Splatty distortion

[bagu]

Hi, I was modding a DOD 250 overdrive pedal and am getting some splatty sounds when i turn up the gain. Here's a sample of the bad distortion i'm getting:

http://www.angelfire.com/al4/bagu/1.mp3

It only happens when the gain is high. The mods were changing the opamp to OPA2134 and the diodes to IRF520 mosfets. I noticed some clear film(grounding paint?) on the pc board had melted around the points i soldered. Could this be the cause of it? Anyone know what causes this unwanted distortion?

[brett]

Hi.  Sounds a bit like clipping by the op-amp.  Are the diodes definately hooked up and doing their thing?

Hey, I've just had a thought...you're using MOSFETs for diodes?  How can they possibly conduct when they've got a layer of glass between the N and P substrates?  (I am a fool sometimes  :roll:  and there might be some obvious reason why this DOES work.)  But *maybe* you need to be using JFETs instead of MOSFETs.

cheers

[bagu]

Hi Brett, thanks for the reply. I tried changing the opamp back to the stock KA4558 but it didn't help much. The splat was still there although slightly less. Changed the diodes again to germanium to see if it solved the problem but it didn't. I wonder if i overheated any component or burnt a trace on the pcb...

As for using the Mosfets, I know nothing about them too, just followed the instructions on Aron's site to connect G and D and use it as a diode.   Used them before on a Marshall BB-2 and it worked beautifully.