Sudden earth loop
 

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

Sudden earth loop
 

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

Sudden earth loop
 

"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!

Sudden earth loop
 

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

Sudden earth loop
 

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

Sudden earth loop
 

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.

Sudden earth loop
 

"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.

Sudden earth loop
 

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

Sudden earth loop
 

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

Sudden earth loop
 

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.

Sudden earth loop
 

On Mon, 26 Jul 2010 00:28:30 +0100, Peter Chant
wrote:

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

This is all very odd. A hum appearing when you plug something in, even
if not switched on, is classic ground loop. And as you describe the
system, each item individually grounded to the mains, is a guaranteed
generator of hum. You may not need to resort to a transformer though.
First try some coax cables with broken grounds for your interconnects.
I don't know how practical/technical you are, but this is certainly
something you can easily do for yourself. Break the ground connection
in just the left hand interconnect first. That should have the same
effect on the hum as disconnecting the mains ground. If it does, stick
with that. There are no safety implications and it is standard
practice.

But the system as you describe it should always have hummed unless you
have recently changed the interconnects.

d

Sudden earth loop
 

"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.

Sudden earth loop
 

Don Pearce wrote:



This is all very odd. A hum appearing when you plug something in, even
if not switched on, is classic ground loop. And as you describe the
system, each item individually grounded to the mains, is a guaranteed
generator of hum. You may not need to resort to a transformer though.
First try some coax cables with broken grounds for your interconnects.
I don't know how practical/technical you are, but this is certainly
something you can easily do for yourself. Break the ground connection
in just the left hand interconnect first. That should have the same
effect on the hum as disconnecting the mains ground. If it does, stick
with that. There are no safety implications and it is standard
practice.

I made the interconnects, so breaking the screen is not an issue. This may
well be an expedient fix before I get around to finding the real culprit.


But the system as you describe it should always have hummed unless you
have recently changed the interconnects.

I'm suspicious of the input board on the amp. With just the offending amp
switched on (all connected) I get hum, but it is much stronger in one
channel than the other. If it were a regular earth loop resulting from the
design I would have thought it would be equal in both channels. I have
previously checked the diodes on the input board of that amp, all seemed
well, as far as I could tell. I'd think that it is not one of the op-amps
or I'd have a dead channel.

Pete

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

Sudden earth loop
 

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

Sudden earth loop
 

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

Sudden earth loop
 

In article , Peter Chant
wrote:


I'm suspicious of the input board on the amp. With just the offending
amp switched on (all connected) I get hum, but it is much stronger in
one channel than the other. If it were a regular earth loop resulting
from the design I would have thought it would be equal in both
channels. I have previously checked the diodes on the input board of
that amp, all seemed well, as far as I could tell. I'd think that it
is not one of the op-amps or I'd have a dead channel.

Not commented previously as I have been trying to get clear which items
have genuine mains-earth wires to their sockets and which do not.

Also, am I correct in thinking that when you swapped over the power amps
the presence/absence of hum followed the physical amp, not the channel?

In general I'd advise only having *one* item in the system with a mains
earth connection and using the connector braids back to it for the ground
definition of all other items. The problem with using multiple mains
grounds is that the results depend on the inter-unit ground wiring and
other items that use it. So what seems OK can change if the mains wiring
alters (e.g. a connection becomes poor) or some other item starts injecting
return into the wires.

It may well be that in a given setup multiple mains earth connections will
sound OK. If so, fine to use them.

For AV systems I tend to regard conventional TVs as worth being mains
earthed as they can tend to float large potentials. This is true with CRT
TVs in my experience, but I don't if flat panels are better in this
respect.

FWIW I've been wondering if the problem is due to a poor connection in thr
ground wiring of the relevant power amp. Either in the amp itself or in the
cable or plug/sockets.

