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Shoog 28th October 2006 05:12 PM

Rising hum when turntable is plugged into preamp ??
 
Hi all,

I've been spending some time tracking down those small hums which really annoy if left alone (like an itch that need scratching). I've managed to get my preamp and my power amp whisper quiet.
Yesterday I decided to try my hand at my Phono preamp. Its the preamp discussed by Theorsten in the "DC phono" thread.

http://www.diyaudio.com/forums/showt...8957&highlight

I have done various things and improved things quite a bit (after making things considerably worse for a stressful while).
When I run the preamp with no input I get a bit of hum - which I attribute to the residual hum of the preamp. Then with the source deck plugged in there is a fraction more hum. The strange thing is that when I plug the turntable in the preamp becomes absolutely quiet, then over the next 30 seconds or so the hum slowly ramps up until it is back at normal levels. Its as if plugging the turntable in discharges a cap which then charges up again over the next 30secs.
It could be a slight difference in earth potential between the phono preamp and the deck causing some effect.


Anyone experienced this and can offer an explanation - if not a solution.

Thanks
Shoog

Gold_xyz 28th October 2006 09:37 PM

IMHO try the DC filament supply and eliminate every ground loop.
also shield all magnetic flow from mains transformer.
also try to uncouple power supply of every tube.

(I need a lot of lessons of English :dead: )

Geek 28th October 2006 10:35 PM

Where the turntable is situated can make difference too. Last winter, I noticed the hum shot up a lot as the electric heater turned on. EMF being induced from the power wires in the adjacent wall were causing this. I moved the turntable and the hum disappeared :)

Tweeker 29th October 2006 05:06 AM

Quote:

Where the turntable is situated can make difference too. Last winter, I noticed the hum shot up a lot as the electric heater turned on. EMF being induced from the power wires in the adjacent wall were causing this. I moved the turntable and the hum disappeared :)
No no no! You need to tear out all the walls and rewire your house with twisted pair. ;)

Geek 29th October 2006 07:19 AM

Hi Tweeker,

Quote:

Originally posted by Tweeker

No no no! You need to tear out all the walls and rewire your house with twisted pair. ;)

My roof leaks and I have water in the walls, you have no idea how probable your joke is :D

Shoog 29th October 2006 07:47 AM

The whole phono preamp is built within two PC power supply cases strapped together, one for the power supply and one for the circuit.
Thinking about grounding a phono preamp takes you to a whole different level than a line stage.
Yesterday I seperated the earth return for the final stage filtering caps from the first stage. That helped quite a bit. I am going to create two star grounds for the two stages, using the valve socket as the final star point. Its almost in this configuration, but needs a slight bit of tidying up.
Filament DC supply shows a nearly flat response, but I suppose there could be current pulsing which wouldn't show up on the scope. I may try lifting its potential a tad and see if that helps .
Still no explanation for the rising hum when the turntable is plugged in.

Shoog

Shoog 31st October 2006 01:41 PM

I have tracked down and eliminated all sources of hum except for one.
There is a low clean 50hz mains hum which I suspect is been picked up off the case. If I cup my hand around the ECC83 the hum is gone, take my hand away and it comes back.Seems as if my hand is working as a screen/drain.

Can't for the life of me work out a perminent fix for this hum - any ideas.

Shoog

Sherman 31st October 2006 02:00 PM

Is the ECC83 inside a shield? If not that is the next obvious thing to try.

Are your two chassis connected with a ground wire? Not one that is part of the circuit, just a wire from one metal case to the other. If not does connecting the two cases increase or decrease the hum?

Also how far apart are the chassis and what type of connection are you using to take power from the PS to the pre?

Shoog 31st October 2006 05:25 PM

The two chassis are bolted together. There is an earth strap between the two chassis, tied at the earth reference point for the turntable.

The ECC83 isn't shielded, and I was think that that was an obvious thing to try, though unfortunately it requires a fairly big rebuild.

I just tried an earth strap from the chassis next to the ECC83 socket to the main star ground, this has improved the hand hum situation a great deal.

I am coming the conclusion that the transformer I am using is c*&p and its radiating leakage inductance all over the case. This is bleeding right across to the valve sockets and causing most of the problem. The conclusion is that I need a new transformer and a bigger case for better isolation. As it is I think I am wasting my time trying to get it quiet.

Thanks all.

Shoog

mark845 1st November 2006 03:30 AM

Turntable hum problems
 
Dear Shoog,

Although others have suggested this also, you should definitely make sure that the ground wire on the turntable is making contact with both the phono pre-amplifier ground point AND the turntable platten. On my DIY Seduction phono pre-amp if I disconnect the ground wire, I get considerable hum. I found that ground wire from my Technics turntable had lost contact with the turntable platten.

mark845


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