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.

I hate when that happens. Do you have another pair of speakers, the more similar the better, that you can "rig" up to the sub and see if it stemming from the amplifier in the sub? We are not talking rocket science here. Forget ohms for right now. Also, hook the NHT's to another amp if you can (rig). Though I doubt it is the speakers really.
It seems to be the sub - have you tried the speakers without the sub.
I did use the speakers w/o the sub and they were fine. I do not have another pair of powered speakers that I can drive off the sub. They would also have to have the 1/4" phone jack input that the S-00 sub uses as output.
Hi Michael,

Sounds like a mystifying one. I don't have any really promising theories to offer, but here are a few things that come to mind to try:

1)Try moving the sub and monitors a goodly distance apart, if they are not presently set up that way.

2)Try putting shorting plugs on the rca inputs of the sub, especially if the previous owner was not driving them with a high impedance source such as the PVC.

3)With power off, insert and then remove a phone plug into the unused 1/4" TRS input jacks on the sub. Do that a couple of times, to make sure that their contacts are not somehow stuck in an incorrect position (which conceivably could affect things even though you are not using those inputs).

4)With power on and the whistling present, lightly jiggle the 1/4" plugs at both the sub outputs and the monitor inputs, to see if there is any effect on the whistle, due to one of them making marginal contact.

5)If you haven't already, see if the whistle occurs with only one monitor connected, and try using each of the two TRS interconnects on the one monitor.

6)Carefully compare the volume levels of the two monitors while playing a source which is either mono or centered, as a way of verifying that there isn't an open connection on one channel, on one side (+ or -) of the balanced signal pair within each TRS cable. That would cause a 6db volume disparity, and conceivably could account for the whistle as well.

If you gather from some of these suggestions that I don't have too much faith in the connection integrity of 1/4" connectors, you are right! BTW, one minor correction just fyi -- they are referred to as phone jacks, not phono jacks.

Hope that helps,
-- Al
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
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!!!
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!
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).
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.
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.
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.
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
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
Thx Force53. I found a bad IC and all is well now.