Problem with high pitch noise (ZVEX SHO, Lovepedal COT50)

[MacAttack]

Hello, first post! 

I have tried searching all around with no luck in finding an answer to my problem.

The problem is when my effects are on (they are true bypass) and the pot is turned to 95-100% gain, there is a very high pitch noise coming out of the amp. You can compare the sound to something like crickets so you can understand how high the pitch are. I use a hammond 1590b case and non-isolated jacks. All the grounds are connected to the input jack sleeve (one wire from pcb-ground, one wire from the pot, one from the led, and one from the output sleeve). The -9v i connected to the input jack ring. Is there something i have missed because i have googled around and read alot about star grounding (still dont understand that concept), and ground loops and that you should/could leave the output jack not connected by wire because it will be grounded through the metal box.

Best regards
Mac

[MacAttack]

Oh, and worth to mention! When i turn my guitar volume all the way down, this noise gets a bit worse! 

/Mac

[Fancy Lime]

Hi Mac,

welcome to the party!

Just to clarify, you are talking about two clones that you made? Or are these commercial pedals? I cannot speak for the Lovepedal but the SHO is prone to noise of all sorts at the highest gains (a design flaw, imho), depending on the precise components and quality of the build. The easiest way to get rid of these is to add a small(ish) series resistor with the gain pot, that limits the maximum gain to "just below noisy". All other options require a bit more info on your part.

The noise getting worse with a closed guitar Vol pot usually has to do with high frequency oscillations produced by the cable capacitance in front of extremely high impedance inputs. Lowering the input impedance by lowering the according resistors might fix that. Please post or link the relevant schematic, so we can talk in part numbers like "R2", "C4" and so on.

Cheers,
Andy

[MacAttack]

Hi Andy, thanks!

Yes they are both clones that ive built myself.

These are the layouts i used:
SHO: https://2.bp.blogspot.com/-iG7wRTYtG0U/UR1l_HsMnEI/AAAAAAAAE4Y/h_oe9Dm-7lw/s1600/ZVex+Super+Hard+On+-+Compact+Layout.png
COT50: http://3.bp.blogspot.com/_y7gcfRKQFgA/TDbtopL8rPI/AAAAAAAAALk/Rt8ek2TUz_Y/s1600/Lovepedal+COT50-794239.png

Yes ive read something like that also with the design being flawed, and the COT50 is a bit similar to the SHO with the BIAS knob, maybe thats where the problem comes from. Thanks for the tip with the resistor on the pot, but i dont understand exactly which lug on the pot to connect it to?

/Mac

[Fancy Lime]

Hi Mac,

ok, thanks. First: did you socket the transistors? I suggest to always do that, especially for MOSFETS. They are a bit sensitive, so when you "solder around" on your board to fix problems, it is best to take them out first. If you haven't socketed them, never mind, but be aware that you might fry them and they might need to be replaced in the end.

The Gain-limiting resistor goes between the gain pot and one of the points where the pot is attached to the board (top right or bottom right). It doesn't matter which point you use because it is in series with the pot in both cases, which is all that matters. For practical reasons I would solder the resistor to the pot and than attach the wire that comes from the board to the other end of the pot. I would suggest trying something like a 220Ω or 330Ω resistor. You will loose a little bit of maximum gain. The Problem with the design is, that at max gain without the extra resistor, the transistor "stands directly on the ground". In this situation some parameters of the transistor become important that are very variable (meaning different between individual transistors of the same kind) because transistor production processes (especially JFETS and MOSFETS) are surprisingly inconsistent (because tiny differences in the processing have a huge effect).

Have a look at the Soulsonic Crackle Not OK: http://tagboardeffects.blogspot.fi/2014/03/soulsonic-crackle-not-ok.html
This design addresses exactly your problems by changing the gain control and lowering the 10M resistors to 2.2M. I would suggest before you try to fix the SHO, try building the Crackle Not OK and compare them. Very educational exercise 

I have to look at the Lovepedal some more but it seems that increasing the 47Ω resistor to something like 100 or 150Ω might help. I have never built this though, so its best to wait for someone to chime in who has.

Cheers and good luck,
Andy

[MacAttack]

I did socket the MOSFET in the SHO actually, but i dont think thats the problem here.

Thanks for that, maybe ill try it in the future, and i think i dont wanna build another one like these, im pretty happy with them except that noise thing, not a big problem for me though as i dont usually use the effects at maximum gain.  Just frustrating that its not working to 100%.

And also i just bredaboarded a simple fuzz that have the same problem as the cot50 and sho with the same noise! Starting to wonder that i have another problem on my hands maybe? I also have built a lpb-1 and a stratoblaster that do not have these noise issues, sigh...  No other clues what could be causing the noise?

Thanks
Mac