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

@filipm I'm looking at one version of the schematic on line...

https://rockgrotto.proboards.com/thread/11529/ear-834p-phono-stage-mods

...and it looks like the earth ground (e) is actually part of the audio circuit. Wow, I've never seen anything like that before. I would think this design makes it super susceptible to picking up atmospheric electrical noise. I don't care how good it sounds, this is a piece of equipment I would personally (and maybe you should too Filip) not incorporate into my audio system. IMO, it's a very poor design, but, I can see you're more than determined to keep using it, and make it work in your electrically noisy environment, so, I'll give it another try...

To fix this would probably entail a complete do-over (let's eliminate that option right now). From looking at this schematic (if this on-line schematic is a true version of *your* model), it looks like anytime you plug this EAR into a grounded outlet, the entire ground system that it's plugged into becomes part of the audio circuit (I hate to think of what this thing would sound like in a high RFI radio station environment, which already has loads of RFI and EMI in the surrounding atmosphere). 

My first suggestion is to (however you can accomplish it) lift the "e" ground at the chasis iec connector (I hope you know how to solder). It has to be done there, and nowhere else. Now how does the unit sound (with the turntable ground connected to the EAR where it should be)? If that still produces noise, I would reconnect the "e" ground, and purchase an Isolation Transformer, like this one:

XtremPro Hi-end Ground Loop Noise Isolator

Connect the XtremPro (since it probably employs a transformer of some type in its design, I don't know how perfectly flat its frequency response is) between the output of the EAR, and your preamp input (while still connecting the TT ground at the ear). How does the EAR sound now? The XtremPro should (hopefully) break the ground loop that's taking place with the design of the EAR. If that works, if needed, you can always shop around for a higher quality RCA isolator. Good luck.  

@filipm wow - now we are getting somewhere - I think @atmasphere is right on track and @dpop has some great suggestions too. Why it sounded good elsewhere is still a mystery...I would have thought if this is the issue you would always hear this noise. Did you use the MC switch at your friends place and just not notice the what would be then lowered noise?

Also, one thing to get out the way, an easy one - are the leads from the R16 replacement touching the chassis under the board (or any of the other leads under the circuit board)?

@ejb14 Hmmm, you just made me think of something. It's possible the chasis is floating (no earth ground connection). Filip, do you see that little silver philips screw on the back panel with the bonding washer (washer with ridges which digs into the panel when screwed down) underneath it? Is there any wire connected to it inside the EAR? 

I have measured the impedance between the ground of the RCA input/output and the chassis. You’re right - it’s 0.01 ohms.

Seems like there’s a poor grounding on the inlet. I wrote to EAR if that’s on purpose or not.

Thanks!

What can I do to find out what causes this 33 ohm resistance?

@filipm Grounding in audio products is poorly understood, especially in the context of meeting safety requirements! In the US its a bit of the Wild West; some know what they are doing and others don’t, often using techniques from the 1950s that are no longer considered kosher in the age of grounded outlets.

That 33 Ohm resistance isn’t a bad thing- its just that its in the wrong place. How it should have worked is that the connection between the IEC and the chassis should have read zero Ohms and 33 Ohms between the RCA grounds and chassis. In this way the chassis is held at ground by the power cord- and noise collected by the chassis is grounded. The audio circuit is thus floating at ground potential owing to the 33 Ohm resistor but shielded from the outside world. In this way ground loops are avoided even though the power cord is grounded.

Right now the chassis is less effective as a shield it can act as a bit of an antenna instead. You don’t get ground loops though, because there is a 33 Ohm resistor between the AC power cord ground and the audio ground. But in practice, this approach is noisier insofar as RFI and the like are concerned.

IMO the preamp needs to be modified- ’fixed’ or whatever you want to call it, but not so much to fix the RFI issue as it is to fix the potential shock hazard should something go wrong. You really want that chassis grounded so if something like a power switch were shorted to the chassis (for example if the unit were dropped and then plugged in), the fuse would instantly blow, protecting the user. Right now what would happen in that scenario is the 33 Ohm resistor would likely fail, leaving the chassis live, causing damage to equipment downstream and also being a possibility of a lethal shock hazard.

This is of course based on what you’ve told me so far.

Had a similar problem and surprisingly 

a power line conditioner solved it - 

this is the model i have.