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

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 .

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

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

@filipm you have 3 tubes? And the noise is in both channels? I’m assuming one of the 12ax7 tubes is a rectifier? I would just replace that one to start with. 

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

Well there may be other tube units that won’t have the same issue. Also, pretty sure you can get a good solid state phono amp that would equal or beat your EAR in performance and noise rejection.