Laser resonates with ceiling speaker to create a horrible ?ground loop?...

D

Don

Guest
Received a phone call from a client in regards to a new medical laser
device. Whenever the laser is activated it causes a sympathetic loud
hum in only one of the Valcom ceiling speakers - located relatively far
away in the waiting room. Speakers closer to the laser show no symptoms.
Haven\'t gone on-site yet. AFAIR, a sheilded audio cable feeds the
speakers, and a LM386 drives them. The built-in Valcom LM386 amp uses a
couple of exposed spade terminators, spaced about an inch apart, to
connect to the audio line.
Is it possible for the terminators to act as an antenna? Or, does
this indicate a broken/incorrect ground?
TIA.

Danke,

--
Don, KB7RPU, https://www.qsl.net/kb7rpu
There was a young lady named Bright Whose speed was far faster than light;
She set out one day In a relative way And returned on the previous night.
 
\"Don\" <g@crcomp.net> wrote:

Received a phone call from a client in regards to a new medical laser
device. Whenever the laser is activated it causes a sympathetic loud
hum in only one of the Valcom ceiling speakers - located relatively far
away in the waiting room. Speakers closer to the laser show no symptoms.
Haven\'t gone on-site yet. AFAIR, a sheilded audio cable feeds the
speakers, and a LM386 drives them. The built-in Valcom LM386 amp uses a
couple of exposed spade terminators, spaced about an inch apart, to
connect to the audio line.
Is it possible for the terminators to act as an antenna? Or, does
this indicate a broken/incorrect ground?
TIA.

Danke,
Click to expand...

We have no idea how the system is wired or where the hum is coming from.

You should go to the site and inspect the problem before posting.



--
MRM
 
On Sun, 30 Oct 2022 14:47:22 -0000 (UTC), \"Don\" <g@crcomp.net> wrote:

Received a phone call from a client in regards to a new medical laser
device. Whenever the laser is activated it causes a sympathetic loud
hum in only one of the Valcom ceiling speakers - located relatively far
away in the waiting room. Speakers closer to the laser show no symptoms.
Haven\'t gone on-site yet. AFAIR, a sheilded audio cable feeds the
speakers, and a LM386 drives them. The built-in Valcom LM386 amp uses a
couple of exposed spade terminators, spaced about an inch apart, to
connect to the audio line.
Is it possible for the terminators to act as an antenna? Or, does
this indicate a broken/incorrect ground?
TIA.

Danke,
Click to expand...

Is the hum line frequency? Someone could use a cell phone and send you
the sound.
 
On a sunny day (Sun, 30 Oct 2022 14:47:22 -0000 (UTC)) it happened \"Don\"
<g@crcomp.net> wrote in <20221030a@crcomp.net>:

Received a phone call from a client in regards to a new medical laser
device. Whenever the laser is activated it causes a sympathetic loud
hum in only one of the Valcom ceiling speakers - located relatively far
away in the waiting room. Speakers closer to the laser show no symptoms.
Haven\'t gone on-site yet. AFAIR, a sheilded audio cable feeds the
speakers, and a LM386 drives them. The built-in Valcom LM386 amp uses a
couple of exposed spade terminators, spaced about an inch apart, to
connect to the audio line.
Is it possible for the terminators to act as an antenna? Or, does
this indicate a broken/incorrect ground?
TIA.
Click to expand...

I once had that problem in a building, differential audio (2 opposite phase and ground),
but somebody had cut one of the audio wires...
As it is far away looks like a possible connection problem.
 
On Sunday, October 30, 2022 at 10:47:30 AM UTC-4, Don wrote:
Received a phone call from a client in regards to a new medical laser
device. Whenever the laser is activated it causes a sympathetic loud
hum in only one of the Valcom ceiling speakers - located relatively far
away in the waiting room. Speakers closer to the laser show no symptoms.
Haven\'t gone on-site yet. AFAIR, a sheilded audio cable feeds the
speakers, and a LM386 drives them. The built-in Valcom LM386 amp uses a
couple of exposed spade terminators, spaced about an inch apart, to
connect to the audio line.
Is it possible for the terminators to act as an antenna? Or, does
this indicate a broken/incorrect ground?
TIA.
Click to expand...

The \"far away\" part might be a clue. More line length= more pick-up. You need twisting and not shielding to neutralize external magnetic field interference.

The source is most likely the laser power supply putting high frequency transients on the mains. Those transients will be currents, and currents produce magnetic fields. Might be a good idea to line filter that too.

