ground-loop problems

[William Sommerwerck]

I'd like to thank everyone who took the time to respond to my question. I
was particularly appreciative of those that told me about noise & grounding
problems I've never even heard of.

I'm not quite ready to put everything on one circuit. So I'm going to call
both Parasound and Pioneer to see what insights, if any, they have to offer.

Again, thank you-all for your help.

PS: As for balanced cables being an OTT solution for home installations... I
have five power amps, four of which sit next to the speakers they drive.
Given the distances, and the fact that the amps draw significant current,
and some are on different circuits, it would not be a good idea to use
unbalanced cables.

 

[Dave Plowman (News)]

In article <h0lmjk$dqd$1@news.eternal-september.org>,
William Sommerwerck <grizzledgeezer@comcast.net> wrote:

I'd like to thank everyone who took the time to respond to my question.
I was particularly appreciative of those that told me about noise &
grounding problems I've never even heard of.

I'm not quite ready to put everything on one circuit. So I'm going to
call both Parasound and Pioneer to see what insights, if any, they have
to offer.

I dunno US electrics, but can't see why that would matter. As far as audio
- or video - is concerned there should be only one ground. And that
doesn't really matter what mains circuit things are fed from as there will
be an isolating transformer between it and the DC. Unless it's some crappy
design where it has to be grounded for safety reasons.

Again, thank you-all for your help.

PS: As for balanced cables being an OTT solution for home
installations... I have five power amps, four of which sit next to the
speakers they drive. Given the distances, and the fact that the amps
draw significant current, and some are on different circuits, it would
not be a good idea to use unbalanced cables.

What distances are you talking about? I have unbalanced runs here of some
80 ft without problems. I do use balanced audio to feed things round the
house - but that is purely to use cheap telephone cable and avoid
crosstalk.

--
*I finally got my head together, now my body is falling apart.

Dave Plowman dave@davenoise.co.uk London SW
To e-mail, change noise into sound.

 

[William Sommerwerck]

I'm not quite ready to put everything on one circuit. So I'm going to
call both Parasound and Pioneer to see what insights, if any, they have
to offer.

I dunno US electrics, but can't see why that would matter. As far as
audio -- or video -- is concerned there should be only one ground.

Should be -- but isn't. Each device has its own ground. Each ground may have
a slightly different AC potential. This is the cause of "ground loops" when
you connct the devices.

All the equipment in my system is transformer-powered. That, in and of
itself, does not prevent ground loops.

PS: As for balanced cables being an OTT solution for home
installations... I have five power amps, four of which sit next to the
speakers they drive. Given the distances, and the fact that the amps
draw significant current, and some are on different circuits, it would
not be a good idea to use unbalanced cables.

What distances are you talking about? I have unbalanced runs here
of some 80 ft without problems. I do use balanced audio to feed things
round the house - but that is purely to use cheap telephone cable and
avoid crosstalk.

You're fortunate to have no problems with 80' runs. Professionals use
balanced lines for a good -- it avoids problems.

 

[Dave Plowman (News)]

In article <h0meak$p4i$1@news.eternal-september.org>,
William Sommerwerck <grizzledgeezer@comcast.net> wrote:

I dunno US electrics, but can't see why that would matter. As far as
audio -- or video -- is concerned there should be only one ground.

Should be -- but isn't. Each device has its own ground. Each ground may
have a slightly different AC potential. This is the cause of "ground
loops" when you connct the devices.

Indeed.

All the equipment in my system is transformer-powered. That, in and of
itself, does not prevent ground loops.

In the UK pretty well all audio equipment has class II insulation - so
doesn't *need* the metalwork grounded to a mains earth for safety reasons.
And has been for many a year. So won't normally have a mains earth
connected. If it did have you'd get earth loop problems - since the DC
side is normally grounded. And that is connected to the co-ax screen. So
the only ground connection is via the interconnect cable - avoiding earth
loop problems. You can sometimes get this with using more than one tuner
off an aerial DA - but not so usually with video equipment which is
designed to be daisy chained off one aerial feed.

