, I am not an Electronics Engineer, just a 54 year old hobbyist/enthusiast
  • Yes, I am using Proteus to learn more about circuit design and electronics
  • No, I do not own a licensed copy of Proteus, I am evaluating it legitimately
  • No, I do not believe in software or intellectual property piracy or theft
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Thanks!laugh

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I also read somewhere many years ago about someone who had a tooth filled with a metal filling which became a radio in his mouth!

 

 

JC

 

Edit:  Story here

 

 

Last Edited: Wed. Jul 21, 2021 - 12:46 AM
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We live a couple mile from WLW and their old half-million watt transmitter...when it ran, it turned many fences and bedsprings for miles around into "radios"

I like that the entire HUGE upside down antenna rests on a pair of large glass bead insulators.    You can still tour the old facility, they are converting the land around it into a shopping center!

The base impedance of the tower is 390 Ohms and is fed with an antenna current of 35.7 Amperes of RF energy on 700 kc.

 

http://j-hawkins.com/wlw.shtml

 

 

 

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

Last Edited: Wed. Jul 21, 2021 - 01:56 AM
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DocJC wrote:
Edit:  Story here

 

Thanks for the link JC, interesting read!

 

avrcandies wrote:
We live a couple mile from WLW and their old half-million watt transmitter...when it ran, it turned many fences and bedsprings for miles around into "radios"

 

Half a million watts! That's a substantial output! I would love to see their linear amplifiers for that kind of wattage! Pretty dangerous stuff though if not respected as it should be. I worked for a guy many moons ago who had a business providing radio services before the advent of cell phones. He isolated an antenna from the radio and climbed a tower to work on the antenna. Unfortunately some idiot felt it was far better to have the antenna connected to the radio and linear amplifier providing 5 000 watts instead of leaving it unplugged. Rocky up the mast had his hand on the active element when the radio went into transmit mode and it burned a chunk of flesh away at the base where his thumb met his wrist. I have had a very healthy respect for RF energy since I saw that back then. Admittedly, Rocky should have provided an isolating notice in red on the radio before doing the, work but still.

 

I have had a few minor RF energy burns over the years too. When I was a kid I installed a center-fed dipole for my 27 Mhz rig. When I keyed the mike the pliers next to me picked up 12 watts on SSB and burned a red stripe onto my thigh. Great fun because RF energy burns instead of shocking.

 

EDIT: Interesting 30 minute video about the WLW installation of 500 000 watts here - https://www.youtube.com/watch?v=CbHjcwIoTiY

 

EDIT 2: They needed 33 volts at 4 500 amps to feed the valves for that output!

Wayne

East London
South Africa

 

  • No, I am not an Electronics Engineer, just a 54 year old hobbyist/enthusiast
  • Yes, I am using Proteus to learn more about circuit design and electronics
  • No, I do not own a licensed copy of Proteus, I am evaluating it legitimately
  • No, I do not believe in software or intellectual property piracy or theft
Last Edited: Wed. Jul 21, 2021 - 07:51 AM
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EDIT: Interesting 30 minute video about the WLW installation of 500 000 watts here - https://www.youtube.com/watch?v=CbHjcwIoTiY

I've occasionally (rarely) ran into a few of those guys in the video at local electronics events.

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!