@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?
Tube PHONO preamp interference - RFI, EMI, bad grounding?
Hello!
My tube phono is picking up interference most probably from the air. It's EAR yoshino 834p, using three 12AX7 tubes. It's sounds pretty amazing and I willing to try everything to keep it.
Here is a sample of the sound -
The rest of the setup is ARC LS16 mk1, Classe CA200, Chord Qutest, Technics SL1200 with Nagaoka MP200, Tannoys D700
I have tried many things already -
- grounding the phono to the preamp, grounding the phono to a socket, covering the phono with a pot, saucepan - no change
-plugging the phono preamp alone into an integrated (Bryston B60) and removing other stuff.
- the important part is I have taken the phono to two other places and it worked perfectly fine, even with the cheapes cables.
- I haven't had any problems with previous phono preamps which were all solid state.
- if I unplug the turntable the signal fades to about 50%
- if I try different RCA cables, there's not much of a change even they are shielded (audioquest mackenzie, supra etc.)
- the signal also fades when I grab the cables. Also works if I grab or squeeze the output cables.
- I have tried to wrap the cables into aluminum foil, I have noticed a difference but it's still unlistenable.
- I have tried pluging in a 5 meters long RCA output cable and walked with the phono preamp around the room. It's simply like carrying an antenna. Placing the phono on the floor helps but again, the interference is still present.
Do you have any suggestions what else to try? Is there some kind of grounding that would prevent the phono preamp acting like an antenna?
I haven't tried a new set of tubes yet.
I think the 12AX7 are simply too sensitive to all the mess in the air. The ARC LS16 preamp was catching the same signal very quietly when I took it's cover of.
Thanks!
Filip
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- 187 posts total
@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. |
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. |
- 187 posts total