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I made a stompbox which is basically a small wooden box with a piezo element inside that when stomped on produces a sound similar to a bass drum except that the thing is much more portable.

The circuit works but I have two problems with it that I don't know how to solve:

  • It's extremely loud. I hooked it up to two different soundboards and even with the gain at 1 this was super loud.
  • The circuit produces a really loud and constant "crackly" hiss. The hiss remains at the same apparent loudness regardless of what the gain pot on the soundboard is.

The circuit in question is this:

(I replaced the MPF102 with a J113 because I didn't have one in hand) enter image description here

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Having read extensively the posts on 'diyAudio.com' about the challenges of achieving low-noise performance with JFETS for RIAA vinyl record playback with MovingCoil phonocartridges, one noise source is hot-carrier currents needing an exit path out the gate. Your exit path is 3.3MegOhms.

Experiment with reducing the Rgate.

Regarding the "super loud", you can reduce the gain by reducing the 1.5Kohm to 560 ohm or 220 ohm. At 220 ohm, your gain is actually 'attenuation', of 10 dB.

The 'diyAudio' NJFET is 2SK170, usually cascaded with bipolar to hold the drain stress to 20 volts or less, for purpose of avoiding hot-carrier-gate-current noise. In this circuit, the J113 exhibits that noise with only 9 volts.

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  • \$\begingroup\$ I just soldered your suggested changes into my prototype and I must say you were spot on (on both suggestions). Thanks! Quick follow up question: would replacing the 1.5Kohm resistor with a variable resistor with an audio taper work as as volume control? If not, what's the proper way to achieve that in this circuit? \$\endgroup\$ – Julian Jun 29 '17 at 4:20
  • \$\begingroup\$ You are welcome. The crew at 'diyAudio' should get the credit for diagnosing the hot-carrier noisy-gate-current effect. Regarding the pot, install the pot in place of the 1.5K resistor, and take your output from the wiper. Please notice this circuit (and your original circuit) has NO POWER SUPPLY REJECTION. Whatever trash is on the +9v line will appear in the output. \$\endgroup\$ – analogsystemsrf Jun 29 '17 at 4:45
  • \$\begingroup\$ The forum on 'diyAudio' is "simplistic NJFET RIAA", moderated by 'salas'. Those people deal with low nanoVolt fidelity issues. Their RIAA vinyl preamplifiers have NO PSRR, like yours, so they use Shunt Regulators to suppress VDD trash+noise down to a few nanoVolts. \$\endgroup\$ – analogsystemsrf Jun 29 '17 at 4:48

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