PS: As for balanced cables being an OTT solution for home
installations... I have five power amps, four of which sit next to the
speakers they drive. Given the distances, and the fact that the amps
draw significant current, and some are on different circuits, it would
not be a good idea to use unbalanced cables.

What distances are you talking about? I have unbalanced runs here
of some 80 ft without problems. I do use balanced audio to feed things
round the house - but that is purely to use cheap telephone cable and
avoid crosstalk.

You're fortunate to have no problems with 80' runs. Professionals use
balanced lines for a good -- it avoids problems.

I'm a sound recordist working in TV so yes I know about balanced
equipment. But its use is more for the common mode rejection of
interference on long and hazardous runs - as well as to provide immunity
from ground loops across a variety of equipment which is likely to be used
in different situations. But shouldn't be needed in a domestic environment
for a permanent installation. And such connections are rare in domestic
equipment. And rightly so, since they add unnecessary electronics at both
input and output - adding to complexity and cost.

--
*The hardness of the butter is proportional to the softness of the bread *

Dave Plowman dave@davenoise.co.uk London SW
To e-mail, change noise into sound.

 

[William Sommerwerck]

You're fortunate to have no problems with 80' runs. Professionals use
balanced lines for a good -- it avoids problems.

I'm a sound recordist working in TV so yes I know about balanced
equipment. But its use is more for the common mode rejection of
interference on long and hazardous runs - as well as to provide immunity
from ground loops across a variety of equipment which is likely to be used
in different situations. But shouldn't be needed in a domestic environment
for a permanent installation. And such connections are rare in domestic
equipment. And rightly so, since they add unnecessary electronics at both
input and output -- adding to complexity and cost.

I've never experimented with single-ended and balanced installation, using
the same equipment, so I can't speak from experience. However, balanced
inputs and outputs have become common in "good" equipment, partly because
they add little to the cost, * and partly because -- especially in A/V
systems -- they make things easy for the manufacturer and dealer.

In a system where the amplifiers don't sit right next to control unit (qv,
my system), the designer & installer don't have to worry about hum and
interference. The customer isn't going to be happy if the dealer has to do
extensive troubleshooting to get rid of noise, or (worse) if the house has
to be rewired to reduce the inter-component ground potential.

I remember, ca 1978, installing a relatively simple Crown system in an
equipment rack. We had all kinds of grounding problems. Some of this was
probably due to the Crown equipment itself (surprising, because the same
products were used professional), but we wound up have to completely isolate
the units from the rack. This involved wrapping the screws in vinyl tubes,
and using faucet washers to lift the ears away from the rack. Our first
attempt used black washers, which contain carbon and are conductive. These
were quickly replaced with non-conductive red washers.

* Nor are there any additional electronics.

 

[msg]

William Sommerwerck wrote:

<snip>

we wound up have to completely isolate
the units from the rack. This involved wrapping the screws in vinyl tubes,
and using faucet washers to lift the ears away from the rack. Our first
attempt used black washers, which contain carbon and are conductive.

Interesting, would have never suspected it; thanks for the heads-up.

Michael

 

[Dave Plowman (News)]

In article <h0mqkb$gl4$1@news.eternal-september.org>, William Sommerwerck
<grizzledgeezer@comcast.net> wrote:

I've never experimented with single-ended and balanced installation,
using the same equipment, so I can't speak from experience. However,
balanced inputs and outputs have become common in "good" equipment,
partly because they add little to the cost, * and partly because --
especially in A/V systems -- they make things easy for the manufacturer
and dealer.

What connectors do they use? There simply wouldn't be room for XLRs on the
back of my AV amp.

In a system where the amplifiers don't sit right next to control unit
(qv, my system), the designer & installer don't have to worry about hum
and interference. The customer isn't going to be happy if the dealer has
to do extensive troubleshooting to get rid of noise, or (worse) if the
house has to be rewired to reduce the inter-component ground potential.

With decent design, you don't ground at each component. You use the screen
of the interconnect to ground everything back to the same point.

