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
>>>I measured 33 ohms between the grounding pin on the chassis and the grounding pin of the inlet.<<< If the piece of equipment you're referring to is the EAR, that figure should be much closer to zero ohms. If you were measuring the resistance of a wire that was maybe 100 feet in length, sure, maybe *that* figure could be 33 ohms, but it shouldn't be when measuring a connection the distance of inches, especially a ground connection. |
You have a very unique setup, where you're mixing balanced (XLR type) and unbalanced (RCA type) pieces of equipment together, and in a situation like that, they don't always want to play nicely. This is when ground loops can take place. Are you familiar with a ground lifter plug? I know we have them in the US, but I don't know if you have them there. I'd be interested in hearing what the EAR would sound like, if you lifted the AC ground to it, while still keeping the TT ground connected to the EAR. |
@filipm the EARs transformer is pretty noisy mechanically at your place; suggesting a power issue. The battery test was interesting and you would think would rule out dirty power, but that may have also just demonstrated the same power issue - have you tried the battery test elsewhere (like at the friends place where the EAR sounds fine)? I suspect the same results. I listened to the noise again - the noise is a pulsing sound over what looks and sounds like a ground buzz. This may be a stretch, but perhaps your EAR's power supply is near end of life and when combined with the transformer issues at your place, is sagging and recovering rapidly, like a motorboat noise. It may not occur elsewhere because the wall power is just better enough there to allow the EAR's power supply to keep up. This would cause the tube circuit to generate the noise in the output regardless of input (search tube motorboating on youtube). Are the filter capacitors in the EAR's power supply original? They are probably over 20 years old at this point if so. I'd take those out and test them or just replace 'em.
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