Recently I made an audio amplifier by following this video(https://www.youtube.com/watch?v=4ObzEft2R_g). The interesting thing is that when I plug the battery in, suddenly I received a very clear radio signal and the audio input acts like a antenna. Can anybody tell me what is going on here?
Electronic – How does a LM386 audio amplifier become to a radio receiver
amplifieraudioradio
Related Solutions
The LM386 amplifier's inputs are an internally biased differential pair, with common mode input range extending to the indicated ground, Vee, or actually 0.4 Volts below ground.
In order to not disturb this internal biasing, the input signal must be capacitively coupled (or perfectly ground-referenced) - a capacitor in series on the input line is needed.
Next, for a voltage gain of 200, the input signal peak to peak needs to be under 1/200 of the output voltage range of the amp. While I could not find this specified in a quick scan of the datasheet, if we assume 7 volts output voltage range with a 9 Volt supply, the input signal needs to be under 35 mV peak to peak, to avoid clipping the signal. Clipping would result in mild to severe distortion of the output - though this does not explain the complete lack of signal you find on output.
If the incoming signal is higher than this 35 mV P-P, a potentiometer as an attenuator on the input is suggested.
Once these fixes are done, please revert with results, so the answer can be added to if needed.
"Actually does the antenna really work with only one end connected onto the breadboard?" No, not this type of antenna. What you have is a loop antenna and it creates a magnetic field. You're now using it as if it is an electrical antenna and that would work IF it was very long, for 60 kHz we're talking 1250 meters (!!!) and not rolled up like yours.
I suggest you connect the open end of the antenna to +5 V BUT to limit the current, use a series resistor of value 1 kohm or so.
If you build this then I think there's a good chance it will work. Note how the Antenna is a coil (long wire with many turns) and both ends of the wire are connected !
simulate this circuit – Schematic created using CircuitLab
Best Answer
It's normal, it happened also using a TBA820M.
The input wires (together with the traces going to the input of the LM386, and LM386's own input capacitance) form a parasitic antenna + resonant LC tuning circuit, which, by chance, has a resonant frequency very close to the station frequency you're listening to. These signals are amplified by 20 (accordingly to your schematics). The small non-linearity of the 386 might act as AM demodulator.
A small capacitor (e.g 6.8 nF) will kill the incoming RF signal, by forming a low-pass filter with the two 1-k Ohm resistors (while passing the full 20kHz audio signal. Actually, if you connect both signals, i.e. if you don't leave one channel open, the bandwidth will be 40kHz). For best effectiveness, put that capacitor (and the two 1-k resistors) close to the input of the LM386.