--
*A picture may be worth a thousand words, but it uses up a thousand times more memory.

Dave Plowman dave@davenoise.co.uk London SW
To e-mail, change noise into sound.

 

[William Sommerwerck]

"Dave Plowman (News)" <dave@davenoise.co.uk> wrote in message
news:5069209fdddave@davenoise.co.uk...

In article <h0mqkb$gl4$1@news.eternal-september.org>, William Sommerwerck
grizzledgeezer@comcast.net> wrote:

I've never experimented with single-ended and balanced installation,
using the same equipment, so I can't speak from experience. However,
balanced inputs and outputs have become common in "good" equipment,
partly because they add little to the cost, * and partly because --
especially in A/V systems -- they make things easy for the manufacturer
and dealer.

What connectors do they use? There simply wouldn't be room for XLRs
on the back of my AV amp.

Even a small power amplifier has enough room for XLRs. Many control units
(what used to be the "preamplifier") are huge -- mine has ten XLRs on the
back, not to mention more than 50 RCAs, plus S and TOSLINK connectors.

In a system where the amplifiers don't sit right next to control unit
(qv, my system), the designer & installer don't have to worry about hum
and interference. The customer isn't going to be happy if the dealer has
to do extensive troubleshooting to get rid of noise, or (worse) if the
house has to be rewired to reduce the inter-component ground potential.

With decent design, you don't ground at each component. You use the
screen of the interconnect to ground everything back to the same point.

I'm afraid to say anything, because I'm not an expert on grounding. But I'm
pretty certain that what you're saying is wrong.

Simply connecting two amplifiers by cable shields does not cause the
amplifiers' grounds to have the same AC potential. This is why components
"far"/"near" each other, not connected/connected to the same outlet,
sometimes/rarely have hum/buzz problems.

 

[Dave Plowman (News)]

In article <h0o5mj$56e$1@news.eternal-september.org>,
William Sommerwerck <grizzledgeezer@comcast.net> wrote:

What connectors do they use? There simply wouldn't be room for XLRs
on the back of my AV amp.

Even a small power amplifier has enough room for XLRs. Many control units
(what used to be the "preamplifier") are huge -- mine has ten XLRs on the
back, not to mention more than 50 RCAs, plus S and TOSLINK connectors.

Well I meant a combined unit like most AV amps. An amp is an amp
regardless of what it's used for.
And 10 XLRs would nowhere cover the connections on mine - that is after
all only 5 inputs before you even consider outputs, etc.

In a system where the amplifiers don't sit right next to control unit
(qv, my system), the designer & installer don't have to worry about
hum and interference. The customer isn't going to be happy if the
dealer has to do extensive troubleshooting to get rid of noise, or
(worse) if the house has to be rewired to reduce the inter-component
ground potential.

With decent design, you don't ground at each component. You use the
screen of the interconnect to ground everything back to the same point.

I'm afraid to say anything, because I'm not an expert on grounding. But
I'm pretty certain that what you're saying is wrong.

Works perfectly here. Even between two rooms which are on different
circuits. But there is only one mains ground - that on the AV amp. I
basically have a SCART cable running from the main system in the living
room to the TV in the kitchen - mainly to avoid the different sound delays
associated with watching even the same prog off digital through different
TVs. The sound from that TV is fed into the audio system in the kitchen.

Simply connecting two amplifiers by cable shields does not cause the
amplifiers' grounds to have the same AC potential. This is why components
"far"/"near" each other, not connected/connected to the same outlet,
sometimes/rarely have hum/buzz problems.

Not in the UK - unbalanced is the usual means of connection - and very
rarely will everything be on the same outlet. The same circuit, though,
most usually. But as I said, if you have a mains ground to every piece of
equipment and a screen connection too you'll likely get an earth loop and
hum - since most equipment has the metalwork connected to the signal
ground.

I do remember some older Pioneer separates all fitted with three core
mains leads and if you plugged them all to the same outlet via a splitter
of some sort and used the supplied interconnects, you got an earth loop.

--
*Laugh alone and the world thinks you're an idiot.

