Skip to main content
1 of 5
Kaz
  • 20.1k
  • 1
  • 41
  • 83

Radio frequencies from AM stations get into audio amplifiers not only through the inputs, but also via the outputs.

The speaker cabling can serve as an antenna to pick up radio signals. This is is coupled back to the output of the amplifier. But the amplifier has a negative feedback loop which feeds the output back to the input. So the output is really just another input.

Production amplifiers usually include a Boucherot Cell on the output, often an output inductor after that. Both of these devices can help block incoming RF, even though it's not their main function. The Boucherot cell is simply a capacitor in series with a resistor, placed between the output and ground. Common values are 0.1uF and 10 ohms. You can see this in many amplifier schematics.

The second problem is that this LM386 probably does not have very good common mode noise rejection, which is essential for guitar inputs. The electric guitar is horrible, even when correctly wired with quality pickups, good shielding and a quality instrument cable. It contains hundreds of meters of wire in its pickups and basically acts as an antenna. The signal between the cable "hot" and the grounded shield can be relatively clean thanks to humbucking pickups and shielding, but there is a lot of common mode noise: oscillating voltages which move both conductors at the same time. Good amplifiers reject this: when the guitar's ground and signal move in the same direction, that common movement is amplified only very slightly. The difference between the amplification of that common movement and of the wanted signal is the common mode rejection ration (CMRR) often expressed in decibels. You want at least 100 dB of this. The data sheet for the LM386 does not even contain the word "common", let alone quote a figure for the CMRR, oops!

Try these tests. Firstly, disconnect the guitar and see if the noise diminishes or changes. After disconnecting the guitar, also try shorting out the input jack. That should absolutely keep almost noise out of the input and so if the radio interference goes away, that tends to confirm that it is coming in via the input. To improve the input, build an input stage which is based on a quality, low-noise audio op-amp with good common mode noise rejection: for instance, the LM4562, NE5532, or NJM2068. (Of course, I realize that these are overkill if the back-end is the measly LM386).

Also test the speaker output similarly. See whether disconnecting the speaker helps, and also try replacing the speaker with a dummy load resistor of around 8 ohms.

Lastly, there is a reason why amplifiers are built into metal boxes and why fuss is made over any internal wiring and in particular grounding! If your power amp is just bare components on a breadboard, don't be surprised if it is susceptible.

Kaz
  • 20.1k
  • 1
  • 41
  • 83