[KenB]

I need some electronics help to understand a phenomenon.

I just received a new STAX SR-313 from Japan wired for 120v (or so it states on the back of the unit). With nothing connected to the unit (no cables other than power), I noticed a HUM when the volume knob was turned above 5 through 9.5 but it stop[ed at 10 (max position). The power chord is 3-pronged into the unit, but 2 pronged to the outlet. There is a ground terminal on the SR-313 although I couldn't figure out what to do with it. I tried plugging the chord directly into a wall outlet and into a surge protector -- hum persisted.

Just fiddling around, I noticed that if I touched, with my fingers, any two metal parts on the amp itself (e.g. front and back metal cabinet), the hum would stop. I then tried touching a wire from the ground terminal to one of the cabinet's metal screws -- hum also stopped. I guessed (probably incorrectly) there must be some internal wiring/grounding problem in the SR-313 and got the bright idea to semi-permanently connect a short wire from the ground terminal to a metal cabinet screw, at least until I could write to Japan and find out what's wrong.

So, I got out a phillips head screwdriver and, the instant I started twisting this one metal screw on the metal housing, the hum instantly stopped and, so far, it has not returned. Geez!

Does anyone have any ideas as to what's happening? Was there some static or other electricity charge built up inside the amp that was dissipated when I fiddled with that screw? I'm not an electrician and am totally clueless -- although perseverant!

At least I'm back to enjoying my new SR-404/SR-313...sans hum!

KenB

 

[Sinbios]

Weird. Maybe the ground was supposed to be secured to the casing, but came loose somehow? Your tightening the screw might have re-secured it.

 

[KenB]

After twisting that cabinet screw, the hum stopped and did not return the remainder of yesterday, even into the evening. This morning, when I turned the system back on, the hum was back, exactly the same symptoms!!!

So, I decided to connect a short wire from the ground terminal to one of the metal cabinet screws and the hum is gone again -- this time, I hope for good. I have to assume, at the moment, that there is some faulty wiring or grounding problem inside the SR-313.

I have written to STAX with my descriptions and will await an answer from the experts in Japan. In the meantime, I can at least enjoy the heavenly music without that disturbing noise. I just hope I haven't done anything that would invalidate my warranty or otherwise do damage to the unit...

KenB

 

[KenB]

The STAX engineer did concur that the chassis/cabinet was not being properly grounded. The recommended procedure was to back out the two screws on each long side of the cabinet, lift the housing, replace snugly, and retighten the screws.

I performed that simple procedure last night, after removing my homemade wire grounding loop, and the HUM completely disappeared! While the cover was off, I did have a look around the inside and didn't see any loose connections or solder breaks. AFAIK, everything appeared to be solid and proper.

I have no idea how chassis grounding is accomplished on the SR-313, but all it took to fix was removing and replacing the housing. Hmmm...

It appears that the hum problem is solved for now -- I just hope it doesn't recur.

KenB