Dave Plowman dave@davenoise.co.uk London SW
To e-mail, change noise into sound.

 

[William Sommerwerck]

<lots of interesting stuff snipped>

The issue -- in my mind -- is the distinction between "mains" (AC) ground
and audio ground. They're not the same -- at least, not in the U S of A.
This is why it's possible, on devices with three-wire power cables, to
sometimes reduce the hum by lifting the ground on one and "rotating" the
plug. Indeed, the Parasound power amps have a "ground lift" switch on the
back.

I've spoken with Magnolia and Pioneer, and neither had the "Aha!" response I
was hoping for. I'll call Parasound today and ask them.

 

[Dave Plowman (News)]

In article <h0ocea$jg1$1@news.eternal-september.org>,
William Sommerwerck <grizzledgeezer@comcast.net> wrote:

The issue -- in my mind -- is the distinction between "mains" (AC) ground
and audio ground. They're not the same -- at least, not in the U S of A.
This is why it's possible, on devices with three-wire power cables, to
sometimes reduce the hum by lifting the ground on one and "rotating" the
plug. Indeed, the Parasound power amps have a "ground lift" switch on the
back.

If they're not the same there would be nothing to gain from an earth lift
switch. It's having two non identical paths for the signal ground that
causes an earth loop - you get a differential current flow. But as I
said, it is common for them to be the same. But don't you have 'double
insulated' equipment that doesn't need a safety mains ground? This is the
norm for Hi-Fi equipment in the UK - even with the higher voltage here.
And assuming it has the normal transformer supplying the DC - I'm not
sure why reversing the mains lead should have any effect - given an
isolating transformer of this type removes any reference from the supply -
even when one leg of the supply is grounded as in the UK. Not so sure
about SMPS, though, and they seem to becoming more common in audio
equipment.

I've spoken with Magnolia and Pioneer, and neither had the "Aha!"
response I was hoping for. I'll call Parasound today and ask them.

--
*I'm really easy to get along with once people learn to worship me

Dave Plowman dave@davenoise.co.uk London SW
To e-mail, change noise into sound.

 

[William Sommerwerck]

The issue -- in my mind -- is the distinction between "mains" (AC) ground
and audio ground. They're not the same -- at least, not in the U S of A.
This is why it's possible, on devices with three-wire power cables, to
sometimes reduce the hum by lifting the ground on one and "rotating"
the plug. Indeed, the Parasound power amps have a "ground lift" switch
on the back.

If they're not the same there would be nothing to gain from an earth lift
switch. It's having two non identical paths for the signal ground that
causes an earth loop - you get a differential current flow. But as I
said, it is common for them to be the same. But don't you have 'double
insulated' equipment that doesn't need a safety mains ground? This is
tje norm for Hi-Fi equipment in the UK - even with the higher voltage
here.

Not that I know of. Only power tools (and similar products) are
double-insulated.

And assuming it has the normal transformer supplying the DC - I'm not
sure why reversing the mains lead should have any effect - given an
isolating transformer of this type removes any reference from the supply -
even when one leg of the supply is grounded as in the UK.

This is common experience, not something I've made up. I'm sure other people
in this group can report similar stories.

Could we agree to drop this discussion for the time being, until we can find
a /real/ expert on grounding?

 

[Dave Plowman (News)]

In article <h0om24$7v6$1@news.eternal-september.org>,
William Sommerwerck <grizzledgeezer@comcast.net> wrote:

If they're not the same there would be nothing to gain from an earth lift
switch. It's having two non identical paths for the signal ground that
causes an earth loop - you get a differential current flow. But as I
said, it is common for them to be the same. But don't you have 'double
insulated' equipment that doesn't need a safety mains ground? This is
tje norm for Hi-Fi equipment in the UK - even with the higher voltage
here.

Not that I know of. Only power tools (and similar products) are
double-insulated.

Right. Thank gawd I live in the UK, then.

And assuming it has the normal transformer supplying the DC - I'm not
sure why reversing the mains lead should have any effect - given an
isolating transformer of this type removes any reference from the
supply - even when one leg of the supply is grounded as in the UK.

