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I am working on a tremolo guitar pedal circuit. A tremolo pedal basically attenuates the guitar signal using a pattern. Most tremolos use a combination of LEDs and LDRs to do this, but since I do not have any LDRs at the moment I chose to design one using a digital potentiometer. Schematic of the tremolo pedal. For simplicity I have not included the digital side of the circuit. This only consists of an Arduino Nano, connected to the digital potentiometer using SPI. I have breadboarded the circuit and it works, except it makes a ticking sound. It is very noticeable on the high gain setting on my amplifier. At first I thought it was an issue with the Arduino, but after removing both the Arduino and the digipot -- and replacing the digipot with a resistor -- the issue persists. I have no idea what is going on. Could it be an oscillation of some sort? Should the schematic work and lies the error in my breadboarded implementation?

Thanks in advance!

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  • \$\begingroup\$ I don't see any sign of a decoupling capacitor (100 nF typically) on the TL074P. It should be mounted very close. Is the ticking the same frequency as the tremolo? Where is the 5 V supply coming from? \$\endgroup\$ – Transistor Nov 26 '18 at 22:59
  • \$\begingroup\$ Thanks for the input. The 5V supply is coming from the 7805 in the power section of the schematic. I have added a 100nF cap between the TL074's VSS and ground but the issue unfortunately persists. I am currently testing without the digipot and instead with a fixed 20k resistor, so there is no tremolo effect, but the ticking is still there. The tickrate is about 300BPM. \$\endgroup\$ – Guido Nov 26 '18 at 23:10
  • \$\begingroup\$ We understand you don't have the tremolo effect hooked up, but if there's something on the digital side that pulls a lot of current in sync with it, that could cause a ticking sound. Hence the question. \$\endgroup\$ – TimWescott Nov 27 '18 at 0:30
  • \$\begingroup\$ Come to think of it -- try disconnecting the Arduino from the circuit, or at least powering it down. If the ticking goes away it's on that side; if it doesn't, it's on the analog side. \$\endgroup\$ – TimWescott Nov 27 '18 at 0:32
  • \$\begingroup\$ why are both inputs of IC1C biased to V/2? \$\endgroup\$ – Tyler Stone Nov 27 '18 at 2:08
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C7 is back to front. Its positive side is at +2.5 V and its negative at +4.5 V. Also the TL074 really needs a supply of at least 10 V (see data sheet). The common mode voltage of the op-amp needs to be at least 4 V above the negative rail too. This is not the case in the second stage. An LM324 might work better although its audio qualities are poor (crossover distortion) but you could try it for starters.

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  • \$\begingroup\$ Thanks for the swift response! I flipped the orientation of C7. The issue still persists however. Thanks for the note about the common mode voltage. I did this in the second stage because the digipot I was using (MCP41050) uses a single 5V supply. Tomorrow I'll try using an LM324, if that's not working I'll try to supply -9V on the TL074 as well! \$\endgroup\$ – Guido Nov 26 '18 at 23:23
  • \$\begingroup\$ I'm not sure if it's relevant but you could also try a larger value for C2 since this node biases your first and last stages but may be affected by the last stage. Two other things to consider are using 5V for bias instead of 4.5 V (but might be noisy) or use the spare op-amp to buffer the 4.5 V supply. \$\endgroup\$ – Steve Hubbard Nov 26 '18 at 23:32
  • \$\begingroup\$ Buffering the 4.5V bias and swapping the op amp to an LM324N fixed the ticking! Thanks a lot for the insight, I learned a lot! \$\endgroup\$ – Guido Nov 27 '18 at 12:52
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Your +9V/2 voltage reference is probably the cause of the oscillations. R3 and R4 are quite big resistors which means that only a small current is 'creating' the voltage reference.

As R10 and R11 are small (at least relatively speaking in comparison with R3 and R4) they allow currents to pass that will overwhelm the small current thus your voltage reference will fluctuate according to the signals that are present.

As the voltage reference is used in several places in your schematic you have created a feedback loop.

Easiest way to fix it is by decreasing the resistance of R3 & R4 and increasing the resistance of R10 & R11 using your spare op-amp as buffer as Steve Hubbard has suggested.

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