In article ,
wrote:
On Monday, August 26, 2013 12:26:06 AM UTC+1, Dave Plowman (News) wrote:

It's the PS for a fairly esoteric moving coil pickup pre-amp which
uses balanced inputs. The quiescent current at least is very similar
on each rail. Basically was chasing the last residue of hum, and
discovered (by accident) if I disconnected the transformer centre tap
it improved. The scope showed the hum to be more on the negative rail.
Changing the neg reg to a different make improved things to the point
I doubt I'll better.

Sounds like a symmetrical load. Adding resistors can help deal with
small imbalance. For low current loads like that, passive power rail
filtering is often enough, and can often help. IME though poor grounding
layout and lack of IC decoupling are as often the problem with homemade
preamps.



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.

The arm assembly is mounted on wood so it's not a ground loop. Moving the
arm over the deck makes no difference to the hum - so I'd guess it isn't
coming from the motor. The wiring within the arm is single wires - but
then the arm itself should screen them? I've checked it is grounded to my
star ground point.

--
*When cheese gets it's picture taken, what does it say?

Dave Plowman London SW
To e-mail, change noise into sound.