On Mon, 26 Aug 2013 14:23:13 +0000 (UTC),
(Andrew Gabriel) wrote:

In article ,
"Dave Plowman (News)" writes:
In article ,
Andrew Gabriel wrote:
In article ,
"Dave Plowman (News)" writes:

Seems the small amount of hum left is coming from the actual cartridge
or wiring to it within the arm. (It's an SME arm) Unplug the connector
at the base of the arm and the hum goes - only a reasonable amount of
hiss left, with the gain full up. Fine at any listening level.

Try connecting just the ground side of the cartridge, and see if that
causes any hum. Could also try fully connecting it, but with the
cartridge output shorted at source.

There's no actual ground side as the pre-amp has a balanced input. The

Ah, that's whay the screening doesn't matter much.
However, it opens up the possibility for some other faults around
the balanced inputs having gone wrong. Check none of them have got
one side shorted to ground in the wiring or connectors. It could also
be that one side of a balanced input circuit has failed.

pre-amp is mounted as close to the arm as possible. The cart has an output
impedance of 2 ohms so as near a short as possible. ;-) But I'll have a
play.

Still worth testing with the outputs shorted - that might identify
a faulty balanced input stage. (BTW, I expect it's the resistance
which is 2 ohms - I'm surprised it's that low, but I hope the
impedance is somewhat higher.)

It sounds like you have a magnetic field interfering with the
cartrige. Try sticking the end of the arm into a grounded empty baked
bean tin. Then if it diminishes the hum see what electrics have been
added lately.

The SME arm is aluminium or an alloy thereof as far as I can recall,
not the best magnetic screen.

If the PSU is still suspect incidentally I would be tempted to try a
smooth DC supply and a couple of 15V Voltage converters. Not sure of
the switching frequency though, you may be swapping 50Hz for 20kHz :-)

AB