I'm using an L298 H-Bridge to drive a DC motor. The motor is spinning a magnet across a guitar pickup at ~30 Hz. That signal is then amplified and sent to a speaker.

However, the output signal also contains a fairly constant (and loud) sound at ~10 kHz. I believe this is coming from the L298 H-Bridge's switching circuitry.

Obviously I don't have the H-bridge connected in the audio circuit. But that switching signal is making its way into the audio at some stage, either from the pickup coil, the op-amp amplifier or the wires themselves.

I don't think a band-reject filter is the answer here. What physical things can I do to get rid of this noise?

  • 1
    \$\begingroup\$ Look for positive stray feedback and suppress with shielding. Then post a schematic and photos \$\endgroup\$ May 5 at 19:02
  • \$\begingroup\$ Who (what) is PWM'ing the L298 ? If you don't need much power, using a linear regulator to adjust the motor speed would be practical. If it is a low cost brushed DC motor, then the commutation (brushes) will of course generate lots of electrical noise. A brushless DC motor would not generate as much noise itself, but likely the controller would be PWM'ed and it would generate noise. \$\endgroup\$
    – jp314
    May 5 at 20:05
  • \$\begingroup\$ You're going to have 10KHz ripple in the motor currents which will result in a magnetic field that will easily couple into your pickup. Can you double the PWM frequency so you can't hear it? (Or if you can hear 20 kHz bump it up a little higher.) \$\endgroup\$
    – John D
    May 5 at 21:04
  • \$\begingroup\$ @TonyStewartEE75 What kind of shielding would work here? The induced current will also fall off with distance, but the whole system is quite packed together :) \$\endgroup\$ May 6 at 0:23
  • \$\begingroup\$ @jp314 An arduino is controlling the PWM. Yeah, it's only a cheap 6v DC motor with an encoder doing PID calculations. I agree that there shouldnt be any noise coming from or into the motor. \$\endgroup\$ May 6 at 0:26


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