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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
 

@ejb14 

Ok, I'll get them as fast as possible. I actually got this recommendation a while ago but I have been trying so many things I have forgotten. Thank you. 

I have tried to find out how the wifi noise sounds like. Are you able to record it? Is it any similiar to what I have posted? (the noise sample on the top of this thread)

If the stoppers help, I'm thinking the RCA cables would have to be placed in a copper sleeve, maybe even two copper sleeves. I'd solder a grounding cable to the sleeves and ground it to either the phono preamp or to a screw on the ARC linestage. 

(I'm only suggesting this from my previous experiment with an aluminum foil - I had wrapped the input cables in several layers of the foil. The result wasn't perfect but it was ok just to see how the phono stage actually sounds in my setup.)

I'll also borrow some cheap integrated amp and a small speaker. There's a socket on the first floor of my apartment building so I'll test it on the hallway. If it won't pick up the noise then I'll do the same on the top floor. That should give me a hint where it's coming from. 

 

ejb14
37 posts
 

Just to be sure - sometimes "stopper" will not have a signal pin (like the AudiioQuest stopper is just an RCA cover with no center pin), but for this test you want shorting plugs with the center pin. You want the signal shorted to the ground so there is no signal on the input (which is also why I said don't put them on the outputs - they will blow the amp on the outputs). 

I no longer have the luxman, and not sure if my current preamp is susceptible -(solid state Sutherland PH3D). I will see if I can try something.

 

theaudioamp
221 posts
 

@filipm 

 

I go back to my original statement. Flawed design or broken. It should not due this. If this happens on both channels, then it is a flawed design. A small capacitor between the signal input and signal ground (or case ground) and the noise will probably disappear.  Cables may make a difference .... or not.

lewm
12,565 posts
 

Aluminum foil is fine for identifying the source of radiation but if it works you’ll need a more practical long term solution, like possibly tube shields or other.

audphile1's avatar
audphile1
4,376 posts
 

At this point the cure is worst than the disease. Dump this phono amp and get one that’s properly designed to not be susceptible to this type of interference at least within your setup. Seriously. You’re going to drive yourself nucking futs. 

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