Slainte,

Jim

--
Please use the address on the audiomisc page if you wish to email me.
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html
Audio Misc http://www.audiomisc.co.uk/index.html

Sudden earth loop
 

Jim Lesurf wrote:

In article , Peter Chant
wrote:


I'm suspicious of the input board on the amp. With just the offending
amp switched on (all connected) I get hum, but it is much stronger in
one channel than the other. If it were a regular earth loop resulting
from the design I would have thought it would be equal in both
channels. I have previously checked the diodes on the input board of
that amp, all seemed well, as far as I could tell. I'd think that it
is not one of the op-amps or I'd have a dead channel.

Not commented previously as I have been trying to get clear which items
have genuine mains-earth wires to their sockets and which do not.


Thanks. This is all useful, can be helpful to discuss things when trying to
sort out problems.

Pre-amp powered - mains earth
2x power amps - mains earth.

cd player - no earth (double insulated)


Also, am I correct in thinking that when you swapped over the power amps
the presence/absence of hum followed the physical amp, not the channel?

Kind of. When the left hand amp is connected both amps hum. The right amp
on its own does not hum. Swapping left and right on the preamp does not
cause this condition to swap - therefore it is not a fault on one of the
preamp channels.


In general I'd advise only having *one* item in the system with a mains
earth connection and using the connector braids back to it for the ground
definition of all other items. The problem with using multiple mains
grounds is that the results depend on the inter-unit ground wiring and
other items that use it. So what seems OK can change if the mains wiring
alters (e.g. a connection becomes poor) or some other item starts
injecting return into the wires.

It may well be that in a given setup multiple mains earth connections will
sound OK. If so, fine to use them.


It sounded fine until recently - a slight hum, but you had to approach the
speakers to hear it with nothing being played. At present it is
unacceptable.

Incidentally, left hand amp (the problematic one) - phono sheild to earth
pin reads 0 ohms, right hand amp which is fine reads 20 ohms. Something odd
going on there.

For AV systems I tend to regard conventional TVs as worth being mains
earthed as they can tend to float large potentials. This is true with CRT
TVs in my experience, but I don't if flat panels are better in this
respect.


Something to do with the flyback transformer? A charge build up? All TV's
I've owned have not been earthed.

FWIW I've been wondering if the problem is due to a poor connection in thr
ground wiring of the relevant power amp. Either in the amp itself or in
the cable or plug/sockets.

I'm suspicious of the input circuitry. I suspect something has failed short
circuit.

Pete

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

Sudden earth loop
 

On 27/07/2010 11:30 a.m., Peter Chant wrote:
Jim Lesurf wrote:

In , Peter Chant
wrote:


I'm suspicious of the input board on the amp. With just the offending
amp switched on (all connected) I get hum, but it is much stronger in
one channel than the other. If it were a regular earth loop resulting
from the design I would have thought it would be equal in both
channels. I have previously checked the diodes on the input board of
that amp, all seemed well, as far as I could tell. I'd think that it
is not one of the op-amps or I'd have a dead channel.

Not commented previously as I have been trying to get clear which items
have genuine mains-earth wires to their sockets and which do not.


Thanks. This is all useful, can be helpful to discuss things when trying to
sort out problems.

Pre-amp powered - mains earth
2x power amps - mains earth.

cd player - no earth (double insulated)


Also, am I correct in thinking that when you swapped over the power amps
the presence/absence of hum followed the physical amp, not the channel?

Kind of. When the left hand amp is connected both amps hum. The right amp
on its own does not hum. Swapping left and right on the preamp does not
cause this condition to swap - therefore it is not a fault on one of the
preamp channels.


In general I'd advise only having *one* item in the system with a mains
earth connection and using the connector braids back to it for the ground
definition of all other items. The problem with using multiple mains
grounds is that the results depend on the inter-unit ground wiring and
other items that use it. So what seems OK can change if the mains wiring
alters (e.g. a connection becomes poor) or some other item starts
injecting return into the wires.

It may well be that in a given setup multiple mains earth connections will
sound OK. If so, fine to use them.


It sounded fine until recently - a slight hum, but you had to approach the
speakers to hear it with nothing being played. At present it is
unacceptable.

Incidentally, left hand amp (the problematic one) - phono sheild to earth
pin reads 0 ohms, right hand amp which is fine reads 20 ohms. Something odd
going on there.

