Tube PHONO preamp interference - RFI, EMI, bad grounding?


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. 




@filipm ,


The dominant signal in the recordings you posted is 50Hz. It is much larger than any other frequency component. I don't know if that is due to noise in your system, the recording, or it is a demodulated frequency from the noise you are experiencing.



The bursts are a packet rate of what looks like 2KHz, which is close to what you would get from GSM, but not close enough. (1.7KHz versus 2KHz).  It is bang on 2KHz.



Maybe there is a wickedly noisy power supply in your vicinity and the RFI in in the building wiring.

Although I don’t recommend tube rolling in order to improve sound quality, in this case it might be worthwhile to change all the tubes in the unit. It could be that one or more tubes is defective, thereby enhancing susceptibility to the noise. Even if that did not help, there is also the possibility that there is a cold solder joint somewhere in the chassis, possibly in a connection to a tube socket. So I would try the inexpensive and simple things first like shorting the unused inputs, and shielding the unit, and shielding the cables, but if that does not work you might want to take it to a knowledgeable technician who can measure the voltages. 

@theaudioamp thanks, this is nice. 50hz means a ground loop, right?

the system is overall quiet with the other inputs. there's only slight white noise on the DAC's input, audible only when I'm really close to a speaker and at high volume levels.

If it was in the building wiring the noise would be gone with that battery we have tried here. 

@lewm I'll have a different set of tubes next week. It's gonna take a while to get the shorting plugs.

I still think the EAR preamp is not defective simply because it works well anywhere else I have taken it. This must be an issue of the location. Soon I'll bring a simple integrated and plug it in in the hallway two floors lower. 



@filipm ,


I am suggesting that your phono stage is picking up RF that is being radiated through the building wiring. using the inverter would not change that. Though I assume that your turntable was unplugged from the AC and/or unplugged from your phono stage when you did that test.