LM386N Audio Amplifier picking up local FM station

Thread Starter

Mitch conrad

Joined Apr 10, 2013
7
Hi I'm pretty new to electronics, still reading through the volumes of info on this site.

I decided to take a break from reading and start building some simple circuits.

So I made a basic audio amplifier with the LM386N using a schematic on the datasheet. I plugged it into my computer and strangely enough im picking up signals (92.7 MHz) from a local radio station that's being amplified. You can also hear it faintly if its not plugged into a audio source.



I used different values for the capacitors and I didn't include a 10 ohm resistor after the .01uf cap cause I don't have one.

Another thing is I used speaker wire and a old headphone jack to make my audio input, I think that's is where I'm getting the leak-in. Could a low-pass filter fix this?
 

#12

Joined Nov 30, 2010
18,224
Yes, the leak-in is at the input. Keep your wires neat and ground the bottom end of pot 1.

Just a ragged estimate, Put 75 ohms after the wiper of the pot, in series with the input signal, then a 1nf ceramic to ground, then the signal goes into the amplifier (to cut the high frequencies). The distance from 20Khz to 97 MHz is so wide that a guestimate should work.

You really must put a few ohms before the .01 cap on the output. It's a frequency roll-off to match the speaker, so 16 ohms will do. Anything from 8 to 16 will be kinda OK. And learn this: Many amplifier chips hate a capacitive load. You have done a no-no.
 
Last edited:

#12

Joined Nov 30, 2010
18,224
ps, it's called a Zobel circuit...the impedance of the speaker and a capacitor to drain the very high frequencies rejected by the inductance of the speaker coil.
 
Top