WD88VA - hiss, hum and interference

[wiggers]

I have a WD88VA factory-built for me in 2008 that I use for the treble in a bi-amped setup. A few months after I got it a capacitor blew, taking out a valve, which John Caswell kindly fixed for me. It's been fine since then.

Just recently, however, the hiss has increased a bit and I've started picking up interference from a PowerLine ethernet device nearby. The mains for the WD88VA comes through a UPS with an AC-DC-AC converter, so it isn't getting in that way. It only seems to be on the left channel and it is independent of the volume setting. I've swapped the valves around and cleaned the contacts but it doesn't make any difference.

I've also started getting a noticeable hum from the second (solid state) amp in my bi-amped setup but only after I've turned on the WD88VA. The SS amp has recently been serviced so I'm a bit suspicious they mucked something up, but the retailer swears it was all fine when he gave it back to me.

Any thoughts on how I can work out what's happening?

[Richard]

Hi Wiggers and welcome

These things can be strange and it may be airborne interference such as we get dialing a mobile near a computer. Many of us too will have heard Long Wave foreign radio stations playing very quietly with an ear near the speaker late in the evening. It may be getting in through the inputs or possibly even the speaker leads.

Don't discount the mains though either as for a long time I had to live with a regular buzz from my next door neighbours old air-blown central heating kicking in. After 20+ years that's finally gone now with new owners and a new heating system!

I'd start by using the power amps separately, hook in just the WD88 as a full range amp, then the solid state, and take it from there.

[John Caswell]

Hi Wiggers,
First things first, remove the inputs to the amp and fit phono plugs with something like a 100R resistor across centre to earth. Does the interference still exist. If not suspect pickup on the source devices and/or wiring
I am suspicious of the UPS as they use switching devices to convert the DC to AC then a step-up Tx to the required voltage.
Remove this from circuit and try again.
I have a lathe whose motor runs on a 1 phase to 3 phase convertor. When powered up it affects FM reception drastically.
Let us know how you get on

John

[wiggers]

Thanks for the suggestions. I think I've tracked the problem down. When I reconnected everything the other day I accidentally ran the left speaker cables close to the mains lead for a gang socket that has the PowerLine receiver on it. Moving the speaker cables away has reduced the interference significantly, now I need to reroute it much further away.

BTW, I've been very happy with the UPS and it has been working well for several years.

[Richard]

Hi, that's good, the amp has a global feedback loop connected at the speaker cable terminals so I think that could happen if the cable is near an interference source.