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

What I see in the photo is that IF the 3rd prong on the IEC is directly connected to the chassis, somewhere not visible in the photo, 

@lewm 

And thats precisely why I suggested he check the chassis for grounding after unsoldering the ground lug ( but keeping the link to the board through the 33 ohm resistor ).- there could be an unknown connection to the chassis from somewhere else - my fear is there might be another connection between the board somewhere other than the grounding post and if he grounds the chassis ahead of the 33 ohm resistor he will end up will multiple earth paths.

So in a nutshell bypass the ground lug, check the chassis is now ungrounded completely ( not 0 not 33ohms ), then he can earth the chassis and ground post directly to the mains input earth lug ahead of the 33 ohm resistor.

cc @filipm 

I see now that there is a difference between your thinking and mine. I perceive the ground lug, which is meant to be used to ground the turntable or tonearm or both to the phono stage, ought to be or can be floated along with the audio circuit with respect to earth ground. By my way of thinking, that actually makes the most sense if you are going to float the audio grounds; you want the upstream gear to be grounded at the same potential as that of the on board audio circuit. It seems that you want the ground lug to be at AC or earth ground, which is certainly not "wrong" either. In my way of thinking (float the ground lug), the photo shows exactly what you’d want; the ground lug for external upstream equipment is floated above earth or AC ground by the 33 ohm resistor shown in the photo and which we are told is attached to the 3rd prong on the IEC. Likewise, the ground lug is therefore at the same potential as the on board audio circuit by virtue of the other green/yellow wire which attaches the ground lug presumably to the ground plane on the PCB of the audio circuit. I see nothing wrong there. For all of the above to hold water, we also need to know that (1) the ground lug is not in electrical contact with the chassis, where it passes through the chassis wall, and (2) the 3rd prong of the IEC is directly in contact with the chassis, keeping the chassis at AC or earth ground. The OP can easily verify those two points by his ohmmeter; there is no need to de-solder anything, if the meter verifies that the IEC is in contact with the chassis and the ground lug is 33 ohms away from the chassis.

From what I have been able to read, this practice of floating the audio ground is "new", in the sense of the past 20-30 years and is not required but only makes for quieter audio. I checked two of the many phono stages I have lying around here or in use. My original Quicksilver has a ground lug that is not floated. It’s about 25-30 years old and sounds excellent when I use it. My Steelhead has two ground lugs, both appear to be isolated from the chassis.

I have a Beveridge system in my basement. Its direct drive amplifiers are about 40 years old, like the speakers. In the original design, everything in the audio circuit was grounded to the chassis, and the chassis was not at all grounded to the AC, because the amplifiers were originally supplied with a hard-wired 2-prong cord. I assume grounding was originally supposed to be handled at the Beveridge preamplifier, designed by Roger Modjeski for Harold Beveridge. This is a bit scary since the amplifiers develop 3200V in the output stage. When I got the speakers up and running, there was a low level buzz that I found very annoying. After some consultation and reading, I cured it by (1) installing a 3-prong IEC connector for AC and (2) connecting its 3rd prong to a pair of back to back high current SS diodes that in turn connect to the chassis. This "yin/yang" topology *(not my term but coined by others who thought of the idea) isolates the AC ground from the chassis and from the audio circuit in a safe way and seems to work to completely eliminate the buzz.

Finally, this EAR unit works fine in other locations. And Tim de Paravicini, the designer of the EAR products was no dummy. For those two reasons, I tend to think this whole brouhaha over the ground scheme is a red herring. The OP has some external source for RFI entering his apartment, IMO.

@lewm 

The ground lug is used for shielding, not signal. Thats why you normally would tie it to mains earth.

The suggested layout is not new - my old 1980's Theta tube gear uses it. The suggested grounding is to EU code.

Having the EAR chassis floating above ground at 33 ohms means that if something goes wrong and the 33 ohms resistor fails, there is a possibility that the chassis could go live and fry your beloved. Not a good idea. I am actually shocked the unit is not to code.

It’s possible that we are arguing because our two countries have different codes re grounding. Everything I have written is based on how things are done here in the US. The 3rd prong of the IEC is earth or AC ground. Correct? We see here that there is a 33 ohm resistance between the 3rd prong or earth ground and a screw that we agree is a ground lug. I take it to be the back side of a typical grounding scheme to be found on many phono stages which is to be used to ground the turntable or tonearm. There is no significant current or voltage on the ground wire coming from a turntable chassis or a tonearm, so there is no danger of blowing that 33 ohm resistor and making the chassis dangerously "hot". This is why EAR could get away with what appears to be a 1/4 Watt resistor or 1/2 Watt at most. By my way of thinking, that 33 ohm resistor is to float the AUDIO ground with respect to earth ground. We also see that the screw is directly connected to the PCB, and I assume it is soldered to the ground plane. This is not dangerous and is a ploy to reduce noise. For me to be correct, I would need to know that (1) the screw we see is NOT touching the chassis, and conversely (2) the 3rd prong on the IEC connector is directly connected to the chassis. (That would mitigate your legitimate fear that the chassis is floated.) So, the lug and the audio circuit are both floated by 33 ohms over the AC or earth ground and the chassis. That is OK in the US, where the AC receptacle is connected to a solid earth ground in every home that is up to code, by the 3rd wire.

I am not an EE, so I would welcome correction if I am off base. I don’t want to be giving advice that may lead to a dangerous situation.