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. 




@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.

@theaudioamp That's possible but the noise if being picked up even when the turntable was unplugged from everything. The signal was just weaker. I can plug any RCA cable to the input of the phono stage to get back the strength of the signal, doesn't matter if the turntable is plugged in on the other side. That's the most confusing. 

I'll know more next week though. Different tubes, I'll do tests around the building with more practical intergrated amp (I'm using separates)  and I'll have the two closest neighbors' wifi switched off.


@filipm , an underlying 50Hz, with a 2KHz packet rate is not WiFi. WiFi is rarely an issue with interference into low frequency analog electronics. The 2.4GHz signal is fast enough that most "things" cannot demodulate the frequency, and the modulation method also helps. I strongly suspect it is something else, though it could be 2 somethings, the 50Hz from building wiring, and the 2Khz from something else. Some home automation could be close enough to a 2KHz tone.


@filipm can you please do the following if you have a multimeter. Measure the resistance from the signal ground on the Phono input to the metal case of the box.


CABLES: I don’t know all the cables you tried, but you don’t need BS marketing crap like how the AQ Mackenzie is described. You need good old fashioned doubled shielded coax. Ignore all the marketing crap and use something that is designed using proper engineering principles like the Blue Jean LC-1.   A cheap $10.00 coaxial 75 ohm video cable is likely to work better than that AQ crap for rejecting RF noise.


If adding the RCA cable (without the turntable) brings back the noise completely that tells me that your probably not using coax and/or (could be both), there is no connection from the signal ground to the metal enclosure. There should be a connection, at least capacitive between the two, so that the "cage" is unbroken and so that the potential (voltage) between the two is the same at RF frequencies so that no noise can transfer between the two.



Although I agree with most of what @theaudioamp has suggested, I'm yet to be convinced this is actually the fault of the phono section, even though the phono section is clearly able to amplify the noise.

First off the 12AX7s have nothing to do with this. They are not somehow inherently sensitive to RFI. How their grid circuits are set up is an entirely different matter...

The length and exposure of the 20 foot XLR cables is a clue.

I think that what is happening is there is noise being picked up and injected into the ground of the ARC (which plays the same sound at a much lower level...). Since the ARC does not support the balanced standard (even though otherwise is balanced) a noise is manifest.

To sort out what is going on first short the inputs of the phono section to be sure its OK. Removing the top cover and no additional noise points to the idea that the phono section really isn't the culprit. But if shorting the phono inputs shuts it up then the phono section needs mods to protect it from demodulating RF (demodulation is what causes nearly all RFI problems). I've seen this sort of thing caused by poor tonearm cables too, but if the phono section is adequately protected from RFI its usually no concern.

If shorting the inputs has no effect I would be trying to use different interconnect cables. A lot of 'high end audio' manufacturers have a poor grasp of how balanced line is supposed to work so I would be trying a cheap microphone cable like you can get at Guitar Center since such a cable will be wired correctly and will also have an effective shield. Note that this cable is between the preamp and amp; the idea here is that noise is being injected elsewhere from the phono section but is able to be amplified by the phono section; the noise is in the grounds of the preamp and is being passed to the phono section.