Nasty hum on phono3S

[bikerhifinut]

Just quickly metered the psu and phono 3.
The phono3 earth is connected to case and signal earth.

The PSU3 like wise has no Earth lift, case and HT- are at mains earth potential.

Could this be a cause?

And is it easy to rectify by dint of a 10 ohm resistor and 0.1uF cap like my WD88VA is between mains earth and signal earth? I seem to remember the resistor went to signal earth/HT- from the back of the earth post with the capacitor in // with it. And If I did that do I assume that I can leave the PHONO3 left as is with the earth post at signal earth potential?

Andy.

[bob orbell]

Andy, I think that the earth lift resistor is R5 in PSU3 and is 100 (ONE HUNDRED) ohms not 10R, mine is almost silent on both MM & MC with or without any input connections. My only thought on the phono is, has the builder connected all six phono socket ground tags together? I do believe that the four inputs are connected together and the outputs are separate, maybe someone can confirm this. BOB

[Richard]

Yes as standard signal ground is lifted 100R inside PSU, using a resistor in line of the earth lead and pcb earth tag, at the pcb end.

You can check much of it with a meter set on 200R but probably not the phono sockets Bob mentions as their grounds will be connected on the pcb. Like him I've built 3 and 3S and they are completely hum free, just a little thermal noise.

Bear in mind there are various ways of arranging the earth paths during a build, I did mine differently, but we're just talking standard build here so far.

Let me know if you need the build sheets and I'll email them.

[bikerhifinut]

Think I have cracked it!
Its definitely an earth loop.

I temporarily removed the earth wire from the Mains lead to PSU3 and hey presto an immense reduction in hum. there's still a bit of residual noise but some of these MM cartridges can pick up a fair bit of 'um Goldring G1042's spring to mind here, mine was always noisier than the AT clones I used before (LinnK9, AT95E).

So I think I won't need to return it to the shop, I shall investigate the resistor 100R. I can't say whether it was there or not, I certainly don't remember seeing one when I peered through the case.

And yes, I do know the hazards of not having metal cases protected to earth.

Although none of my Rega kit is Earthed, its all double insulated. Not an option on the WD stuff with perforated cases.

watch this space and very much thanks for allowing me to rant and bounce ideas.

Andy

[Greg.]

Hi Andy,

I had a loan of a Rega cartridge once and found I got increasing hum the closer the cartridge got to the centre spindle and therefore motor on my Garrard 401.

Greg

[bikerhifinut]

Originally Posted by Greg.

Hi Andy,

I had a loan of a Rega cartridge once and found I got increasing hum the closer the cartridge got to the centre spindle and therefore motor on my Garrard 401.

Greg

Sounds about right, I dont have that problem on the GyroSE but in my Linn Axis days I did notice the diffenence between MC and MM cartridges and different MM carts. I have no experience of Rega's own cartridges but I found the Goldring1000 series I have was a s*d for picking up hum whereas the AT stuff including the Linn cartridges which were badge engineered AT's were much quieter. They had metal bodies rather than the plastic moulded one of the Goldring.

Anyway I'm a happy Bunny now. As long as it's only me using it, I did wonder about just running a length of Green/Yellow wire between the earth posts of the WD88VA and the PSU.

Another thing has just sprung to mind, I have been using the LEAK Stereo20 and that is most definitely earthed to the chassis. I wonder..................
Chasing one's own tail is a bind but sometimes even those of us who should know better get caught out.

I'm not keen on butchering the PSU until the warranty period is up just in case it does develop a fault. So may just settle for the external earthing wire until then. I can't see any real safety issues that way and my gear is always switched off and unplugged when not in use.

Thanks again Gents, you sent me along the correct route to diagnosis.

Andy.

[bikerhifinut]

I opened the psu case up and sure enough theres no ground lift resistor everything is earthed back to the earth pin on the IEC socket.

The easiest way would be to disconnect the earth wire on the pcb and solder a 100R resistor in series there. I guess that would do the trick.
Aha! I just noticed R5 marked case gnd at the back of the board, It's never been connected up. Easy peasy!

[Greg.]

Good but still consider your warranty. Would it not make sense to discuss what you have found with your retailer before you get the iron in there?

[bikerhifinut]

Bit late for that now. ooops.
A couple of minutes with the soldering iron and the Ground lift resistor is now in circuit and the result is not a lot short of miraculous!
Who'd have thought 100 ohms of resistance would transform the beastie?

I don't think this can have been a WD build, I can't see their staff making that sort of error?

Anyway all's well, its wired correctly and what hum there is is what the AT95E is picking up. With no arm plugged in the only noises are a bit of valve Hiss.

I've plugged in my nearly new NOS Amperex ECC83's (Dutch made so i suspect phillips/mulllard manufacture?) with a Jan/Philips 5751 in the cf as per Bob's suggestion. So far so nice.................... YAY!