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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
filipm's avatar
filipm OP
55 posts
 

I guess there's nothing that could go wrong making the shorting plugs. I simply cut a piece of cheap RCA cable and connected the signal wire to the shielding wire. It still acts as a regular cable though, basically an antenna. 

I measured it with a multimeter - connecting the signal pin and the shield makes a beep. 

If there's something wrong in the EAR itself, how's that possible it's working everywhere else except my home? 😫

There's only one thing that made a difference so far - covering everything - the unit and the cables, mainly the cables - into a metal pot. Or making a thick aluminum sleeve for the input cables. 

Is it possible that there's an interference between the power cord and the output cables? But again. I tried the same cables at different places. Also, the EAR preamp has been powered with a battery. 

atmasphere's avatar
atmasphere
11,753 posts
 

I would make sure the EAR is grounded via the power cord.

@filipm 

That it makes more noise with the inputs shorted is telling.

Of course you are living in a high RFI zone; that is why everything works fine elsewhere. Could you clarify this comment:

About the ARC - I have tried the same only with a Bryston B60 integrated, the noise has always been there.

Was the ARC in the system with the Bryston and is that with or without the EAR?

filipm's avatar
filipm OP
55 posts
 

@atmasphere 

I bought the EAR before the ARC LS16 and it's been tested with other things, Bryston B60 was one of them. 

I mentioned it because it's a solid state integrated. When I plugged the EAR into the Bryston the noise was still being picked up. 

There's an interesting point about the ARC though. When I changed the tubes (it's mk1), uses 4 pieces of 6922, the noise was picked up by the ARC too. It was just very quiet and only when the cage was opened. That set of tubes was used, I think it was either JJ or the stock electro harmonix. I'll try if I can make the ARC to pick up the noise again. Anyways, I think it was only apparent for about a minute and then it disappeared. 

I'll have a different set of tubes for the EAR on Sunday. 

atmasphere's avatar
atmasphere
11,753 posts
 

@filipm OK. That's 99 44/100% the EAR is at fault. What happens when it is not running off of the battery?

ejb14
37 posts
 

@filipm 

Hmm - perhaps I am missing something but the statement:

"It still acts as a regular cable though, basically an antenna. " - I am not sure is correct once you connect signal to ground. I believe all the energy on the input should be draining to ground at that point. 

I just ran this same test (turns out I also created shorting plugs this way once way back when) with an admittedly quiet solid state phono preamp, and with an wifi router right next to the cut wire ends. When the signal wire was not connected to ground wire, I was picking up all kinds of noise from the router through the phono stage - move the wire further away from the router, the noise got quieter. 

When I twisted the wires together  - absolute silence - nothing, with the wires resting on the router, not even the usual phono stage ocean noise at max volume on the amp. 

I wonder if your ground connection on your input RCAs (internally) are not connected (in which case, the shorting plugs would definitely act like an antenna). I suspect the EAR connects all grounds to a single point from the PCB to the case. Is that connection still good?

 

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