This is common experience, not something I've made up. I'm sure other
people in this group can report similar stories.

It makes no sense, unless there is a direct connection between supply and
the DC side. Last time I can remember this was with 'live chassis' valve
stuff.

Could we agree to drop this discussion for the time being, until we can
find a /real/ expert on grounding?

Fine. But I've certainly got enough 'experience' to know that balanced
connections are total overkill for domestic line level installations.
Different matter with low level signals from microphones, etc, or for very
long runs.

If you've been sold that they're needed, that's your problem.

--
*Don't byte off more than you can view *

Dave Plowman dave@davenoise.co.uk London SW
To e-mail, change noise into sound.

 

[William Sommerwerck]

Could we agree to drop this discussion for the time being,
until we can find a /real/ expert on grounding?

Fine. But I've certainly got enough 'experience' to know that
balanced connections are total overkill for domestic line level
installations. Different matter with low level signals from
microphones, etc, or for very long runs.

If you've been sold that they're needed, that's your problem.

Well... <ahem> Balanced inputs and outputs have been common on the "better"
audio equipment for around 20 years. They're particularly desirable if
you're a nut about fully balanced circuitry. And you pay for them whether or
not you want them.

Balanced ins and outs became really popular with the introduction of the A/V
system controller, precisely because they eliminate, a priori, grounding and
hum problems, without adding a lot of money to the system's cost.

I previously owned Brand K electronics, which were fully balanced, and I
never had hum problems. I could crank the volume all the way up, well past
normal listening levels, and the system was dead-quiet (other than through
the phono input). Why would I want to use unbalanced lines?

Balanced cables cost only a little more than unbalanced cables. Unlike RCA
plugs (which should have been banned decades ago), they rarely pop loose,
and you can plug and unplug them without getting a "big blast o' hum". *

* The better RCA cables have "long" collars, so the ground is made before
the hot side when plugging in, and vice-versa when pulling the cable.

 

[Dave Plowman (News)]

In article <h0oume$jts$1@news.eternal-september.org>,
William Sommerwerck <grizzledgeezer@comcast.net> wrote:

Could we agree to drop this discussion for the time being,
until we can find a /real/ expert on grounding?

Fine. But I've certainly got enough 'experience' to know that
balanced connections are total overkill for domestic line level
installations. Different matter with low level signals from
microphones, etc, or for very long runs.

If you've been sold that they're needed, that's your problem.

Well... <ahem> Balanced inputs and outputs have been common on the
"better" audio equipment for around 20 years. They're particularly
desirable if you're a nut about fully balanced circuitry. And you pay
for them whether or not you want them.

Plenty of the very good audio equipment doesn't have balanced inputs.
They're completely unnecessary in the average domestic setup to avoid hum,
etc.

Balanced ins and outs became really popular with the introduction of the
A/V system controller, precisely because they eliminate, a priori,
grounding and hum problems, without adding a lot of money to the
system's cost.

So you say. I've got 'a very good' AV amp too with not a balanced
connection in sight. Oh - and no hum.

I previously owned Brand K electronics, which were fully balanced, and I
never had hum problems. I could crank the volume all the way up, well
past normal listening levels, and the system was dead-quiet (other than
through the phono input). Why would I want to use unbalanced lines?

Up to you but I can do the same with unbalanced.

Balanced cables cost only a little more than unbalanced cables. Unlike
RCA plugs (which should have been banned decades ago), they rarely pop
loose, and you can plug and unplug them without getting a "big blast o'
hum". *

Err, plugging and unplugging *anything* with the level control wound up is
very bad practice - but if you don't care about your speakers and just
want to prove some point or whatever, fine.

Oh - an XLR can't 'pop' lose unless there's something very wrong - they
have latches to stop this happening. But that's because they are basically
for microphone etc use where they will get strain on them. Unless you wave
your amp round your head, not necessary in the home.

* The better RCA cables have "long" collars, so the ground is made
before the hot side when plugging in, and vice-versa when pulling the
cable.

