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Bob Simon[_2_] Bob Simon[_2_] is offline
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My Panasonic TV (TH-50PH9UK) is a video monitor only; all audio is routed through my NAD preamp and amp. Last week, a hum started without having made any changes to any systems. Today, I tried unsuccessfully to eliminated it by plugging the TV into the same power strip that the preamp and amp are plugged into. Lifting the TV's ground eliminated the hum.

Does this suggest that a capacitor shorted last week in the amp, preamp, or TV?

As a practical matter, have I created an unsafe or inappropriate condition by lifting the TV's ground pin?
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Phil Allison[_4_] Phil Allison[_4_] is offline
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Bob Simon wrote:

-----------------

My Panasonic TV (TH-50PH9UK) is a video monitor only;
all audio is routed through my NAD preamp and amp.
Last week, a hum started without having made any changes to any systems.


** That makes no sense - how do you get a TV pic.

You have no cable or antenna connection ?



..... Phil

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geoff geoff is offline
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On 27/10/2019 4:34 pm, Phil Allison wrote:
Bob Simon wrote:

-----------------

My Panasonic TV (TH-50PH9UK) is a video monitor only;
all audio is routed through my NAD preamp and amp.
Last week, a hum started without having made any changes to any systems.


** That makes no sense - how do you get a TV pic.

You have no cable or antenna connection ?



.... Phil


You must have missed the bit "My Panasonic TV (TH-50PH9UK) is a video
monitor only" !

geoff

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Phil Allison[_4_] Phil Allison[_4_] is offline
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geoff the Kiwi Jerk wrote:

---------------------------


My Panasonic TV (TH-50PH9UK) is a video monitor only;
all audio is routed through my NAD preamp and amp.
Last week, a hum started without having made any changes to any systems.


** That makes no sense - how do you get a TV pic.

You have no cable or antenna connection ?




You must have missed the bit "My Panasonic TV (TH-50PH9UK) is a video
monitor only" !



** Nup, a TV set can be a monitor with a digital STB plus antenna or cable box supplying the video.

Important to know, as those are prime causes of hum loops.



..... Phil
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John Williamson John Williamson is offline
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On 27/10/2019 05:26, Phil Allison wrote:
geoff wrote:
You must have missed the bit "My Panasonic TV (TH-50PH9UK) is a video
monitor only" !



** Nup, a TV set can be a monitor with a digital STB plus antenna or cable box supplying the video.

Important to know, as those are prime causes of hum loops.

The OP also states that the audio goes nowhere near the monitor.

Tciao for Now!

John.


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Phil Allison[_4_] Phil Allison[_4_] is offline
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John Williamson wrote:

---------------------------

You must have missed the bit "My Panasonic TV (TH-50PH9UK) is a video
monitor only" !



** Nup, a TV set can be a monitor with a digital STB plus antenna
or cable box supplying the video.

Important to know, as those are prime causes of hum loops.


The OP also states that the audio goes nowhere near the monitor.


** Fake quote - just like fake news.

Complete bull****.

Ask Mr Trump.




..... Phil

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Scott Dorsey Scott Dorsey is offline
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Bob Simon wrote:
My Panasonic TV (TH-50PH9UK) is a video monitor only; all audio is routed t=
hrough my NAD preamp and amp. Last week, a hum started without having made=
any changes to any systems. Today, I tried unsuccessfully to eliminated i=
t by plugging the TV into the same power strip that the preamp and amp are =
plugged into. Lifting the TV's ground eliminated the hum.


The TV has multiple grounds. That's the problem. Likely what you mean is
"lifting the TV's power line safety ground."

Does this suggest that a capacitor shorted last week in the amp, preamp, or=
TV?


No, it suggests that the grounding system on the cable TV line has likely
changed, since this is the most common source of these problems.

As a practical matter, have I created an unsafe or inappropriate condition =
by lifting the TV's ground pin?


