"Peter Chant" wrote in message
...
Any thoughts on the following?

Cambridge C70 pre and two A70 power amps. I've suddenly stated getting
hum
from both power amps when:

* Preamp is plugged into mains (whether set to active or passive)
* If preamp is passive and not connected to the mains and I plug it into
Arcam Alpha 5 tuner, which I assume is earthed.
* I replace preamp with my own passive box with switch and volume pot.

Thing is this set up was fine for quite a while and I find it hard to
beleive that a loop has suddenly appeared affecting two amps.

Further information, this happens irrespective of whether it is plugged
into
TV - but nowadays I have isolating transformers between TV and preamp.
This
should be of no consequence to the discussion but I started getting hum on
my rear surround speakers that are connected to a media PC without
isolating
transformers. These started humming at the same time. This latter one I
suspect is easy, Virgin have done something with the cable feed and I need
more isolating transformers.

Now, the latter issue seems to have occured at the same time as the first
one, but I cannot see how it is related due to my use of isolating
transformers between TV and Cambridge amps.

The only thing that seems logical is that the Cambridge power amps both
might have differential inputs to avoid earth loop problems, and that they
both failed simultaneously. But this does not seem credible.

Any thoughts on this?



Lift the earth on an extension lead and plug the pre into that and check all
of the above scenarios to see if you still get hum. - for starters.

http://www.petezilla.co.uk


Nice photos!

On Sat, 24 Jul 2010 10:50:31 +0100, "Keith G"
wrote:


"Peter Chant" wrote in message
...
Any thoughts on the following?

Cambridge C70 pre and two A70 power amps. I've suddenly stated getting
hum
from both power amps when:

* Preamp is plugged into mains (whether set to active or passive)
* If preamp is passive and not connected to the mains and I plug it into
Arcam Alpha 5 tuner, which I assume is earthed.
* I replace preamp with my own passive box with switch and volume pot.

Thing is this set up was fine for quite a while and I find it hard to
beleive that a loop has suddenly appeared affecting two amps.

Further information, this happens irrespective of whether it is plugged
into
TV - but nowadays I have isolating transformers between TV and preamp.
This
should be of no consequence to the discussion but I started getting hum on
my rear surround speakers that are connected to a media PC without
isolating
transformers. These started humming at the same time. This latter one I
suspect is easy, Virgin have done something with the cable feed and I need
more isolating transformers.

Now, the latter issue seems to have occured at the same time as the first
one, but I cannot see how it is related due to my use of isolating
transformers between TV and Cambridge amps.

The only thing that seems logical is that the Cambridge power amps both
might have differential inputs to avoid earth loop problems, and that they
both failed simultaneously. But this does not seem credible.

Any thoughts on this?



Lift the earth on an extension lead and plug the pre into that and check all
of the above scenarios to see if you still get hum. - for starters.

http://www.petezilla.co.uk


Nice photos!



I had considerable trouble with a cable feed causing hum and bizarre
behaviour with a Linn CD player and power amp. Does your hum disappear
when you disconnect the cable RF connector? If it does, the hum may be
a result of powerline frequency hum currents coming through the
RF/coax grounding.
I had this problem, and I solved it by using an RF isolation
transformer at the on the cable feed itself. Since the ordinary RF
transformers (baluns or splitters) do not isolate grounds, they will
not do the job. I ended up breaking open a cable splitter, and using
the tiny little toroidal core inside. I used a couple of turns for the
primary, and the same number of turns on the secondary. You could get
fancy and wind the primary and secondary "bifilar". The little ferrite
toroids in the splitter are ideal for the wide range of
frequencies.... I wouldn't recommend any other types unless you are
real comfortable with RF stuff. Of course, you need to make sure that
the grounds on secondary & primary are isolated. (I mounted the 2 RF
connectors and toroid in a little plastic tube.) Lead lengths must be
kept to a minimum.
It's cheaper than powerline isolation, and will keep out most of the
high frequency crap that can irritate some of the electronics.
I am not aware of any common commercial RF isolator for this
purpose. Maybe its an opportunity for someone to market it as a cure
for some of the weird effects you get when tied into cable.

Paul G.

"Paul G." wrote

I had considerable trouble with a cable feed causing hum and bizarre
behaviour with a Linn CD player and power amp.

You have a cable feed to a CD player?

I am not aware of any common commercial RF isolator for this
purpose. Maybe its an opportunity for someone to market it as a cure
for some of the weird effects you get when tied into cable.


So called "Braid-breakers" have existed for many years to help keep out
interference picked up on the braid of a TV or FM radio downlead. They work
in a similar way to what you have described, ie. a small ferrite-cored
transformer.

David.

On Sun, 25 Jul 2010 08:13:20 +0100, "David Looser"
wrote:

"Paul G." wrote

I had considerable trouble with a cable feed causing hum and bizarre
behaviour with a Linn CD player and power amp.

You have a cable feed to a CD player?

No..... but the audio system shares a ground with the FM tuner
which has a cable connection. The CD player (Linn Karik & Numerik)
would do strange things now and then, until the cable connection was
removed. Our cable must have some pretty weird stuff on it, as many
other audio devices would hum or behave oddly when they shared a
ground with the cable.

I am not aware of any common commercial RF isolator for this
purpose. Maybe its an opportunity for someone to market it as a cure
for some of the weird effects you get when tied into cable.


