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Newly developed ground loop hum in a Stax SR-007A's right channel


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Hi y'all,

I have a Stax SR-007A e-stat (with the blu-tac port mod) energized by a Stax SRM-T1S with the CCS mod running a pair of NOS Tung-Sol 6SN7GTB tubes through angled socket adapters. The signal from my DAC (RME ADI-2) is being fed through custom-made balanced XLR cables and the energizer is powered by an Audioquest NRG-Z3 power cord. The system has been running fine for at least a year.

Yesterday I removed the earpads for a routine cleaning of the driver (dusting off the dust covers, etc.), but after that the 007A developed what seems to be a ground loop in the right channel. The right driver started having this 60 Hz hum that remains even if the energizer is turned off. I've performed the following diagnosis steps:

  • If you remove just the signal interconnects or the power cord the hum remains, it goes away only if all cables are removed.
  • The hum disappears if I touch the ground pole in the back of the energizer with my hand, regardless of whether the energizer is on or off.
  • I also have a Stax Lambda SR-407. It has no hum at all, regardless of whether I plug it alone to the energizer or together with the 007A (the latter's right driver still hums, of course).
  • I've checked the soldering points at the right driver by measuring the resistance between each soldering point and the corresponding pin in the plug (no broken wiring) and also between each pair of soldering points (no cable or driver shorts). Everything seems fine in that regard.
  • I've replaced the blu-tac in the port mod just in case there was something conductive mixed into the old blu-tac. That didn't change the problem at all.

I insist, I've been using this rig for more than a year with no problems, the hum only started yesterday and the only unusual thing I did was the aforementioned cleaning of the drivers. There are no other background noises (squeaking, buzzing, etc.), just the 60 Hz hum in the right channel. The left channel is fine.

What could be causing this? It's clearly an earspeaker issue, but I have no other clue besides that. Any help is deeply appreciated!

P.S.: in due time - my building has no grounded power outlets. Not much I can do about it, but, as I said above, that has never been a problem before with this system.

Edited by plaurids
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  • plaurids changed the title to Newly developed ground loop hum in a Stax SR-007A's right channel

I've found a checklist of possible causes of single-channel hum in Stax e-stats in a 2009 HC post by spritzer. Long story short, if the culprit indeed is the headphone itself (and not some defective output tube in the energizer) then he states that debris inside the driver may cause it through bias leakage, or perhaps a faulty bias connection.

Taking into account my previous diagnosis steps listed above, it all points out to debris having entered the right driver, which means a full inside cleaning of said driver was in order, together with checking the dust covers for punctures and/or tears. Luckily for me, I do have some experience in disassembling SR-007 drivers for cleaning (I also have a SR-007 Mk1 which got mold inside the right driver through a dust cover tear about 9 months ago, but that's another story... 😔 ), so I did just that. Curiously (and fortunately), there were no holes or tears in the dust covers, but the cleaning really did the trick. The hum is gone! Yay!

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Plot twist: now the left channel got the hum, albeit quite faint. Right channel still ok after the above cleaning. Cause is probably the same - the above diagnosis steps led to the same conclusions and I did hear some squeaking a few hours before the hum started, which also points to debris inside the driver. Time to gut and clean the insides of the left driver as well, then... Will report later how that goes.

Edited by plaurids
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