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

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.

I’ve spent considerable time reading this thread, but it has been worthwhile. @atmasphere got the ball rolling on a solution, @budlite22 confirmed it, and @ejb14 moved things along, while @dpop chimed in too. It really was a drama proving that solutions grow quickly and surely when one starts by asking the right questions and answering them by proven tools and methods. Reminds me of a Sir Arthur Conan Doyle mystery, “Facts, Watson, we must have facts and reason shall lead us to a solution. What can we deduce from the facts before us?” Well done, all. I’m nearly as grateful as I’ sure @filipm is to finally have solved the mystery. And all I can say to @ahal1 is wow! I’ve got to get me one of those! (Where were you earlier?)

@oldrooney ...but has the problem really been resolved? We haven't heard from @filipm for a few days now. 

I thought of that while I was writing my response, but surely this is it, right? [Famous last words.] I hope @filipm has it resolved, or another preamp, at least. He sure has hung in there where lesser men would have thrown in the towel by now.