I guess you are forever pulling things apart and re-plugging? I don't have
to - I have more than enough inputs for my needs. Something that simply
wouldn't be possible if using XLR connectors. The amp would be bigger than
the room...

--
*Acupuncture is a jab well done*

Dave Plowman dave@davenoise.co.uk London SW
To e-mail, change noise into sound.

 

[William Sommerwerck]

This is precisely the kind of discussion I did not want this question to
devolve into.

I don't understand why you think an audio system with components separated
by 20' or more, and connected to different power lines, can be wired with
unbalanced interconnects, on the naive assumption that there will never be
any problems with hum or noise.

For what it's worth, all my prior systems had the power amps sitting next to
the preamps. The components were connected with unbalanced cabling, and I
never had problems with hum, noise, etc. Nor did it ever cross my mind that
any of these systems would be "better" with balanced wiring.

My Parasound components -- and the Apogee electronic crossovers -- have both
balanced and unbalanced inputs and outputs, and can be freely connected as
desired. Other than the fact that balanced cables cost slightly more than
unbalanced -- why would anyone in their right mind use unbalanced cables?
Balanced cabling is in every way "superior".

I'm friends with the manager of the Bellevue Magnolia. I'll ask him what
their installation department's opinion on this matter is, and /why/ they
hold that opinion (whatever it might be).

 

[isw]

In article <506940fa09dave@davenoise.co.uk>,
"Dave Plowman (News)" <dave@davenoise.co.uk> wrote:

In article <h0ocea$jg1$1@news.eternal-september.org>,
William Sommerwerck <grizzledgeezer@comcast.net> wrote:
The issue -- in my mind -- is the distinction between "mains" (AC) ground
and audio ground. They're not the same -- at least, not in the U S of A.
This is why it's possible, on devices with three-wire power cables, to
sometimes reduce the hum by lifting the ground on one and "rotating" the
plug. Indeed, the Parasound power amps have a "ground lift" switch on the
back.

If they're not the same there would be nothing to gain from an earth lift
switch. It's having two non identical paths for the signal ground that
causes an earth loop - you get a differential current flow.

That's one way to get hum pickup (voltage drop due to current flowing in
a wire), but there's another: an actual *loop*, say unit "A"'s ground to
the power line, power line to unit "B"'s ground, and unit "B" back to
unit "A" via the shield of the signal cable (a real "loop").

That loop is a single-turn transformer winding, and if the three wires
are arranged so the loop has an enclosed area (which is almost always
the case), any magnetic lines cutting one of the wires differently from
another will induce a signal into the wire of the loop -- that's why
twisted pair works; the area enclosed by the wires is very close to zero.

That kind of hum problem is easily dealt with by transformer coupling
the signal, because (not counting the impedance of the interwinding
capacitance), there's no more loop to couple hum into.

Isaac

 

[Dave Plowman (News)]

In article <h0pha3$oa2$1@news.eternal-september.org>,
William Sommerwerck <grizzledgeezer@comcast.net> wrote:

This is precisely the kind of discussion I did not want this question to
devolve into.

I don't understand why you think an audio system with components
separated by 20' or more, and connected to different power lines, can be
wired with unbalanced interconnects, on the naive assumption that there
will never be any problems with hum or noise.

Because it is the common domestic way to do it. In the UK at least. You
may well have to take care with cable runs to avoid picking up
interference from mains wiring etc - but then the same applies to balanced
circuits too, although they are more robust.

For what it's worth, all my prior systems had the power amps sitting
next to the preamps. The components were connected with unbalanced
cabling, and I never had problems with hum, noise, etc. Nor did it ever
cross my mind that any of these systems would be "better" with balanced
wiring.

Well they can't be 'better' since you're introducing extra electronics to
balance and unbalance the signal - not many devices are balanced
internally.

My Parasound components -- and the Apogee electronic crossovers -- have
both balanced and unbalanced inputs and outputs, and can be freely
connected as desired. Other than the fact that balanced cables cost
slightly more than unbalanced -- why would anyone in their right mind
use unbalanced cables? Balanced cabling is in every way "superior".