Danke,

--
Don, KB7RPU, https://www.qsl.net/kb7rpu
There was a young lady named Bright Whose speed was far faster than light;
She set out one day In a relative way And returned on the previous night.
Click to expand...
 
Fred Bloggs wrote:
Don wrote:
Received a phone call from a client in regards to a new medical laser
device. Whenever the laser is activated it causes a sympathetic loud
hum in only one of the Valcom ceiling speakers - located relatively far
away in the waiting room. Speakers closer to the laser show no symptoms.
Haven\'t gone on-site yet. AFAIR, a sheilded audio cable feeds the
speakers, and a LM386 drives them. The built-in Valcom LM386 amp uses a
couple of exposed spade terminators, spaced about an inch apart, to
connect to the audio line.
Is it possible for the terminators to act as an antenna? Or, does
this indicate a broken/incorrect ground?
TIA.

The \"far away\" part might be a clue. More line length= more pick-up.
You need twisting and not shielding to neutralize external magnetic
field interference.

The source is most likely the laser power supply putting high frequency
transients on the mains. Those transients will be currents, and currents
produce magnetic fields. Might be a good idea to line filter that too.
Click to expand...

You got me back on-track Fred, thank you. It helps to show up onsite
with a battle plan.
It\'s been years (not long enough in other words) since the last time
Valcom speakers were debugged by me. And, of course they use the
cheapest twisted pair available - not the luxurious audio line imagined
by my mind.
So, yes, somewhere in the ceiling the twisted pair for the obnoxious
speaker must parallel the electrical outlet used by the medical laser.
The Valcom speaker system is no longer used, so the simplest solution is
probably to disconnect the twisted pair from the Valcom LM386 amp.

For the benefit of others in this thread, a \"ground loop\" implies a
frequency of 60 Hz.

Danke,

--
Don, KB7RPU, https://www.qsl.net/kb7rpu
There was a young lady named Bright Whose speed was far faster than light;
She set out one day In a relative way And returned on the previous night.
 
On Sun, 30 Oct 2022 17:05:52 -0000 (UTC), \"Don\" <g@crcomp.net> wrote:

Fred Bloggs wrote:
Don wrote:
Received a phone call from a client in regards to a new medical laser
device. Whenever the laser is activated it causes a sympathetic loud
hum in only one of the Valcom ceiling speakers - located relatively far
away in the waiting room. Speakers closer to the laser show no symptoms.
Haven\'t gone on-site yet. AFAIR, a sheilded audio cable feeds the
speakers, and a LM386 drives them. The built-in Valcom LM386 amp uses a
couple of exposed spade terminators, spaced about an inch apart, to
connect to the audio line.
Is it possible for the terminators to act as an antenna? Or, does
this indicate a broken/incorrect ground?
TIA.

The \"far away\" part might be a clue. More line length= more pick-up.
You need twisting and not shielding to neutralize external magnetic
field interference.

The source is most likely the laser power supply putting high frequency
transients on the mains. Those transients will be currents, and currents
produce magnetic fields. Might be a good idea to line filter that too.

You got me back on-track Fred, thank you. It helps to show up onsite
with a battle plan.
It\'s been years (not long enough in other words) since the last time
Valcom speakers were debugged by me. And, of course they use the
cheapest twisted pair available - not the luxurious audio line imagined
by my mind.
So, yes, somewhere in the ceiling the twisted pair for the obnoxious
speaker must parallel the electrical outlet used by the medical laser.
The Valcom speaker system is no longer used, so the simplest solution is
probably to disconnect the twisted pair from the Valcom LM386 amp.

For the benefit of others in this thread, a \"ground loop\" implies a
frequency of 60 Hz.
Click to expand...

Not necessarily. A 50 Kilowatt switching power supply can generate
all manner of frequencies. I\'ve also encountered 180 Hz, caused by
swapping a power neutral (white in the US) with safety ground (green)
on a 19: relay-rack cabinet.

Joe Gwinn
 
On Sunday, October 30, 2022 at 9:48:25 AM UTC-7, Fred Bloggs wrote:
On Sunday, October 30, 2022 at 10:47:30 AM UTC-4, Don wrote:
Received a phone call from a client in regards to a new medical laser
device. Whenever the laser is activated it causes a sympathetic loud
hum in only one of the Valcom ceiling speakers - located relatively far
away in the waiting room.
....
The source is most likely the laser power supply putting high frequency transients on the mains. Those transients will be currents, and currents produce magnetic fields. Might be a good idea to line filter that too.
Click to expand...

Yes, and if the laser has a simple line cord, a common-mode clamp on bead would be the first thing to try.
 
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