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

@filipm 

From your experiment, the cables might be more an issue than the EAR.

Try some Gotham Audio Double Reussen shielded interconnects - TT to phono and phono to ARC. - fairly cheap, but good quality and very good shielding, much better than many audiophile cables.

@filipm

Have you tried turning off your cellphone - not flight mode - turned off completely ?

Is there any test I can do inside the apartment? I invited an electrician and he said the power line should be fine. 

Beautiful city Prague.

If you are in an older building, you should get an electrician to verify the male earth pins in the building are in fact grounded. They may not be, or the grounding may be compromised with poor connectivity - corrosion etc.

Also if both the EAR and ARC are grounded through the power cables, then when you connect the 2 units with an RCA cable then you have an earth loop - have you tried lifting the ground on the EAR ( making sure iit is grounded via the ARC by putting a multimeter across the 2 chassis.?

@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.

@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.

@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.

@filipm

I think if I unsoldered the green wires from the pin and soldered them together, I’d just ground the board while the cage would be left absolutely unshielded.

Yes, but I’m not suggesting you leave it like this.

We want to get to a place where the chassis is grounded (no 33ohm resistor ) but leave the board floating with the 33ohm resistor. This I believe is the optimum solution.

If you can prove the chassis ground can be detached from the board ( by unsoldering the ground post earth and checking that there is no continuity between chassis and circuit board ground ) then you can safely ground the chassis and ground post to the mains ground pin ahead of the 33 ohm resistor with an additional wire. In other words ground the chassis and grounding post without the 33ohm resistor but still run the 33ohm resistor to the circuit board.

You could do as you suggest and just eliminate the 33 ohm resistor and leave everything grounded at 0. Only downside is it may be suboptimal for the circuit itself in terms of noise floor.

@lewm

Without de-soldering anything, just put one probe on the 3rd prong of the IEC, which is earth ground or AC ground, and the other anywhere on the chassis where there is bare metal. You ought to see continuity.

It’s very simple - from @filipm tests - both the chassis and board are floated at 33 ohms above ground. That is a problem as @atmasphere has pointed out.

If you look at the IEC earth pin there is only 1 33 ohm resistor attached from the earth pin to the board and ground post.- nothing else.

So the suggested fix is to isolate the chassis from the board and ground it to the IEC earth pin directly bypassing the 33 ohm resistor as suggested by @atmasphere and according to EU regulations.

In order to achieve this @filipm needs to be sure that there are no other ground connections between the board the the chassis, otherwise he will end up with other issues.

The ground lug should be disconnected from the board and earthed to the chassis.

If you go back and read my posts you will get it.

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 

@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.

@lewm 

So that 33 ohm resistor we see in the photo, which is in series between the threaded screw and the 3rd pin of the IEC, according to the OP, is either doing nothing or the chassis is floated by the 33 ohm resistor which I certainly agree is not the best idea. The OP has dropped out so we may never know. 

We knew this days ago when the OP confirmed the chassis is 33ohms above ground. He measured it. 

@atmasphere and I have been trying over the past few days to guide him in the right direction - ground the chassis and ground lug to 0 and leave the circuit ( board ) floating at 33 ohms.

@lewm 

Indeed, if the 33 ohm resistor was in series between the IEC and the ground lug, on the one hand, and the PCB ground, 

 

Wrong.

With the EAR the ground lug is connected to the chassis.

The 33 ohm resistor must go directly to the board. - not the ground lug.

The ground lug/chassis should be connected directly to the ground pin with NO 33 ohm resistor in the path.

There is 230 volts floating around - you should be more careful if you are going to give advice on this forum.

@filipm

Would you suggest trying to move resistor between the audio and the ground plug? While chassis > IEC would be connected directly.

Yes I was going to suggest that. Run a wire from IEC earth to ground post, and then 33 ohm resistor from ground post to board.

Then just measure resistance between chassis and IEC earth pin - hopefully should read 0.

If it still measures 33 ohms between chassis and IEC earth pin that would mean that the chassis is connected to the board somewhere else other than the ground post.

Glad you are still with us - its worth getting the grounding sorted.

Good luck.