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
That noise to me sounds like something is generating on the building AC power service line or on the apartment circuit. I would like to see what's else is on the same circuit. Sounds like a motor driven fan or compressor and even a celling fan or something turning. "Maybe even the buildings boiler room generator or vibrating power massage chair"! The vibrating chair was only for humor but something is oscillating on this circuit... Bill M
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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.
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@atmasphere yes, I was referring to 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 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)? |
