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I am designing a guitar pedal for overdrive/distortion from scratch and the circuit creates a lot of noise. The circuit also doesn't immediately start working, there is a delay before the signal makes it through the circuit and into my speaker.

Basic Circuit Schematic for Guitar Pedal Pre-Amp I've omitted the other parts of the circuit and just kept the basic pre-amp to show the basic set up. Not shown here, I am also using coupling capacitors at the 9V pin of my op amps of the recommended values from the datasheet.

The issue I'm running into is that when first powering it up with 9V, it takes about 7 seconds each time to start transmitting the guitar's signal through my circuit and into the speaker. And then it takes another 12 seconds of playing the guitar to get rid of the cracky and broken sound within the signal, before the signal finally comes out clean or distorted how it should be sounding from the beginning.

As for the background noise, it's really just one loud static hum, but no cracks or pops that would indicate that the power supply is the problem. And this background noise comes in at the very end when the signal finally starts sounding like what it's supposed to.

Just for reference I've also included how I'm supplying the 9V and 4.5V (below).

Circuit Schematic for Powering the Guitar Pedal

The 1000uF capacitor above is my decoupling capacitor to limit the oscillations of the 4.5V supply. Anything smaller was showing visible oscillations in my 4.5V supply. In addition I am using one of the unused op amps as a buffer for the 4.5V source (shown in the voltage-follower configuration above).

The op amps I'm using are the TL074H's, so I believe 9V to 0V rails are allowed. The green capacitors signify polarized electrolytic capacitors and the golden capacitors in parallel signify the MLCC capacitors used to lower combined ESR/ESL. I am using 3904 NPN BJT's for the buffer amplifiers in the signal chain.

I am not sure how I can fix that 7 second delay before the pedal starts responding, how to get rid of the messy cracked/broken signal noise before a full 20 seconds has elapsed and the background noise shows up, and how to limit the background noise even more. I will not be able to shield the circuit as my enclosure is made of wood so it would be awesome if I could somehow attenuate external signals as well to further limit the background noise.

I suspect it has something to do with RC constants since my capacitances and my resistances are both high, but I kept the capacitances in the signal path high so that they wouldn't attenuate the higher frequencies. Originally I was using 47uF capacitors instead of 100uF but the high (treble) frequencies were not as pronounced as the low (bass) frequencies in my speaker.

How do I get rid of the noise and the signal's delay?

If there are any other blatant issues with my schematic feel free to point them out, this is my first time attempting to design an audio circuit.

Edit:

I'm providing the digital circuit schematic: Entire Circuit Schematic Keep in mind I've omitted the bypass switching mechanism for now that toggles either the overdrive or distortion stages.

And this is the internals for the pre-amp Pre-Amp circuit schematic

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  • \$\begingroup\$ Your hand-drawn schematics look very nice! \$\endgroup\$
    – JYelton
    Aug 7, 2023 at 21:10
  • \$\begingroup\$ ... except that there are no component designators, R1, R2, etc., which makes it very difficult to discuss. Can you fix them up? \$\endgroup\$
    – Transistor
    Aug 7, 2023 at 21:59
  • \$\begingroup\$ I agree, I added them now \$\endgroup\$
    – Da Mahdi03
    Aug 7, 2023 at 22:43
  • \$\begingroup\$ I think you're clearly right about RC time constants being an issue somewhere. The component values you've written should take a second or two, max, to start up. I wonder if either: (1) some component is not hooked up as you intended, or doesn't have the value that you intended it to have; or (2) some pin on some IC, which you didn't intend to be using at all, should be grounded and is floating, or something, and is taking awhile to get to a reasonable value by leakage alone. \$\endgroup\$ Aug 8, 2023 at 0:43

1 Answer 1

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First fix: On your 4.5 V reference voltage circuit move the capacitors to the non-inverting input. All you need to do is hold the input steady while the 9 V supply is jumping around and the output will be steady. As shown the op-amp input will fluctuate with the supply voltage and the op-amp will be fighting the capacitors.

The 470 Ω 4.5 V divider will consume about 10 mA. There's no need for that. Raise the resistances to 10 kΩ and reduce the capacitors to 10 μF and the 47 nF caps should be fine.

Second fix: The output potentiometer is a bit strange. Since you have a 10 kΩ emitter resitor you could just change that to a 10 kΩ log pot and feed the output decoupling capacitors from the wiper.

Report back.

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  • \$\begingroup\$ Which capacitors are you saying to reduce? All the 100uF to 10uF? \$\endgroup\$
    – Da Mahdi03
    Aug 7, 2023 at 22:28
  • \$\begingroup\$ Do you see why we need component designators? \$\endgroup\$
    – Transistor
    Aug 7, 2023 at 22:34
  • \$\begingroup\$ I added my LTSPice schematics to the question \$\endgroup\$
    – Da Mahdi03
    Aug 7, 2023 at 22:46
  • \$\begingroup\$ How does lowering the capacitances in the signal path help lower the noise? \$\endgroup\$
    – Da Mahdi03
    Aug 7, 2023 at 23:28
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    \$\begingroup\$ The 100uF capacitors feeding 10k resistors pass frequencies down to 1Hz and take a long time to charge and discharge. A guitar plays down to maybe 50Hz so use 3.3uF capacitors. Hum is usually caused by wiring a circuit on a breadboard and using connecting cables that are not shielded. The crackling noise is probably the opamp making the 4.5V reference fighting the capacitors to ground on its output. An electric guitar usually has a preamp with an input impedance that is 1M or more. Your preamp input impedance is 25 times too low at 10k. \$\endgroup\$
    – Audioguru
    Aug 8, 2023 at 0:58

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