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?

Pete


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

On Sat, 24 Jul 2010 10:43:21 +0100, Peter Chant
wrote:

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?

Pete

Have you tried unplugging things to see what is causing the hum? Try
and arrive at the minimal setup that results in hum - even if that
means disconnecting one of the power amps - then describe the circuit.

A sudden ground loop can't happen all by itself, and you have
undoubtedly done some small thing you didn't even notice.

d

"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!

Don Pearce wrote:


Have you tried unplugging things to see what is causing the hum? Try
and arrive at the minimal setup that results in hum - even if that
means disconnecting one of the power amps - then describe the circuit.

Pre amp - plugged in
2XPower amps

I'll have to try them separately.

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

On Sat, 24 Jul 2010 16:34:28 +0100, Peter Chant
wrote:

Don Pearce wrote:


Have you tried unplugging things to see what is causing the hum? Try
and arrive at the minimal setup that results in hum - even if that
means disconnecting one of the power amps - then describe the circuit.

Pre amp - plugged in
2XPower amps

I'll have to try them separately.

Are the power amps separate items, each with its own mains lead? This
is a very minimal setup to still be causing hum, and I suspect maybe
you have a power supply fault in the pre-amp.

There is always a bit of a battle between minimal hum and safety.
Minimum hum demands a single earth connection for the entire system,
while safety says each item should have its own. If the preamp or
power amps are double insulated, then they won't need a safety ground
and the signal grounds in the coax connections will do.

Anyway, temporarily disconnect the grounds from the mains plugs in
each item and see if you get a cure. If you do, then some more
isolation transformers will be a good idea. Maplin sell them for car
stereos. If there is no cure, then it isn't a ground loop but a fault
in the preamp.

d

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.

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

Don Pearce wrote:



Are the power amps separate items, each with its own mains lead? This
is a very minimal setup to still be causing hum, and I suspect maybe
you have a power supply fault in the pre-amp.

Two power amps and a pre-amp. As far as I'm aware the power amps and pre-
amps were designed to work together ie Cambridge A70 power amps and C70 pre-
amp - I've not mixed and matched. All have mains leads.


There is always a bit of a battle between minimal hum and safety.
Minimum hum demands a single earth connection for the entire system,
while safety says each item should have its own. If the preamp or
power amps are double insulated, then they won't need a safety ground
and the signal grounds in the coax connections will do.

Neither the pre nor power amps appear to be double insulated, all have IEC
leads and working earth connections in their leads.


Anyway, temporarily disconnect the grounds from the mains plugs in
each item and see if you get a cure. If you do, then some more
isolation transformers will be a good idea. Maplin sell them for car
stereos. If there is no cure, then it isn't a ground loop but a fault
in the preamp.

I'm using one power amp for the left speaker, one for the right.

With pre-amp connected to the mains, active powered with one of the power
amps, the one I use on the right, I have no hum. As soon as the left power
amp is connected - even if not switched on - I get hum. This hum is audible
in both amps and the headphones in the pre-amp.

Disconnecting the earth to the left hand power amp stops the hum. Swapping
the power amps over, left and right, and the hum still occurs when the same
amp is connected. It therefore appears that the fault is not in the preamp.
I am rather suspicious of the input circuity of the left hand amp. I am
rather confused though why hum is the only issue if there is a fault on the
input.

I'm loath to chuck transformers at the problem when it appears that there is
a genunine fault.

Pete

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

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.