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Strange Grounding Problem?

I have a unique grounding problem unlike any in my past experience.

I'm using a pair of the latest NuForce Reference 9 amplifiers and a dbx 14/10 equalizer that has a two prong AC plug with a floating ground. The NuForce amps and preamp use 3-prong grounded AC plugs.

I have a kind of low-level spitty high-frequency noise in the left speaker when using the dbx. It is audible a few feet from the speaker if you listen on axis. It can't be heard at the listening position and doesn't affect the sound quality, but I'm a nit-picky kind of guy.

If I turn the left monoblock on first, there is no noise, but when I turn the right amp on, the spitty hissy noise appears in the left channel as soon as the relay kicks in. There is no hum, so that's not an issue. Even more oddly, if I turn the power off on the EQ the noise gets louder and can be heard in the right channel as well. If I unplug the dbx the noise diminishes. If I bypass the dbx EQ and hook my solid-state preamp directly to the amps, both channels are stone dead silent. Problem is, I have a broad bass peak in my room at 125Hz and I need the EQ for the system to sound natural.

I was thinking of buying a Behringer EQ, which should be more quiet, but I'm not convinced it would sound as transparent as the dbx. I was just wondering if anyone has experienced this type of odd problem and if it could be easily resolved.

I switched the amps from left to right and that did nothing. I also tried grounding the dbx chassis and that did nothing. I use a PS Audio UPC-200 power conditioner and my AC power is relatively clean. I must be missing something, but I'm not sure where to look at this point. The only thing I'm fairly sure of is that the problem is somehow centered on the dbx. The negative sides of the input/output jacks are linked to the chassis by small capacitors for RFI rejection, but I don't think that's the problem. Any help or insight would be appreciated.
plato
wrp
42 posts
 
Hi Plato,

I am experiencing a similar problem with a standalone solid state phono stage, and the phono stage of my Thor TA-2000 tube preamp.

I live near the NuForce office, so Jason Lim of NuForce offered to diagnose the problem for me. I dropped off my Thor for testing and the conclusion was that some noise may be travelling back from the NuForces to upstream components, such as my Thor.

Apparently the problem is quickly resolved by implementing star grounding. FYI, the linestage section of my Thor did not experience this problem. It is DEAD quiet. Neither did my passive preamp. Apparently phono circuits (perhaps your EQ?) are more susceptable to the noise the NuForces dissipate across the ground plane.

Contact the folks at NuForce. They are more than happy to help you.
plato OP
1,543 posts
 
Will and Wrp,

Thanks for your replies. I've been corresponding a bit with Jason and Casey at NuForce about this. I believe that for whatever reason, the noise is traveling back upstream from the NuForce amps.

I'm going to give those guys another call and see if they can suggest star grounding or some other easy fix.

If I do get resolution on this issue, I'll post back and let folks know how the problem was addressed.

Many Thanks!
jea48
4,425 posts
 
Even though the EQ unit has a two prong non grounding power cord you still could have a ground loop problem. The polarity on the primary of the power transformer could be reversed.

Just for the heck of it one thing you might try is reversing the polarity on the two wire plug of the EQ unit. I would imagine the plug is the polarized type so you can not just flip it 180 in the 120V wall outlet. If you have a ground lifter, cheater, trim the wide blade with a pair of tin snips. This way you will be able to plug the EQ unit into the ground cheater and reverse the cheater in your wall outlet.

If you check the archives Sean goes into great detail on checking the polarity of the power supply transformer.
plato OP
1,543 posts
 
Hi Jea48,

That's interesting, but actually, I did reverse the plug polarity, but there was no change in the noise.

Maybe reversing the plug and then grounding the chassis to the preamp would work, but it seems like a long shot.

I'm still waiting for a call back from NuForce, so I'll see what they have to offer, if anything.

Best regards,
Frank
plato OP
1,543 posts
 
Just to follow up, Jason at NuForce did offer to check over my equalizer if I send it to them. However, since I can only hear the hiss up close to the speaker and don't particularly want to take my chances with shipping right now, I think I'm going to leave well enough alone.

Thanks to all who replied and Happy Holidays!
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