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Help wanted. My NHT M00-S00s are whistlingI

OK, boys and girls; time to put your thinking caps on and help me solve a problem w my powered speaker/PC set up, although I'm pretty sure its in the speakers. I just bought a used pair of NHT M-00s (powered monitors), NHT S-00 powered sub and PVC passive volume control. They are hooked up as follows- USB digital output from my Music Vault server using Windows Media player to Keces DAC. Analog out via RCAs to PVC controller. RCAs out of PVC to RCA full range input on S-00 powered sub. Only output on sub is 1/4" phono jacks to phono jack input on M-00 monitors. Sounds great, but what's that very high pitch whine or whistle coming from the monitors? Hmmm, it's there, regardless of whether there is anything playing on Media Player or not. Volume and pitch of whine/whistle does not vary w PVC volume control position. Just under the upper limit of audibility to me. Try re=plugging all cables to no effect. Try changing AC outlets to no effect. Shut off wireless router to no effect. As I am plugging and unplugging I realize that I think it's there, even though the RCA cables are NOT plugged in to the subwoofer. So I cycle the sub off and on a few times. Each time its the same. Turn on the sub (which turns on the monitors). Silent. In approx 30 sec or so, I start to hear the whistle. It "spools up" in pitch almost like a hard drive spools up, but starts higher pitched and ends up just below the upper limit of my hearing. Turn off sub and sub and monitors shut off w a pretty good thump and the whistle stops immediately.

In summary, its a very high pitched whistle or whine that is present in the monitors whenever I power up the sub, regardless of whether the amps are getting an audio signal, or just AC power, and regardless of whether they sub/monitor set is hooked up physically to the DAC, and regardless of whether the wireless is operating. Not affected by position of PVC volume control.

Any ideas? Seller is long time A'gon member w great feedback. He says it was working perfectly when shipped. I'm not asking about responsibility, I just want to see if your collective wisdom can give me anything else to try. Given the weight of the sub, I'd really don't want to start shipping the sucker around.

Thanks, all.

128x128swampwalker
casouza
239 posts
 
Swampwalker, 30 seconds to start amplifying noise and then silence after powering down looks like a pretty normal power up / power down sequence. So, IMO, the sub amp is amplifying noise.
The real question here is to find out from wher is that noise coming???

I find it unlikely that both amplified monitors have the same defect, so whatever it is, it is coming from the preceding components/cables.

You can easily find out if the noise comes from within or outside the subwoofer amp by trying the shorting plugs connected to the woofer inputs. There is no other easy way out, unless you have acccess to an oscilloscope to track down the noise source.
Otherwise, we are stuck.
swampwalker's avatar
swampwalker OP
5,138 posts
 
OK Casouza. As I mentioned, I have no tech training. What made me suspicious was the "ramping up" of the whistle or whine in pitch and volume coupled w the load thump at turn off. Once I get my Sunday chores done, I will see if I can cobble together some shorting plugs.
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almarg
9,670 posts
 
My thought was that since the whine kicks in after about 30-60 sec and ramps up in pitch and level, then stops instantly when the woofer is shuts down (about 5 sec after hitting the switch, w a significant thump, that it might be a component failing in the woofer amp
Could be, but during the initial seconds and minutes after turn-on, internal temperatures and consequently the operating parameters of all kinds of internal devices are changing significantly, which very conceivably could change circuit responses to externally induced noise or interference.

And I would guess that the turn-off thump is unrelated, and is just a consequence of some of the circuitry in the path through the woofer and the monitors de-energizing more quickly than the output stage of the amplifiers in the monitors. You might want to ask the previous owner if he had encountered the turn-off thump.

My instinct is that the odds are the problem is not a component failure, since it all apparently worked ok before it was shipped to you.

Good luck!
-- Al
force53
1 posts
 
on this system i would check all interconnect cable but with a tester just to make sure there is no feedback on them it sound like some lose ground. also check all rca connection leave the unit on than move the rca side by side to see what happen. i had this problem befor and it was a bad rca input on the speakers it self, i like to use xlr better. okay thanks
swampwalker's avatar
swampwalker OP
5,138 posts
 
Thx Force53. I found a bad IC and all is well now.
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