I'm having some nasty 60Hz hum which I don't know how to avoid. I'm
hoping someone here can help.

I've reduced the problem to this:



RCN Coax +-------------------+ RCA Audio +----------+ /|
========| Digital Cable Box +=+=========+|Audio Amp +=[] |
RF in | ("off") | | : +----------+ \|
+-------------------+ | :
+-+ :
Ground A | .......:
| : Ground B
| :
+---------+-+---+
|TV ("off" but |
| plugged in)|
+---------------+


Ground A and B are just a single conductor connecting the outer
conductor of any video input on the TV to the shield at either end of
the RCA cable feeding audio from the cable box to the audio amp. When
Ground B is connected we get a nasty 60Hz hum.


* It doesn't matter whether Ground A is connected or not -- I just
put it in the diagram to show that connecting that ground doesn't,
by itself, cause the problem -- you need to have a ground
connection between the TV and the audio out cable but not right at
the cable box. Halfway down the audio cable the ground still
causes the hum.

* Disconnecting the TV's power plug makes the problem go away

* Disconnecting the RF input fromt the cable box makes the problem
go away.

* We've heard some hum even when the cable box's power plug is
unplugged, though it gets worse when it's plugged in.

* The problem happens with any audio amp or speaker.

* I wrote "off" above because as you know most devices these days
are never entirely off. On the TV, for example, there's an LED
glowing on the front even when it's off.

* We're using only high-quality monster cable to connect everything.

In case you're wondering, we need to connect ground B if we want to
switch video from the cable box and a VCR through our receiver, which
does video routing.

Thanks in advance,
--
Dave Abrahams
Boost Consulting
www.boost-consulting.com

I suspect that the biggest problem is the ground loop leg going up the RCN coax since it is grounded so far away. The fact that removing it corrects the situation is evidence of this. Nearly all of the problems that occur similar to yours can usually be fixed by isolating the coax ground using a quality wide-band isolation transformer such as Jensen's VRD-1FF. There is a picture of one and you can order them from:

http://www.jensen-transformers.com/

Hope this helps

- Jeff


David Abrahams wrote:

I'm having some nasty 60Hz hum which I don't know how to avoid. I'm
hoping someone here can help.

I've reduced the problem to this:

RCN Coax +-------------------+ RCA Audio +----------+ /|
========| Digital Cable Box +=+=========+|Audio Amp +=[] |
RF in | ("off") | | : +----------+ \|
+-------------------+ | :
+-+ :
Ground A | .......:
| : Ground B
| :
+---------+-+---+
|TV ("off" but |
| plugged in)|
+---------------+

Ground A and B are just a single conductor connecting the outer
conductor of any video input on the TV to the shield at either end of
the RCA cable feeding audio from the cable box to the audio amp. When
Ground B is connected we get a nasty 60Hz hum.

* It doesn't matter whether Ground A is connected or not -- I just
put it in the diagram to show that connecting that ground doesn't,
by itself, cause the problem -- you need to have a ground
connection between the TV and the audio out cable but not right at
the cable box. Halfway down the audio cable the ground still
causes the hum.

* Disconnecting the TV's power plug makes the problem go away

* Disconnecting the RF input fromt the cable box makes the problem
go away.

* We've heard some hum even when the cable box's power plug is
unplugged, though it gets worse when it's plugged in.

* The problem happens with any audio amp or speaker.

* I wrote "off" above because as you know most devices these days
are never entirely off. On the TV, for example, there's an LED
glowing on the front even when it's off.

* We're using only high-quality monster cable to connect everything.

In case you're wondering, we need to connect ground B if we want to
switch video from the cable box and a VCR through our receiver, which
does video routing.

Thanks in advance,
--
Dave Abrahams
Boost Consulting
www.boost-consulting.com

David Abrahams wrote:

I'm having some nasty 60Hz hum which I don't know how to avoid. I'm
hoping someone here can help.

I've reduced the problem to this:



RCN Coax +-------------------+ RCA Audio +----------+ /|
========| Digital Cable Box +=+=========+|Audio Amp +=[] |
RF in | ("off") | | : +----------+ \|
+-------------------+ | :
+-+ :
Ground A | .......:
| : Ground B
| :
+---------+-+---+
|TV ("off" but |
| plugged in)|
+---------------+


Ground A and B are just a single conductor connecting the outer
conductor of any video input on the TV to the shield at either end of
the RCA cable feeding audio from the cable box to the audio amp. When
Ground B is connected we get a nasty 60Hz hum.


* It doesn't matter whether Ground A is connected or not -- I just
put it in the diagram to show that connecting that ground doesn't,
by itself, cause the problem -- you need to have a ground
connection between the TV and the audio out cable but not right at
the cable box. Halfway down the audio cable the ground still
causes the hum.

* Disconnecting the TV's power plug makes the problem go away

* Disconnecting the RF input fromt the cable box makes the problem
go away.

If you would have read ANY of the threads here within the past month
that address ground loops, you wouldn't have had to take the time to
post all of this.


* We've heard some hum even when the cable box's power plug is
unplugged, though it gets worse when it's plugged in.

* The problem happens with any audio amp or speaker.

* I wrote "off" above because as you know most devices these days
are never entirely off. On the TV, for example, there's an LED
glowing on the front even when it's off.

* We're using only high-quality monster cable to connect everything.

In case you're wondering, we need to connect ground B if we want to
switch video from the cable box and a VCR through our receiver, which
does video routing.

Thanks in advance,

Jeff Wiseman writes:

I suspect that the biggest problem is the ground loop leg going up
the RCN coax since it is grounded so far away. The fact that
removing it corrects the situation is evidence of this. Nearly all
of the problems that occur similar to yours can usually be fixed by
isolating the coax ground using a quality wide-band isolation
transformer such as Jensen's VRD-1FF. There is a picture of one and
you can order them from:

http://www.jensen-transformers.com/

Hope this helps

Thanks; I was beginning to think that this would be the solution, but
it really helps to have someone else say so with a reasonable degree
of confidence. I really appreciate your guidance.

--
Dave Abrahams
Boost Consulting
www.boost-consulting.com

CJT writes:

If you would have read ANY of the threads here within the past month
that address ground loops, you wouldn't have had to take the time to
post all of this.

[insert cat noise here]

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