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So I have a Marsha PCB v3 with a high frequency that doesn’t go away. More gain – more noise. I even treated this like one of the more high gain preamp circuits and isolated and shielded the power and grounds. That usually does the trick but I’m lost here. I thought there might be a short between the power and audio paths… I do read 4v along my audio path! 4v at all pot connections, D3/D4 and the R12 jumper, just a few random spots that I checked all had 4v BUT… NO continuity between the power jack and any spots on the board in the audio path. So that’s weird. I took to a FB group and someone pointed out that R14 doesn’t have a cap. I was advised to add a 47uF cap in parallel to R14. Thoughts?…
The 4V readings throughout parts of the audio path are normal for this type of op amp circuit and do not indicate a short between power and audio. It is also normal to have no direct continuity between the power jack and those points since the VB reference voltage is established through the resistor network rather than through a direct connection.
It may be worth experimenting adding a capacitor in parallel with R14 since this is a fairly high gain circuit and since it is a very easy modification.
Squealing can also be commonly related to wiring layout, grounding, lead dress, shielded wire routing, or even the op amp being used.
One thing to keep in mind is that taking a verified circuit to random forums or FB groups can sometimes generate a lot of conflicting theories from people unfamiliar with the actual design.
If that doesn’t work out I would do all of the common troubleshooting steps.
- Verify the issue only with two known good cables and a proper isolated power supply, and with no other pedals connected in the chain.
- Since this is a fairly high gain circuit, avoid turning every control fully up. Start with the Tone control near center since it is also part of the Active gain structure, then see if there is a range of settings where the squeal disappears or improves. Try gently moving the wiring around with a pencil to avoid power and audio wires running parallel or too close together.
- power it using the battery test
- Also, do not underestimate the possibility of a cold or questionable solder joint. High gain circuits can amplify even very small connection issues into squealing or unstable behavior. I would strongly suggest reflowing all solder joints on the PCB, pots, jacks, and off-board wiring even if they visually appear OK.
If after testing everything as suggested above and reflowing the entire board and you still have squealing you will need to post some clear, well lit photos showing both sides of the PCB and the off-board wiring so we can try to see what is going on..
Here is a Marsha Tone as part of a dual combo of high gain builds which may be helpful for comparison.
If you’re going to try a parallel capacitor I’d say 100n for hf noise there’s already the 100u in parallel with the voltage divider
I’d use the unscientific method of dabbing your finger on and off the op amps to see if you can narrow it down without hearing it we don’t know if it’s oscillation
Do you have the R12 jumper on a switch if so does it make the noise in the normal position
Post an image somebody might see something I know I’ve missed incorrect values etc after multiple checks it’s easily done
What pickups are in your guitar is your power supply filtered and regulated
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