Split rail PS

[Dave Plowman (News)]

I'd expect to see a level reduction on that channel if it were one legged.

Since both are the same it would be two identical faults?

The output *impedance* is quoted as 2 ohms by the maker.

[Archibald]

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

[Dave Plowman (News)]

Normally if that's the case simply moving the arm will alter the hum. It's the same in both the workshop and living room, so it's not a local thing.

Since there's no hum at all with the input to the pre-amp disconnected I'm fairly sure the PS is ok.

[charles]

is the pickup, in some way, loading the PSU?

[meow2222]

Well, I don't know the solution in this case. But I know one thing. A low o utput 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 standa rd 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 picku ps 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'r e 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 hea dshell should be screened somehow - perhaps a little copper foil cover.

If absolutely all attempts fail, you might just convert the place to dc mai ns :)

NT

[tony sayer]

In article , Dave Plowman (News) scribeth thus

Right..

You say this amp is a "balanced" input is that transformers or electronically balanced?.

Either way what happens when you simply short out the inputs with a total short then say 10 ohms then 100, any reduction or rise in the "hum" level?. Do that at the input terminals presumably XLR or similar then repeat the same on the cartridge leads on the SME arm where the cart goes.

Of course each lead from each pickup coil left and right that is, is connected via two isolated from any ground leads is it not?..

[Dave Plowman (News)]

I used it before with the supplied transformer which went at the amp end. There was some hum - but about par for the course with any pickup. Better than some, like the Decca FFSS.

I suppose the more the number of turns, the greater the mass.

The shell simply grounds to the arm. There is a separate ground wire which comes out of the arm. I dunno where that is connected inside the arm - but adding another makes no difference.

The leads within the shell are unscreened. But if they were picking up the hum I'd expect moving the pickup around to alter the hum?

[Dave Plowman (News)]

Electronic. Uses an SSM 2017 at the front end.

No XLRs. The pre-amp board is mounted only a few inches from the pickup arm. The SME arm has a plug and socket connector, which is hard wired to the PCB. Unplug that, and the hum totally disappears. No need to provide a dummy load.

Each coil goes to a balanced input. No other connection.

What I might try is removing the internal wiring to the arm, and see if it is twisted. I suspect not.

[charles]

I never had hum with my FFSS.

[tony sayer]

In article , Dave Plowman (News) scribeth thus

OK...

Well in the process of elimination it would be a good idea to try that just in case that isn't as "Balanced" as it ought to be.

Then connect the pick-up by a short, well a few feet of balanced twin screened, just wrap the inner wire very lightly round the pins. That then should also be quiet at those frequencies at least then move that around to see if its inducting a magnetic field from anywhere if it is then sort that, if not then do check that the wiring of the SME arm is exactly as it ought be..

Don't earth any of the 2017 input leads from that the only earth should be a single lead from the arm back to a "tech" earth;)....

Shouldn't really matter..

[Dave Plowman (News)]

With the usual Garrard 301?

[charles]

what else? But I was told, by a man from Garrard at a Radio Show, to run a

60w lightbulb in series with the motor to reduce its magnetic field.

[Dave Plowman (News)]

It reduced but not eliminated it here.

[charles]

In those days, I had good hearing and I couldn't hear any.

[Dave Plowman (News)]

Had a bit more of a play. Disconneced the cartridge - hum goes. So it's apparently being induced into the cartridge itself, rather than wiring. No large transformers anywhere near - and it's more or less the same on the workbench, in a different room. Next thing to try is removing the mains to the turntable, I suppose.

[Dave Plowman (News)]

I'd imagine your hearing is still ok for mains hum? ;-)

I wouldn't say it was audible in normal use but definitely there is you whacked the wick up while not playing a record.

[charles]

I can certainly hear it when it turns up on sound sysytems.

I remember an alleged problem when RDS first came out. Went to see someone near Wrotham, but on the back of a hill. He turned up the volume on one of R3 long silent pauses to demonstrate, the next minute we were blown out of our seats by the opening chord of something.

[tony sayer]

In article , Dave Plowman (News) scribeth thus

Dave.. seriously suggest a small value resistor connected across the inputs and that via a short bit of twin screened and see if the hum is still there or not.

Then connect the cart to the same cable and move that around it shouldn't under normal conditions "hum" unless its picking that up from a transformer or motor. If so then you should be able to track that easily.

In a way this is much the same as a moving coil microphone and they can and do as I'm sure you know be used on rather long leads....

[Dave Plowman (News)]

Had a play this morning before reading this. Turns out to have been caused by the transformer in the PS - even although it's a weedy 6VA and 600mm or so from the pickup. Nor does the hum change when moving the pickup around

- which is why I didn't go for this before. But running the pre-amp from an external PS removes the hum totally. (the hum and noise on the output of the original PS and external one are near identical on the scope)

I've ordered up a toroidal transformer and Eddystone box to put it in from CPC. Still free carriage. ;-)

[tony sayer]

In article , Dave Plowman (News) scribeth thus

A Toriod tranny?, best bet for hum field reduction. Make sure there no shorted turns thru it;)..