In article , Sumatriptan
wrote:

I tried a 10 Ohm resistor and it did give a small reduction of around 1
dB according to Audacity, so I will keep it. Every bit helps.

I also tried an (enormous 500VA) isolating transformer in the turntable
mains feed and this reduced the hum by quite a large amount.

Total reduction in hum from the 3 changes: ground lift, shielded
isolation transformer and electrically shortened turntable leads amounts
to around 10 dB overall reduction of the 50Hz component. The isolation
transformer contributed much larger reductions of the more troublesome
higher order harmonics and general hash. Some were eliminated entirely.
Compare the original he

http://www.nu-ware.com/Misc/Screenshots/Dual505A.png

With this:

http://www.nu-ware.com/Misc/Screenshots/Dual505B.png

Interesting. What has struck me is how much your initial 'dirty' hum looks
a lot like the kind of pickup you can get simply by touching the probe of a
high impedance scope input.

I'm wondering if in Germany[1] the domestic mains system doesn't have the
UK's 'unbalanced' line-neutral. i.e. the 505 isn't designed to cope with a
situation where the line and neutral return both have non zero voltages but
with values that differ. I'm also wondering of you have a fair bit of 'dc'
on your mains. That can upset mains transformers that aren't well specified
and enhance hum harmonics.

Many years ago I went though a period where I and others at Armstrong took
home a wooden board and assorted transformers each night to try them out
and assess the *mechanical* buzz they made when presented with mains power
waveform/size/line-neutral imbalance, etc. They varies a lot from one
design to another, and from place to place and time to time. Alas, many
transformer makers seemed to have no interest in this. Ditto for good
sheilding.

cf comments in another posting.

Jim

[1] I'm assuming Dual is German, but my memory may be wrong here as I've
never used or owned one.

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