Humming Plexi-Drive

[Frank_NH]

Hi all,

I haven't asked for debug help, but I'd like to get some advice as I've tried some things but the problem persists.

I built a Wampler Plexi-Drive (the original circuit - no trimmers) and it works great except that I'm getting some humming after Q1, specifically right after the 470 pF cap on the Q1 drain.  This point is connected to the gain pot (lug 3) and the 220 pF treble bypass cap.  Nothing I've tried seems to affect the hum.  I checked for connectivity and cold solder joints, though I didn't try reflowing the solder connections.  I check for ground loops (nothing there that I could see).  Replaced the pot.  

I was wondering if the proximity to the 100 uF electro cap would be a cause, or if the 470 pF ceramic disk cap itself is bad.  I haven't tried replacing either yet.  I'd like to hear some opinions before I start resoldering everything.  If nothing works, I'll try another fresh build - this circuit is just too good and my hand-selected JFETs bias great.  

[IvIark]

I'd desolder each in turn and see if it affects the hum.  Either could be a potential source of noise

[Frank_NH]

Thanks IvIark.  I'm suspecting the 470 pF ceramic cap.  May replace it with a 1 nF film cap.  The electro is a high grade audio cap so I hope it's OK, but you never know.  I'll report my finding after playing with it tonight.

[motterpaul]

Frank, I got lucky on this one and it sounds great, no hum  tons of gain, so if you want me to send pics or test anything let me know.

EDIT: 00PS - I thought you said "with trimmers" - I highly recommend the one with trimmers, that was the one I did and it was actually one of easiest to tune in and best sounding pedals I have built to date.

[Frank_NH]

Update - I couldn't eliminate the hum in my original build so I did what any self-respecting DIYer would do - I built another one! (Using my own vero layout no less)

Sadly, some hum remains, but it may be mostly due to the circuit being unboxed at this point.  I'm going to play around with it a bit more, mainly to understand the effects of grounding (i.e. avoiding ground loops).  I'll probably also try shielding the input and output wires since I have some shielded cable.

The Plexi-Drive sounds fabulous so it's definitely a worthwhile build .