Following Richard Hess's suggestions, you need to get the building
engineers involved, describe the problem and it's time frame to them.
It's easy enough
to put an oscilloscope across the power at the HVAC and see if it
originates there.There's a possibility that high power SCR's are the
culprit. Alot of systems these days are computer contrilled using high
power SCR's to control high current motors. HVAC systems and elivators
are only a couple of examples. I had a similar problem with an
installation in the early 90's that involved high power SCR's used in
the elevator system. The HF hash generated by these high frequency, hi
current devices found its way through a 30KVA center tap grounded
isolation tansformer! The solution was to filter the sub-panel
feeding the elevator system. There are commercially made filters that
can be installed at the sub-panel feeding the offending system.
Interestingly, filtering the AC source to the building elevators
removed a number of troublesome noises elsewhere in the (5 story)
Corey Bailey Audio Engineering
Quoting "Richard L. Hess" <[log in to unmask]>:
> Hello, Martin,
> When you were describing the noise, I was thinking variable speed
> motor drives--which would be completely in keeping with a revamping
> of the HVAC systems.
> How accurately did you measure the 10,330 Hz? Could it be 10,320? If
> that's the case, then it's locked to the 60 Hz and is the 172nd!!
> I don't think there is anything that low in digital cable, and it is
> less likely that any digital cable (which is supposed to stay INSIDE
> the coax shield and does unless it is grossly mis-installed) would
> radiate enough energy around the building to cause the widespread
> havoc you describe.
> I would have the electricians look for potential mis-wires in the
> HVAC system that have some of the load side of the variable speed
> drives (VSDs) being grounded in some way or other.
> Ideally, the power to the VSDs should be twisted and installed in
> EMT or similar conduit. We used to always specify the load
> connectors between stage light dimmers and the outlets to be twisted
> to reduce radiation of the SCR hash from the dimmer.
> I've seen this with SMPTE Timecode as well as power-related issues
> where the return side of some TC signals were grounded by unbalanced
> inputs -- not pretty -- it was all over the place. I've also seen
> neutral-to-ground shorts wreck havoc in a plant. Neutral and ground
> should only be tied in one place.
> With that said, power line filters do put (in sum) a reasonable
> amount of current back into the ground. I have measured it in a
> large TV facility at about 1 A. The client had me fly from Burbank
> to SeaTac on a Saturday morning to do a survey and I was able to
> show him the small, but significant currents injected by each
> filter. Add that up on the back of the proverbial envelope and
> you're close to 1 A...polluting your clean ground. Back in Glendale
> for dinner. Joy.
> On 2011-08-30 2:07 PM, Martin Fisher wrote:
>> Hi gang,
>> FYI, the following is cross posted to two list serves.
>> A mysterious squeal appeared in the transfer chain here in the
>> audio lab about two months ago. It's a tone centered around 10,330
>> hz and seems to be coming in on the AC power lines for a good
>> portion of the building. This has been determined by taking a
>> piece of equipment (in this case a Soundesign 8 track deck with
>> integrated amplifier and a pair of headphones) and plugging into
>> several outlets in the building. The tech power outlets for the
>> recording studios are clean as well as some outlets on the 2nd
>> floor but many of the 1st floor outlets have the offending tone
>> I've tried isolation transformers and low cost IsoBar type line
>> conditioners with absolutely no luck. The tone appeared at
>> approximately the same time of a revamping of the air conditioning
>> system for the building and stringing of lines for some sort of
>> digital cable. One of the campus electricians did some switching
>> around of breakers and various things but nothing has helped.
>> Before I go out and get a high end line conditioner to try out I
>> thought I'd tap the collective wisdom of two of the best informed
>> list serves. Any ideas?? I'm sure I've not provided the best
>> description in the world so I'll try to answer any questions that
>> Thanks. :-)
> Richard L. Hess email: [log in to unmask]
> Aurora, Ontario, Canada (905) 713 6733 1-877-TAPE-FIX
> Quality tape transfers -- even from hard-to-play tapes.
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