Yes, totally. They don't call it a "safety ground" for nothing. Isolate
the ground on the cable TV line, not on the power line.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."
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Bob Simon[_2_] Bob Simon[_2_] is offline
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On Sunday, October 27, 2019 at 8:02:04 AM UTC-5, Scott Dorsey wrote:
Bob Simon wrote:
My Panasonic TV (TH-50PH9UK) is a video monitor only; all audio is routed t=
hrough my NAD preamp and amp. Last week, a hum started without having made=
any changes to any systems. Today, I tried unsuccessfully to eliminated i=
t by plugging the TV into the same power strip that the preamp and amp are =
plugged into. Lifting the TV's ground eliminated the hum.


The TV has multiple grounds. That's the problem. Likely what you mean is
"lifting the TV's power line safety ground."

Does this suggest that a capacitor shorted last week in the amp, preamp, or=
TV?


No, it suggests that the grounding system on the cable TV line has likely
changed, since this is the most common source of these problems.

As a practical matter, have I created an unsafe or inappropriate condition =
by lifting the TV's ground pin?


Yes, totally. They don't call it a "safety ground" for nothing. Isolate
the ground on the cable TV line, not on the power line.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."


Scott,
Thank you for your reply. Yes, I lifted the TV's safety ground.

The cable demarc is attached to the garage and is around 50' from the grounding rod for the electrical panel. The demarc has a separate grounding rod connected by some thin wire (perhaps 16 AWG). If I ground the demarc with 10 AWG solid copper wire and a brand new clamp, will that likely eliminate the multiple grounds issue?

I bought the Jensen ISOMAX years ago because I was told it would eliminate the cable ground issue. Apparently, it isn't. Do you have any idea why not?
Bob
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Scott Dorsey Scott Dorsey is offline
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Bob Simon wrote:
I bought the Jensen ISOMAX years ago because I was told it would eliminate =
the cable ground issue. Apparently, it isn't. Do you have any idea why no=
t?


Maybe you have a different issue. Disconnect the cable. Does the hum stop?
If so, it's not the cable.

The isolation transformer should effectively stop any ground loop through the
cable system. But test it and make sure.

"Don't guess, measure."
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."
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Bob Simon[_2_] Bob Simon[_2_] is offline
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On Sunday, October 27, 2019 at 8:55:36 AM UTC-5, Scott Dorsey wrote:
Bob Simon wrote:
I bought the Jensen ISOMAX years ago because I was told it would eliminate =
the cable ground issue. Apparently, it isn't. Do you have any idea why no=
t?


Maybe you have a different issue. Disconnect the cable. Does the hum stop?
If so, it's not the cable.

The isolation transformer should effectively stop any ground loop through the
cable system. But test it and make sure.

"Don't guess, measure."
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."


Scott,
Thanks! You were right! It had nothing to do with the cable feed. Following your advice to draw all the connections enabled me to identify the root cause.

I had forgotten that there is a high-level connection from the amp's main (speaker) outs to a powered subwoofer which was plugged into a different circuit. When I used a 3-wire extension cord to plug the sub into the power strip for the rest of the system, the hum went away. Now I have to figure out how to route an extension cord across the room in a way that my wife can tolerate. As an alternative, I wonder if an affordable power isolator exists for this type of issue. Are you aware of something suitable?


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Scott Dorsey Scott Dorsey is offline
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Bob Simon wrote:
I had forgotten that there is a high-level connection from the amp's main (=
speaker) outs to a powered subwoofer which was plugged into a different cir=
cuit. When I used a 3-wire extension cord to plug the sub into the power s=
trip for the rest of the system, the hum went away. Now I have to figure o=
ut how to route an extension cord across the room in a way that my wife can=
tolerate. As an alternative, I wonder if an affordable power isolator exi=
sts for this type of issue. Are you aware of something suitable?


