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

@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. https://www.bluejeanscable.com/store/audio/LC1-design-notes.htm   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.

@filipm 

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.

@atmasphere Thanks for the long answer. First of all, I totally agree it could be a matter of poor wiring (Even though I have tried the same TT and cables elsewhere and it was fine). But it can’t be the XLR part.

Is there a way to short the inputs without the input shorting plugs? (I have ordered them but it’s gonna take a while).

About the ARC - I have tried the same only with a Bryston B60 integrated, the noise has always been there.

The 20ft XLR cables were just an idea so I could move the phono section as far as possible from the speakers (or just to the other side of the room, possibly further from the neighbors wifi router). Otherwise I’m using 3ft Nordost Red dawn XLR between the ARC and the power amp.

@theaudioamp

It’s not only adding the RCA cable to the input but also moving the output cable around (without anything plugged into the EAR preamp)

If I have measured the right place, it’s a zero. I’m not sure what you mean by the signal ground of the phono input. If it’s the pin where the TT ground should be attached then it’s zero.

From the ground pin to the rest of the chassis the meter shows 33 ohms.