On Monday, August 26, 2013 12:56:39 PM UTC+1, Dave Plowman (News) wrote:
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.

Well, I don't know the solution in this case. But I know one thing. A low output pickup, as all modern ones are, with 2 ohm impedance is giong to be a major nightmare to keep quiet. Such a system will need a far better standard of screening than usual for 50k pickups.

I can only think of one reason to go with 2 ohms, using a ribbon transducer.. Why do that with mc?

I am wondering what the shell of the pickup is connected to. With 50k pickups it goes to one of the ground output pins, with a balanced setup it would need separate connection to amp ground. Does the arm provide for that?

I forget what your screening arrangements are, but I'd want to check they're watertight all over. All cables need a dense screen, maybe foil and braid, audio plugs should be screened, and the leads to the cartridge in the headshell should be screened somehow - perhaps a little copper foil cover.

If absolutely all attempts fail, you might just convert the place to dc mains


NT