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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, 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
swampwalker's avatar
swampwalker OP
5,138 posts
 
Thanks, all. To update those of you who have so generously offered detailed suggestions:
1. I worked the phone plugs (I knew it was not phono but had a senior moment) on the sub output and the speaker input several times, and then sprayed the jacks and plugs w Caig De-oxidit5, cleaned w Q tip and then re-plugged.
2. Started up system again, same issue BUT it was about 50% lower in volume AND one speaker seemed to be lower in volume than the other.
3. Shut off Wireless router and music server, both of which could be source of RFI. Shut off flourescents. Had tried both before, but lets start w fewer variables and build from there.
4. Powered up M00 and S00, whine still there.
5. Contemplated my navel and the mysteries of the universe for about 5-10 minutes, cursing softly under my breath (wife sleeping).
6. Right speaker pops and whine stops.
7. Left speaker pops and whine stops.
8. Breath big sigh of relief. Turn wireless and flourescents back on and power up again. Notice power pilots on monitors are off. Cycle sub off and on (auto-off feature on monitors). Monitors light up, whine comes back.
9. Resume cursing.

A couple of questions for Almarg and/or Casouza, and/or anyone else that cares to offer their $0.02-
1. The sub and the monitors are both powered but the power supplies are internal. The monitors have conventional captive 3 prong cords; the sub has an IEC type inlet but w only 2 blades, and a 2 prong/slot cord. Does that affect your advice in any way?
2. Should I try to run the rig from a DAC/headphone amp with a volume control that controls RCA outs as well as headphone socket, w/o the PVC (PC model)? I'd like to avoid any software based volume control but was wary of the DAC/headphone amp overloading the phone jack inputs of the amps in the speakers. I thought one of the benefits of the PVC was supposed to be transformer isolation of audio from the PC?
3. Since my home uses cable for internet, TV and phone, can a ground loop develop btwn the cable and the M00/S00 even though they are not physically connected in any way?

All comments welcomed. None will be mocked!!!
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!
shadorne's avatar
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shadorne
7,477 posts
 
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.

Exactly my thought - I think we have a capacitor or other component failure in the sub which has introduced instability or resonance - probably on the the power supply. Capacitors are the kind of devices that cause problems after 30 seconds (once they charge up or breakdown).
swampwalker's avatar
swampwalker OP
5,138 posts
 
Casouza- Yes it was a false negative.
I will first try powering all the components from the same outlet.
Then I will try the shorting plugs.
I do have cordless phones in my office and will also try first w the phones shut off (but if so, would be a major hassle).
I have a ground iso xformer on my TV, but not on the cable internet that is hooked up to my other computer (not the music server).

Shadorne- I don't have any technical skills, but 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.
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