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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

Showing 3 responses by casouza

casouza
239 posts
 
Swampwalker, Almarg's suggestions are excellent, specially the shorted RCA inputs.
You can quickly rig shorted ICs by wrapping any thin copper wire (telephone wire, cable TV center wire, a spare piece of network wire, even solder) around the RCA outer metal contact AND around the inner (positive) contact at one end of a pair of ICs.
Connect the other end of said ICs to the subwoofer input and switch it on. If no more noise, it looks like your system has radio frequency interference - RFI - you need to ditch the passive volume control and/or buy better shielded ICs.
If the noise continues with shorted inputs, I believe it's the woofer's (most certainly) switched power supply "beating" with some other nearby noise generator: a wall wart power supply, light dimmer or fluorescent lights. Unplug stuff until you find the culprit.
If it is a switched power supply, get a linear (old, bulky, heavy but non-noisy) PS from Parts Express or other electronics vendors. SMPS means switched, those are the ones you want to avoid.
if it is a dimmer, get rid of it.
If it is flourescent lighting, replace with old-fashioned regular incandescent lamps, which are quiet.

Coil the phone cables from the subwoofer to active speakers. If the noise increases, you need better shielded phone cables, easily found at music instrument stores. Ask for shielded microphone cables with 1/4 phone plug terminations.
Also, try to coil 3-4 turns of the phone cables around a couple of ferrite rings (Radio Schack or Parts Express), near the speakers.
I hope this helps
casouza
239 posts
 
OK, if I undertand correctly the silence during steps 6 and 7 was a "false positive", the monitors went into automatic power-off, correct ?

Your questions:
1- If the monitors are powered from separate/distant outlets, there may be differential noise riding on top of the Ground or Neutral wiring, due to the wiring resistance between the two outlets.
RFI theory and Ohm's law tell us that there is a chance that some of the RFI MAY develop a voltage across that resistance. The RFI will be amplified as a whine (high-pitch sound).
Solution; power the monitors + woofer from the same outlet. You may use an outlet strip, no problemo.

The woofer has no ground wiring to avoid ground loops, which are a common source of hum (not whine) when using separate woofers.

2.Before you spend any money in a different DAC, try the shorted IC or shorted RCA plug trick on both woofer inputs. If the noise disappears, it is coming from your ICs, DAC or PVC. Move the shorted RCAs ons step back in the chain, the another step, to find the culprit. When the noise resumes, the component AFTER the shorted RCAs is the source of noise.

Only TVCs (transformer volume controls) provide galvanic isolation of RFI noise. Autoformer volume controls and potentiometer-based passive preamps do not provide noise isolation. TVC brand / model ?

3. Yes, a ground loop will develop if the cable or cable TV box are connected to your receiver/preamp/amplifier or computer, however, cable TV ground loop generates hum, not whine.

4. I forgot to mention the cable TV box, wireless phones and microwave ovens as possible RFI sources.

You will find the source of trouble with two tricks: shorted RCA and unplugging everything in the house.

In my place, I can hear treble distortion (another form of RFI) when the computer, cable box, DVD or microwave oven are in Stand-by!
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.
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