FLANGER (weird hiss issue)

[deadastronaut]

no can do, its on pcb...

i thought about the virgin sacrifice, but no joy round here....

[anotherjim]

Ok, I think it might be due to the high impedance of the opamp input and the resistors & tracks acting like antennae.
See if a 1458 opamp makes a difference.

[danfrank]

In reply to post number 138 of the OP... If that speed switch is the one you're talking about with a "center off" position, you can't leave the inverting input of TL062B just "hanging", connected to nothing when the switch is in center off position. You need to tie it to pin 5 (non-inverting input) so it doesn't "float in the wind".

[anotherjim]

^Yes, of course!
I've been thinking of how the inverting input can be referenced in the "off" condition without loading down the input resistors - well that 4.7M anyway. Ideally, a 47M between pins 5 & 6; but 47M is silly and maybe hard to get and would still be prone to interference. 20M might be enough to do it.
Alternatively, rescale the resistors 1/10th and the cap x10.
So...
R10 =470k
R24 =47k
C10 =1uF NP (film or *mlcc might fit)
and...
R101 =4M7 between pins 5 & 6 or 2 as it's the same connection.
* mlcc? It is only an LFO.