Don't break power line grounds. Break signal grounds. In the pro audio
world we use balanced connections which make this easy. With consumer gear
you will need an isolation transformer to do the same thing. The cheap
Edcor stuff will likely be fine for you.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."
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Bob Simon[_2_] Bob Simon[_2_] is offline
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On Sunday, October 27, 2019 at 8:55:36 AM UTC-5, Scott Dorsey wrote:
Bob Simon wrote:
I bought the Jensen ISOMAX years ago because I was told it would eliminate =
the cable ground issue. Apparently, it isn't. Do you have any idea why no=
t?


Maybe you have a different issue. Disconnect the cable. Does the hum stop?
If so, it's not the cable.

The isolation transformer should effectively stop any ground loop through the
cable system. But test it and make sure.

"Don't guess, measure."
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."


Scott,
Thanks! You were right! It had nothing to do with the cable feed. Following your advice to draw all the connections enabled me to identify the root cause.

I had forgotten that there is a high-level connection from the amp's main (speaker) outs to a powered subwoofer which was plugged into a different circuit. When I used a 3-wire extension cord to plug the sub into the power strip for the rest of the system, the hum was reduced to a very low level. Now I have to figure out how to route an extension cord across the room in a way that my wife can tolerate. As an alternative, I wonder if an affordable power isolator exists for this type of issue so I won't need an extension cord (which adds resistance). Are you aware of something suitable?
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Phil Allison[_4_] Phil Allison[_4_] is offline
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Bob Simon wrote:

----------------


I had forgotten that there is a high-level connection from the
amp's main (speaker) outs to a powered subwoofer which was plugged
into a different circuit. When I used a 3-wire extension cord to
plug the sub into the power strip for the rest of the system, the
hum was reduced to a very low level.


** With the TV still de-earthed ?

FYI:

Hum loops are caused by TWO separate things.

1. Different AC circuits being used with independent safety grounds with varying levels of stray AC voltage - generally only millivolts.

2. Magnetic field injection into a ground LOOP or loops formed by the various items in a system being connected to each other as well as a single safety ground.

The mag field comes from any power transformer inside a loop, so part of the equipment.

Picking a single item, usually the main or power amplifier, and making that be the ONLY safety grounded thing is the usual fix.

With a home entertainment system that includes a TV signal arriving at the premises from outside, that is another source of ground hum just like situation #1 described above. Hence using a balun isolator is normal practice.

---------------------------------------------------


As an alternative, I wonder if an affordable power isolator exists for this type of issue so I won't need an extension cord (which adds resistance). Are you aware of something suitable?

** Most folk just break the ground pin off the plug or fit a plug with no such pin. Neither is safe or legal.

I know how to modify items to eliminate the problem safely, but it would not meet regulations so a device that did the same cannot go on sale.

For the curious - it involves fitting a 25amp bridge in series with the safety ground conductor. This allow the item to float up to a volt or so above ground with NO current flow.

The bridge is wired as pairs of inverse parallel diodes and can cope with a massive fault current if need be until a fuse blows or a supply breaker trips.

My workbench scope has this scheme fitted inside, for example.


..... Phil
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On Sunday, October 27, 2019 at 8:55:36 AM UTC-5, Scott Dorsey wrote:
Bob Simon wrote:
I bought the Jensen ISOMAX years ago because I was told it would eliminate =
the cable ground issue. Apparently, it isn't. Do you have any idea why no=
t?


Maybe you have a different issue. Disconnect the cable. Does the hum stop?
If so, it's not the cable.

The isolation transformer should effectively stop any ground loop through the
cable system. But test it and make sure.

"Don't guess, measure."
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."


Scott,
Thanks! You were right! It had nothing to do with the cable feed. Following your advice to draw all the connections enabled me to identify the root cause.

I had forgotten that there is a high-level connection from the amp's main (speaker) outs to a powered subwoofer which was plugged into a different circuit. When I used a 3-wire extension cord to plug the sub into the power strip for the rest of the system, the hum was reduced to a very low level. Now I have to figure out how to route an extension cord across the room in a way that my wife can tolerate. As an alternative, I wonder if an affordable power isolator exists for this type of issue so I could eliminate the extension cord. Are you aware of something suitable?
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