You also have a balanced output radio tuner? Record deck? Etc?

I'm friends with the manager of the Bellevue Magnolia. I'll ask him what
their installation department's opinion on this matter is, and /why/ they
hold that opinion (whatever it might be).

Domestic equipment is fitted with balanced in and out because it looks
professional. Or so it may be used in a pro application, as at the power
amp side of things there is some crossover between them. But not really
with AV stuff.

If your equipment is designed for balanced operation it's quite likely the
unbalanced connections are an afterthought and the screen connected to
mains ground. A simple check with a DVM would confirm this. If this is the
case they ain't suitable for unbalanced connections directly. But that's
their design fault - nothing to do with principles.

I'm not trying to deny that balancing is necessary for long cable runs -
especially with low level signals, like from a microphone. Just that it's
an unnecessary extra expense and complication in the domestic environment.

--
*Don't worry; it only seems kinky the first time.*

Dave Plowman dave@davenoise.co.uk London SW
To e-mail, change noise into sound.

 

[Dave Plowman (News)]

In article <isw-67AA6D.22295310062009@[216.168.3.50]>,
isw <isw@witzend.com> wrote:

If they're not the same there would be nothing to gain from an earth lift
switch. It's having two non identical paths for the signal ground that
causes an earth loop - you get a differential current flow.

That's one way to get hum pickup (voltage drop due to current flowing in
a wire), but there's another: an actual *loop*, say unit "A"'s ground to
the power line, power line to unit "B"'s ground, and unit "B" back to
unit "A" via the shield of the signal cable (a real "loop").

Yes- that's what I mean. Two bits of equipment where the audio ground and
mains ground are common - like most domestic equipment fitted with a three
core mains lead - and connected via a phono lead

That loop is a single-turn transformer winding, and if the three wires
are arranged so the loop has an enclosed area (which is almost always
the case), any magnetic lines cutting one of the wires differently from
another will induce a signal into the wire of the loop -- that's why
twisted pair works; the area enclosed by the wires is very close to zero.

With a balanced pair the signals are out of phase so any interference on
the line is cancelled out when you bring it back into phase at the other
end.

That kind of hum problem is easily dealt with by transformer coupling
the signal, because (not counting the impedance of the interwinding
capacitance), there's no more loop to couple hum into.

Yes - although removing one of the ground connections will usually work
too. But as I said - in the UK at least - very few audio devices have a
mains ground, with the exception of the power amp itself. They are Class
II (double insulated) so don't need a safety ground. This has been the
case for many a year. Otherwise you'd get a hum loop...

--
*If your feet smell and your nose runs, you're built upside down.

Dave Plowman dave@davenoise.co.uk London SW
To e-mail, change noise into sound.

 

[William Sommerwerck]

For what it's worth, all my prior systems had the power amps sitting
next to the preamps. The components were connected with unbalanced
cabling, and I never had problems with hum, noise, etc. Nor did it ever
cross my mind that any of these systems would be "better" with balanced
wiring.

Well they can't be 'better' since you're introducing extra electronics to
balance and unbalance the signal -- not many devices are balanced
internally.

This is not correct. Changing the circuit from an unbalanced input or output
to a balanced input or output does not add a gain stage.

The power amps were designed by John Curl, one of the "inventors" of
full-complementary push-pull amplification. I will ask him what topology the
Parasound A21 uses.

My Parasound components -- and the Apogee electronic crossovers -- have
both balanced and unbalanced inputs and outputs, and can be freely
connected as desired. Other than the fact that balanced cables cost
slightly more than unbalanced -- why would anyone in their right mind
use unbalanced cables? Balanced cabling is in every way "superior".

You also have a balanced output radio tuner? Record deck? Etc?

The tuner does, but I use the unbalanced outputs. We were talking about
power amplifier connections, anyway.

If your equipment is designed for balanced operation it's quite likely the
unbalanced connections are an afterthought and the screen connected to
mains ground.

This is common. It is not generally considered poor design.