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Background

I'm a complete noob with electronics, trying to design a guitar effects pedal. There is a bit of crackle when I connect a pair of headphones to the output, but the audio signal is still clearly audible. When connected to a valve amplifier, there is a much louder crackle, and it's constant. The crackle in both cases reminds me of what I used to think was dirty potentiometers, but now believe is related to varying the DC component in an audio signal.

The circuit looks like this

block diagram

The VCA and buffer are from the VCA datasheet (though I've swapped OP275 for TL072)

SSI2161 single supply suggestion

The clean boost is from here

clean boost circuit

The microcontroller uses an LM78L05 for power, and the only common ground with the analog side runs through that LM78L05. The MCU supplies a control voltage to the Control Voltage Booster (another TL072), which boosts the voltage before sending it to the VCA (which has a virtual ground, hence the need to boost the control voltage).

Question

What might be causing the huge increase in noise when connecting to an amplifier instead of headphones? My first thought was the impedance of the headphones versus that of the amplifier. But then I thought that the TL072 boost should be equally comfortable sending its output signal to an amplifier.

Here's the pedal, for reference. The screen is blank because, despite my ignorance with electronics, I have a pretty cool idea for the software, and I don't want to spill the beans before my product is ready to launch.

photo of the pedal

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    \$\begingroup\$ What do you get signal wise before the booster? What does it look like on an oscilloscope? \$\endgroup\$
    – Ian Bland
    Jul 15 at 1:26
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    \$\begingroup\$ upvote for the nicely presented question \$\endgroup\$
    – jsotola
    Jul 15 at 1:48
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    \$\begingroup\$ Your headphones are essentially balanced-line since what passes for a ground terminal has no connection to ground whatever. This tells us that the noise is not inherent in the output of your device. The valve amplifier very likely has the ground terminal of the input connected to earth. You could very likely eliminate this using an audio transformer between the pedal and the amp, although that doesn't answer the 'where is it coming from question. I'd suggest doing that as a matter of course, since it means you can earth each piece of equipment for safety without sacrificing sound quality. \$\endgroup\$
    – Frog
    Jul 15 at 2:12
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    \$\begingroup\$ @OutstandingBill You'll want the primary of the transformer to be between the pedal output and the pedal's ground; the secondary between the amp input and amp ground; that way there's no DC path between the pedal and the amp. I'm not sure whether that's what you've suggested. A smart pedal designer such as yourself might incorporate the transformer into the pedal itself :-) \$\endgroup\$
    – Frog
    Jul 15 at 3:33
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    \$\begingroup\$ "it's just a mass of noise" - what does the noise sound like? Do you still get the noise when the 'boost' pot is wound down? \$\endgroup\$ Jul 15 at 5:25
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Thanks to everyone for kindly commenting with their helpful suggestions. In the end it was a really basic mistake. I think it was Frog's comment about the ground that got me there.

The signal jacks are Switchcraft open frame. I had assumed that the ground of each would make a decent electrical connection with the aluminium enclosure, and therefore a wire between them would be unnecessary (perhaps even undesirable if it created a ground loop). So only one was connected to the PCB's ground.

But my multimeter tells me there was around 80 Ohms between them. Once I'd connected the two jack grounds with a wire most of the problem disappeared. There's still a bit of crackle occasionally, much less noticeable now, and there's no crackle for a few minutes at a time - I'm wondering if it's when the fridge switches on - but now the headphones and valve amp give a comparable level of crackle.

Edit

The device jonk and Tony Stewart EE75 are referring to works like this:

my interpretation of jonk's suggestion

The idea is that you connect some audio signal to the signal in and some listening device such as headphones or amplifier to the signal out. The device is then inserted into the circuit just before signal out. The headphones should then be silent. Then the device is moved backwards through the circuit until the noise returns. The noisy part of the circuit is then the part just to the right of the probe.

This device would be difficult to use with a circuit which is all on a PCB, since it needs to be inserted in series with the signal path, rather than in parallel. I do have a PCB, so I'm going to try to replicate the noise on a breadboard. Thanks for the advice everybody!

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    \$\begingroup\$ @TonyStewartEE75, thank you for the additional information. I think a detailed description of to use such a device could benefit others, providing it's correct. Would you mind confirming if I've described the implementation correctly? Sincere thanks \$\endgroup\$ Jul 16 at 1:20
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    \$\begingroup\$ yes it duplicates what jonk said \$\endgroup\$ Jul 16 at 1:21
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    \$\begingroup\$ @OutstandingBill Hi. I just noticed all this stuff, now. Anyway, I think you have the basic idea for the left-handed screwdriver. ;) You just start at one end and work towards the other end. Either way you go, there will be a point when "things change, suddenly." When that happens you have some clues. By the way, if you have a long cable between two boxes, you should use the adapter at one end of the cable, then pull it off that end and move it to the other end. The cable itself is also part of the testing process. Not just the boxes/modules. \$\endgroup\$
    – jonk
    Jul 16 at 2:26
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    \$\begingroup\$ @OutstandingBill As far as shot noise, this occurs whenever electrons cross through a randomizing barrier. Copper wire is not a randomizing barrier. But a PN junction is. That means BJTs and diodes generate shot noise. But resistors, wires, and the drain-source of a FET (if you can ignore the built-in diodes in the darned things) do not generate shot noise. Johnson noise and kT/C noise are from the exact same thermal statistics cause, really, and can both be developed from the same starting equations. Shot noise is different. That 80 Ohm thing was certainly part of the problem, eh? \$\endgroup\$
    – jonk
    Jul 16 at 2:29
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    \$\begingroup\$ Batteries are certainly quite significant audible sources of shot noise. Put a 1,5V across any speaker and listen to it. \$\endgroup\$ Jul 16 at 2:35

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