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Tube PHONO preamp interference - RFI, EMI, bad grounding?

Hello!

My tube phono is picking up interference most probably from the air. It's EAR yoshino 834p, using three 12AX7 tubes. It's sounds pretty amazing and I willing to try everything to keep it. 

Here is a sample of the sound - 

 

The rest of the setup is ARC LS16 mk1, Classe CA200, Chord Qutest, Technics SL1200 with Nagaoka MP200, Tannoys D700

I have tried many things already -

- grounding the phono to the preamp, grounding the phono to a socket, covering the phono with a pot, saucepan - no change

-plugging the phono preamp alone into an integrated (Bryston B60) and removing other stuff.

- the important part is I have taken the phono to two other places and it worked perfectly fine, even with the cheapes cables.

- I haven't had any problems with previous phono preamps which were all solid state. 

- if I unplug the turntable the signal fades to about 50%

- if I try different RCA cables, there's not much of a change even they are shielded (audioquest mackenzie, supra etc.)

- the signal also fades when I grab the cables. Also works if I grab or squeeze the output cables. 

- I have tried to wrap the cables into aluminum foil, I have noticed a difference but it's still unlistenable.

- I have tried pluging in a 5 meters long RCA output cable and walked with the phono preamp around the room. It's simply like carrying an antenna. Placing the phono on the floor helps but again, the interference is still present. 

 

Do you have any suggestions what else to try? Is there some kind of grounding that would prevent the phono preamp acting like an antenna? 

I haven't tried a new set of tubes yet. 

I think the 12AX7 are simply too sensitive to all the mess in the air. The ARC LS16 preamp was catching the same signal very quietly when I took it's cover of. 

Thanks!

Filip

128x128filipm
ejb14
37 posts
 

@lewm - I have followed and appreciated your investigation and effort to make sense of the evidence, and I agree this is strange. The EAR has rave reviews, and it just seems improbable that Tim de Paravicini with his reputation would make this kind of mistake. Perhaps in the repairs to this EAR the OP talked about something wasn't put back together correctly - or it is a manufacturing defect and they got it backwards in production and put the resistor in the wrong place.

My Creek Integrated is wired up like atmasphere and dover were saying is the correct way, and my other phono stages are either battery powered or do not have an IEC third prong (powered by wall wart) - so I have own nothing personally to verify with - but you mentioned the Manley Steelhead -  which in its current iteration has three ground connectors, and they do make a distinction between circuit and chassis ground, with separate ground lugs, and allow the user to make their own choice about the grounding scheme; though I strongly suspect they would not leave the chassis floating in any of them. So I think there is definitely something to what you were talking about at least for some phono stages. 

 

atmasphere's avatar
atmasphere
11,851 posts
 

The EAR has rave reviews, and it just seems improbable that Tim de Paravicini with his reputation would make this kind of mistake.

Tim passed away (RIP) about 2 years ago. The simple explanation is this unit was built incorrectly.

lewm
13,160 posts
 

Indeed, if the 33 ohm resistor was in series between the IEC and the ground lug, on the one hand, and the PCB ground, leaving everything but the PCB containing the audio circuit at earth ground, that would be OK. But I still don't think we've hit upon the primary cause of the noise that the OP recorded and included in his initial post. (I realize Dover said all this before me.)

atmasphere's avatar
atmasphere
11,851 posts
 

@lewm This has to be corrected though. As I pointed out earlier, if the chassis isn't directly grounded it will act as an antenna rather than a shield.

Back in the 70's and 80's in South Dakota where TV stations were rare, farmers used to park an old tractor on a hill near their house and then point their TV antenna at that rather than in the direction of the TV station. They were using the tractor to collect RF energy which is then re-radiated.

We don't yet know if this sound is air borne, but if it is it will be important for the chassis to be grounded. Its also important for electrical shock hazard safety.

dover
2,249 posts
 

@lewm 

Indeed, if the 33 ohm resistor was in series between the IEC and the ground lug, on the one hand, and the PCB ground, 

 

Wrong.

With the EAR the ground lug is connected to the chassis.

The 33 ohm resistor must go directly to the board. - not the ground lug.

The ground lug/chassis should be connected directly to the ground pin with NO 33 ohm resistor in the path.

There is 230 volts floating around - you should be more careful if you are going to give advice on this forum.

 

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