WEM PA100 DC Offset

[Loudthud]

Check the ripple on the filter caps. Is it 100Hz as it should be, or 50Hz if the amp is operating as two half wave power supplies.

[stratfordade]

J M Fahey thank you for your analysis and experience which has probably saved me chasing an incurable hum forever!

I did think the layout prone to interference but didn't know about the supply peak charging issue but your explanation is clear. The only thing in WEM's defence is that sensitive input 1" from mains switch is actually a slave in parallel with preamp output so may be a little more robust? But yes the filter caps are connected by a thin unscreened wire travelling right next to the big transformer!

Thanks very much for detailed and helpful post. If it's all inherent to the design then I'll button this up (and see if David Gilmore wants to add it to his collection).

[stratfordade]

Check the ripple on the filter caps. Is it 100Hz as it should be, or 50Hz if the amp is operating as two half wave power supplies.

The ripple (about 0.6V) is 100Hz

[stratfordade]

I've managed to reduce the volume and character of the noise (from obvious buzz to quieter, discreet hum) by grounding the hitherto ungrounded metal front panel. It has some sort of insulating varnish layer on it so I removed one of the Treble Boost slide switches (top left of photo) and scraped a little off with sandpaper around a screw hole. Replaced the screw but with a tag sandwiched between switch and now conducting area and wired the tag to one of the preamp signal grounds from input socket. It's a massive improvement!  A real pleasure to play through t now.