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

"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.

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

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

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

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

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

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.

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

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