Hum from Pre3 Cathode/Follower and WDKT88

[G Willis]

I have upset something moving my equipment. Since the move I have been having a problem with hum from my WD-KT88XL and Pre3 CF. The hum comes and goes.

Powering up today I am hearing a steady hum on both channels which sounds like a combination of G2 and G3 on a piano. G2 is 97Hz so I think given my imperfect pitch this is mains hum.

With my pre-amp powered down a louder, lower 50Hz hum predominates. Switching the pre back on, the 50Hz the hum diminishes and the higher hum is heard again.

The volume or input selector of the pre-amplifier has no effect on the hum.

Unplugging the interconnects causes the hum to almost vanish – there is some transformer noise which is audible over a quiet 50Hz mains hum. I think this is about the same level of quiescent noise produced by the equipment before the move.

Powering down the power amplifier causes the noise to momentarily become louder.

With my DAC plugged directly into the power-amp using the same interconnect cables there is no hum above the normal quiescent level. (It has a built in pre-amplifier)

Any ideas of how to go about looking for this? The obvious culprit is the CF but I cannot see anything wrong with it. I understand that earth loops are caused by two connected devices having different ground potentials but I am not sure how this could arise.

[bob orbell]

It is possible that the umbilical lead is not making a good all round connection, try a wiggle test here. Bob

[Phil Y]

Hi Graham,

You have moved your gear you say, so are you using different mains sockets to before or perhaps a different extension lead? A poor earth connection is a possible cause, check things over.
Earth loops are in deed caused by earth currents, something as simple as using 2 sockets on opposite sides of the room on your system can cause problems.
Earth loop problems can be infuriating, being methodical is the only way.

Does either the amp or Pre have an earth lift fitted?

Phil.

[G Willis]

Thanks both. I tried wiggling the umbilical to no avail. I continuity tested it and it seems good. I also visually inspected the PSU an that seems fine.

I did measure about 0.5v AC difference between the ground on the CF and the ground on the power amplifier. I don't know if that means anything.

I unplugged everything and started turning things on one by one but the problem seems to have gone away, at least temporarily.

Is it a good idea to link the groundings of the power amplifier to the pre-amplifier or will having two earth paths potentially make the situation worst?

[G Willis]

I connected the two amplifiers ground posts together and noise decreased

[G Willis]

All red herrings. It turned out to be the bloody kitchen light. I shall have to ban its use. Any recommendations for mains conditioning etc?

[bikerhifinut]

Originally Posted by G Willis

All red herrings. It turned out to be the bloody kitchen light. I shall have to ban its use. Any recommendations for mains conditioning etc?

Check the kitchen light and its associated wiring would seem to me to be the first thing if all other possibilities have been eliminated. You may have detected a fault that might be safety related. Or just a duff light fitting. Especially in these days of cheap Chinese LED lighting using borderline rated SMPS.
Switching a light on should not cause any piece of competently designed and constructed audio equipment to show any audible hum. Or for that matter any measurable hum.

A.