For AV systems I tend to regard conventional TVs as worth being mains
earthed as they can tend to float large potentials. This is true with CRT
TVs in my experience, but I don't if flat panels are better in this
respect.


Something to do with the flyback transformer? A charge build up? All TV's
I've owned have not been earthed.

FWIW I've been wondering if the problem is due to a poor connection in thr
ground wiring of the relevant power amp. Either in the amp itself or in
the cable or plug/sockets.

I'm suspicious of the input circuitry. I suspect something has failed short
circuit.

Pete

Check the faulty amp to see if it has a ground lift resistor or capacitor
that has gone short circuit. This would explain the sudden onset of hum
from that amp. It shouldn't be that hard to find as you have another amp to
compare with.

Sudden earth loop
 

In article , Mike Coatham
wrote:

Check the faulty amp to see if it has a ground lift resistor or
capacitor that has gone short circuit. This would explain the sudden
onset of hum from that amp. It shouldn't be that hard to find as you
have another amp to compare with.

That was also my reaction. However you may be unlucky. It may be that your
amp's transformer has an earthed 'electostatic shield' and it has become
shorted to the windings in a way that gives 0 Ohms. That can mean
significant earth currents that should be flowing in the Neutral. Not good.

Slainte,

Jim

--
Please use the address on the audiomisc page if you wish to email me.
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html
Audio Misc http://www.audiomisc.co.uk/index.html

GOT IT!!!!! Sudden earth loop
 

Jim Lesurf wrote:

In article , Mike Coatham
wrote:

Check the faulty amp to see if it has a ground lift resistor or
capacitor that has gone short circuit. This would explain the sudden
onset of hum from that amp. It shouldn't be that hard to find as you
have another amp to compare with.


No ground lift resistors diodes etc. Why it was not humming before I don't
know as there appears, at least on the power amp side, no attempt to address
the issue. Suspect it was small enough to generally not be significant.

Cable box is the culprit, perhaps, via the audio and video connections with
the media PC and TV. Suspect somewhere there is a big fat earth-loop via
the cable feed that was not there before, and it has been dumping current
into the multiway adapter that the TV, cable box, media PC and left hand amp
are plugged into. Result, unhappy pre-power combination as the left hand
amp's earth jumps up and down relative to the rest. Removing the cable feed
from the cable box removes the hum The hint was that just earthing the
screen of the pre to left hand interconnect to the respective power amp's
chassis caused hum - yet doing the same thing on the right hand channel did
nothing significant.

Plugging in the left hand amp into the same 4 way adapter as the pre and
other power sorted it.

This explains why by rears were humming at the same time as the fronts when
effectively a separate setup - it was also connected to the same multiway.
BTW, multiway's earth checks out fine.

But it is odd. I can measure no significant voltages or currents anywhere.
0.3V, open circuit between cable feed and earth, but that is small.

That was also my reaction. However you may be unlucky. It may be that your
amp's transformer has an earthed 'electostatic shield' and it has become
shorted to the windings in a way that gives 0 Ohms. That can mean
significant earth currents that should be flowing in the Neutral. Not
good.

Fortunately not that. Toroid, have part number but there is no spec on it.
Looks fairly standard - could probably replace if I needed to - though I
cannot forsee, unless I want to reuse it in a home made welding set up, why
it would fail.

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

Sudden earth loop
 

Peter Chant wrote:


Incidentally, left hand amp (the problematic one) - phono sheild to earth
pin reads 0 ohms, right hand amp which is fine reads 20 ohms. Something
odd going on there.

Simple answer. Both amps outwardly the same. Main circuit boards are
different, design must have been revised. 20 ohm resistor between chassis
(and mains earth) and 0V on right hand amp only.

Pete


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

Sudden earth loop
 

Sounds more likely that somewhere there is a bad earth which is causing the
earth currents to flow around another way in these situations.

Brian

--
Brian Gaff....Note, this account does not accept Bcc: email.
graphics are great, but the blind can't hear them
Email:
__________________________________________________ __________________________________________________ __________


"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?

Pete


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

Sudden earth loop
 

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.