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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
 

I meant if I lift the device, the signal gets stronger. If I put it on the floor, it gets weaker. Just like a radio with an antenna.

 

Yes, I have just tried it again - turned of the only wifi router I have, along with a cellphone. It’s very simple over here, I don’t have any tv, more computers etc.

The device works well anywhere else I have tried it so far, that’s why I’m not suspecting the tubes. But I’ll try that. I just didn’t want to buy three more 12AX7 tubes before I try all the other options.

The building has 5 floors, I’m on 4th. The ceilings are 10 feet and the walls fairly thick. It’s in a city center.

I have just moved the stage to the other side of the livingroom, it’s about the same.

The best results are when I leave it on the floor - unplugged. As soon as I plug in any source - a TT, powered DAC without a source or just the RCA cables, it gets stronger.

 

- I’ll try a different turntable, the ferrite rings and then the tubes.

Going back to ss doesn’t sound very appealing :)

// oh, and the model is 834p deluxe without the volume knob. 1996

lewm
14,704 posts
 

I wouldn’t waste money on new tubes. However if the 834 does not already have tube shields I would look into purchasing some aftermarket cylindrical shields designed to block EMI and RFI. Place one over each tube. Well designed ones also improve cooling and dampen microphonics .

esarhaddon
137 posts
 

It may NOT be HAM radio but HAMs have been using PACKET radio for many years. It was just one more place to look for a source of trouble.
Packet Radio : Software for packet radio use - The DXZone.com

theaudioamp
221 posts
 

Learn something new every day! Thanks @esarhaddon 

filipm's avatar
filipm OP
55 posts
 

I was told to try the tubes first, simply because the tubes in the 834p are not exposed. They are covered by a thick piece of metal that should work as a Faraday cage. 

Do you have any tip for the cylidrical shields? I have only found something on AliExpress or Ebay, looks like it's the exact same thing. 

thanks

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