So called "Braid-breakers" have existed for many years to help keep out
interference picked up on the braid of a TV or FM radio downlead. They work
in a similar way to what you have described, ie. a small ferrite-cored
transformer.

David.


There must be a variety of types of "braid-breakers"... Most of what
I see are mentioned on the internet are (ASCII circuit) :

C1
in1-------.-----||------.------- out1
) )
)L1 )L2
) )
) )
in2-------'-----||------'------- out2
| C2 |
| |
| |
'-/\/\/\/\/---'
R1
R1= a few megohms (needed to bleed current to avoid hi-voltage
buildup)
C1,C2=5-10pf
L1,L2= few turns of wire (not coupled to each other)
To see the ASCII circuit properly, you need a fixed width font like
"courier"
This circuit works at the UHF frequencies, but doesn't respond well
to VHF or lower frequencies. It will definitely clean up any powerline
frequencies.

The RF isolation transformer I mentioned above is referenced a few
times as a braid breaker. With a suitable toroid transformer (like
what you can pull out of a splitter) you should get very good
isolation and uniform response from VHF to well beyond UHF.

I asked around for such a device at TV/electronics stores here in
Nova Scotia (Canada), no one had a clue what I was talking about.

Paul G.

"Paul G." wrote in message
...
On Sun, 25 Jul 2010 08:13:20 +0100, "David Looser"
wrote:


You have a cable feed to a CD player?

No..... but the audio system shares a ground with the FM tuner
which has a cable connection.

I guessed it had to be something like that ;-)

The CD player (Linn Karik & Numerik)
would do strange things now and then, until the cable connection was
removed.

That points to poor design of the CD player. Ground-loop hum is one thing,
"strange behaviour" suggests that the items affected have poor immunity to
EM interference.

Our cable must have some pretty weird stuff on it, as many
other audio devices would hum or behave oddly when they shared a
ground with the cable.

Ditto.



There must be a variety of types of "braid-breakers"... Most of what
I see are mentioned on the internet are (ASCII circuit) :

C1
in1-------.-----||------.------- out1
) )
)L1 )L2
) )
) )
in2-------'-----||------'------- out2
| C2 |
| |
| |
'-/\/\/\/\/---'
R1

Yup, when I did an internet search for "braid-breaker" in response to your
post that's mostly what I found too. I couldn't find any commercially
available unit with a transformer which suprised me, as they were certainly
available 20 years ago.

David.

David Looser wrote:

C1
in1-------.-----||------.------- out1
) )
)L1 )L2
) )
) )
in2-------'-----||------'------- out2
| C2 |
| |
| |
'-/\/\/\/\/---'
R1

Yup, when I did an internet search for "braid-breaker" in response to your
post that's mostly what I found too. I couldn't find any commercially
available unit with a transformer which suprised me, as they were
certainly available 20 years ago.

I'm surprised that they'd not built these into the cable boxes or the
pillars in the street to avoid any nastys on the line.

--
http://www.petezilla.co.uk

Paul G. wrote:

The RF isolation transformer I mentioned above is referenced a few
times as a braid breaker. With a suitable toroid transformer (like
what you can pull out of a splitter) you should get very good
isolation and uniform response from VHF to well beyond UHF.

When you say "tiny little torroid" in your previous post I presume you don't
mean small like 1/2" but the really tiny ones that are like beads?


Pete

--
http://www.petezilla.co.uk

On Mon, 26 Jul 2010 08:11:17 +0100, Peter Chant
wrote:

Paul G. wrote:

The RF isolation transformer I mentioned above is referenced a few
times as a braid breaker. With a suitable toroid transformer (like
what you can pull out of a splitter) you should get very good
isolation and uniform response from VHF to well beyond UHF.

When you say "tiny little torroid" in your previous post I presume you don't
mean small like 1/2" but the really tiny ones that are like beads?


They are about 4mm long, and 4mm in diameter (about 1/8 inch). They
have a very good "mu" throughout their frequency range, same sort of
thing that makes a good audio transformer, but at MUCH higher
frequencies. Same issues of magnetizing inductance & leakage
inductance, loss, etc. The older types of ferrite formulations don't
handle the extreme range of frequencies... so I'm told. 50-1000Mhz is
not that easy to do.
They aren't as small as the old core memory boards... I used to
have a set of boards for a 16KB memory plane, you needed a microscope
to see the ferrite toroids. The hole was just big enough for the 4
tiny wires to pass through the toroid. Students were quite in awe of
how small these things were, and always had a laugh at the puny memory
size. It was being used for real time video analysis in the '70's,
something that was then very difficult. It was handled by a CDC
mainframe, an ECL logic computer. The mainframe had its own core
memory, much bigger in capacity, and much faster (see
http://en.wikipedia.org/wiki/Magnetic_core_memory ).
The ferrite cores used in memory are unsuitable for RF.

Paul G.

Paul G. wrote:


I had considerable trouble with a cable feed causing hum and bizarre
behaviour with a Linn CD player and power amp. Does your hum disappear
when you disconnect the cable RF connector? If it does, the hum may be
a result of powerline frequency hum currents coming through the
RF/coax grounding.

Interesting.

I've actually got two problems. Yes, there is definitely an earth loop with
the cable feed but there is also definitely a problem with the amps when
the TV etc is disconnected. I've got audio transformers between the TV and
amp to try to keep TV/cable induced nastys out of the hifi. Before I had
this there were sufficient nastys to blow the FETs in the mute circuitry of
my tuner.

Pete

--
http://www.petezilla.co.uk