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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
shadorne's avatar
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shadorne
7,477 posts
 
It seems to be the sub - have you tried the speakers without the sub.
swampwalker's avatar
swampwalker OP
5,138 posts
 
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.
almarg's avatar
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almarg
9,670 posts
 
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
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!!!
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