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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

filipm
filipm's avatar
filipm OP
55 posts
 

So we powered the tube phono preamp with this battery. There was no change at all. Some added 50hz hum as we were messing around with the grounding. 

I can now say for sure it's coming from the air. 

@sbank Hey Spencer! :) When I read your posts I though to myself, well, I won't give up on my tube phono. I have just realized I haven't heard a vinyl for about two months and I don't think I can do two more. 

I am using really shitty power cables so my next thought is they might not be properly shielded. I scratched this option off when it was more and more obvious the rca cables and the input shows some changes. Plugging in any RCA cables even well shielded definitely extends the antenna effect.

 

I'll try to borrow better cables. Even though at my friend's place we used the cheapest crap we could find and it was absolutely quiet. 

I'm gonna ask my neighbor in the next building if I could test it out at his place. 

Then I'll buy the tubes.

About the tube shields - can they be substituted with several layers of aluminum foil? Just to see if there's any effect. It would take forever to obtain the tube shields from ebay.

 

ghdprentice's avatar
ghdprentice
9,492 posts
 

OP, wow. What an journey. Maybe it is time to start wondering if there is a problem with that particular Phono stage… you have conducted so many tests. 

lewm
14,701 posts
 

There is no need to buy tube shields from eBay. There are several reputable vendors in the US who sell them, and will probably help you out with installation. One business is tubes and more. Another business is antique electronics in Arizona. Sure, you could wrap your entire unit in heavy duty foil. See if that helps. Probably you want to ground the foil in some way.. 

The foil must completely envelop the unit, top and bottom, sides and rear.

 

theaudioamp
221 posts
 

Yes aluminum foil would be fine as a shield.

filipm's avatar
filipm OP
55 posts
 

@lewm I think what I have done with the baking pot proves that I'd have to use a sleeve for the cables as well. I have used 20 feet XLR cables to reach the other side of the room, closer to the router. Then I placed the router right next to the phono stage and put the phono preamp and the cables inside. Problem solved. Except it's not the most practical solution. :) The pot wasn't grounded but most of the cables must be inside. Once they pop out, they pick up the interference. 

 

There's actually a hint - even the output cables react to the interference. When I squeeze them, the noise changes. 

Let's see what an aluminum foil on the tubes does..

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