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I've been fiddling with a twin-T percussion oscillator with success. It's built on a breadboard and it sounds exactly how it should sound (damped sine wave, big 808/909-kind of kicks). However it's noisy. I've tried adding a big electrolytic cap between V+ and GND. It makes difference yet it's still very noisy. Also, the circuit tends to 'buzz' when I touch the pots or a wire. Any idea how to kill that? The whole circuit is powered from a split power supply from a switchable adaptor (I mean the op amp, the rest of the circuit is powered from 0-9V).

Simulatable schematics of the circuit here (Open in Editor-> Menu->Run Last Simulation):

schematic

simulate this circuit – Schematic created using CircuitLab

This is the power supply used:

enter image description here

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The "buzz" is the usual mains hum, picked up by the body as an antenna and transferred when you touch it. Breadboards tend to be very good at picking up ambient noise anyway, and capacitatively coupling signals that should not be coupled.

Try running the whole thing off a battery; that will tell you whether PSU noise is a problem. (Are there two power supplies or does the 9V wall supply appear twice?)

The resistors in the filter seem very large, which intuitively suggests poor noise rejection around the negative input of the opamp. That's the location I'd focus on for noise avoidance. Tidy up the breadboard in that area - keep wires short.

The long "pulse" wire might also be a noise antenna, and might benefit from a high value resistor to ground.

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  • \$\begingroup\$ Fantastic, you're a star! Thanks very much! So far I've been using a wall adapter that powers both the circuit (0-9V) and the op amp (-+4.5V). \$\endgroup\$ – alkopop79 Apr 27 '13 at 8:02
  • \$\begingroup\$ I've changed the 10M resistor in the feedback network for a 3.9M and it reduced the noise significantly. I still hear hum in the signal. \$\endgroup\$ – alkopop79 Apr 27 '13 at 9:13
  • \$\begingroup\$ Hum could be a traditional ground loop to the output amplifier, or PSU noise, or could be improved by putting the whole thing in a metal case. See electronics.stackexchange.com/questions/36701/… \$\endgroup\$ – pjc50 Apr 27 '13 at 9:56
  • \$\begingroup\$ I tried it with battery and I could still here the hum. Any ideas? \$\endgroup\$ – alkopop79 Apr 27 '13 at 16:06
  • \$\begingroup\$ Given that the change in the resistor reduced noise, I'd redesign the filter network with resistors about ~100k. Not quite sure how you'd change the capacitors to match that. \$\endgroup\$ – pjc50 Apr 27 '13 at 16:19

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