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

@filipm 

IF the first statement is correct and about the EAR (and succeeding posts suggest this is so) THEN the EAR is incorrectly grounded.

So we need to double check:

The ground pin of the IEC connection should tie directly to the chassis of the EAR. To not do so puts the unit at odds with EU directives for voltage safety.

The 33 Ohm resistance would then occur between the chassis (or ground pin of the IEC connection since they are the same) and the ground of the RCA connectors. 

So here's another measurement to take. What is the resistance between the RCA connectors and the chassis? If zero Ohms or very nearly so, I think we found the problem despite remonstrations from EAR.

@atmasphere is correct.

Chassis should be at ground.

In my Music Reference RM4 mc head amp the chassis is grounded ( no resistor ) to the mains input ground.

There is a 12ohm resistor from the "ground" on the circuit board to the chassis.

In other words the circuitry is floated above ground. This is not uncommon.

When you measure the resistance between the -ve RCA and chassis you get 12ohms.

The binding post is connected to chassis earth ( measures 0 to ground pin of mains inlet..

By contrast the EAR appears to float everything, chassis and circuitry, above ground. Bizarre and dangerous as @atmasphere has pointed out in an earlier post.

One thing you need to be aware of is the grounding post on the EAR - sometimes they are insulated from the chassis and sometimes not. It appears that the EAR binding post is connected to the circuit board "ground" (after the 33 ohm resistor ) not the chassis.

In my view you should get a tech to ground the chassis directly to the ground pin on the inlet, but leave the board floated above ground., The tech will need to find how the chassis is currently earthed to the board and remove that connection to ensure the circuitry is still floating above ground, the chassis is at 0  and there are no earth loops.

If you are not comfortable with this get rid of it.

@dover It’s the other way around on the EAR I guess.

Here is a picture of the ground binding post. The wire on the right is connected to the circuit board. The one on the left leads to the IEC through the 33 ohm resistor.

That means the chassis relies on the grounding pin and not so much on the ground to the IEC, correct? Nevertheless, I have tried grounding the chassis using this pin and separate wire leading to the socket ground pin, a heater, chassis of the ARC preamp.. none of these made a difference. Maybe I was just using a poor wire (the was almost no resistance on that wire).

I guess things could change if this 33 ohm resistor was moved to the wire on the right - between the grounding pin of the chassis and the circuit board ground. Then the ground of the RCA connectors would be closer to the ground of the inlet and not the circuit board.

@filipm 

Yeah so I can see that both the ground post and circuit board are all earthed through the 33ohm resistor. If the chassis and earth post measures 0 ohms then the chassis is lifted above ground ( not what you want ).

The chassis could be connected to the circuit board ground anywhere - it could be the binding post;  ?you would need to look around - are there any wires to the chassis ? It could be through circuit board legs ?

I would do the following test -

If you desolder the 2 green and yellow wires from the grounding post but leave them soldered together - then the circuit board would is still grounded through the 33 ohm resistor ( assuming you have reattached it ).

Now measure from the green wires to the chassis. If that measures open circuit then you would know that the chassis is earthed through the ground post mount.

Double check by measuring the chassis to mains earth tag - should be either 33ohms or open circuit. If it is 33 ohms the chassis must be earthed via the board somewhere else. If it is open circuit then we now know the chassis is earthed through the ground post mount only.

Depending on your test results I could suggest a very easy fix for you to the earthing issue. This may or may not fix your noise issue though, but in my view would be worth trying.

@dover the chassis and the board are connected by those green wires. 

board > right green wire > binding post < left green wire - 33ohm resistor - ground pin of the IEC. (which means the left and right wire are meeting in the binding post)

also, there are screws holding the board but they are not touching the circuit of course. 

There is a possibility that the joint of the board ground (where the circuit board and the grounding wire meet) needs to be resoldered. 

If I desolder the 2 wires it only means the chassis would be completely lifted from the ground. The chassis and these wires wouldn't be even electrically connected. 

What's your suggestion?

Let's not forget the error may not be in the preamp. It's working fine anywhere else but my place. Maybe it just needs better grounding from the binding post. But where to attach it if not the socket ground pin? I have no idea. 

(It's possible the grounding problem leads further - maybe the sockets are poorly grounded. Maybe there's a problem in the main switch. We still haven't measured that, if it's even possible. I'm waiting for the electrician to show up.)

@filipm 

If I desolder the 2 wires it only means the chassis would be completely lifted

Thats likely correct - but it would be prudent to verify that. I say  this because most of the time ground posts are insulated from the chassis, and there may be a different path we cant see.

If proven to be correct, and the chassis is now floating, then you would be able to earth the chassis to the incoming mains earth pin before the 33ohm resistor and achieve much better shielding - as per @atmasphere suggestion in his earlier post.