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10 metres audio cable going into PC = too long?

L

Laurence Payne

Jan 1, 1970
0
Two boxes with 3-wire plugs?

Could you elaborate? I thought a ground loop was due to difference in
potential of the ground connections of 2 different pieces of equipment.

Different resistance/impendence between different parts of the
equipment and "ground". Whatever that is.
 
T

tony sayer

Jan 1, 1970
0
David said:
tony sayer spake thus:


Yes, what's so strange about that? We get three wires coming into our
houses: one neutral and two hots. The hots are each 120 (nominally) with
respect to the neutral, with 240 between the hots.
Thats not strange as such, just the general lack of 3 phase
distribution.

Why do you still use 115-120 supplies what with the extra current
demands, or is there still a perceived electric shock issue?.....
 
T

tony sayer

Jan 1, 1970
0
Roy L. Fuchs said:
Here, the pole transformer secondary (service side) is a center
tapped 240 volt output (there are buck and boost taps on some).

Homes here get the full 240 with the centertap (neutral), and within
the home, it is routed about as a single side hot-to-centertap, 120
volt run. The oven, furnace, dryer, inline hot water, etc (high power
devices) typically gets the full center tapped 240 feed. The center
tap is ground at the service panel with a ground rod, and all fault
returns (third wire) also come back to this grounded terminal bus.

All the 240 volt branches get dual breakers and all the 120 volt runs
get a breaker installed on that side of the service panel it will be
drawing from.

That makes any single run in the house 120 volts from ground (or
neutral).

Anyway, the pole transformer feeds several (4?) houses, then another
transformer is used for the next quad of houses The HV feed at the
top of the power distribution poles in Ohio was like 11kV IIRC (not
sure), and I don't know if it was 3 phase or not. I do know that our
3 phase is not like California's. They are Delta. I think Ohio is
Wye.

They may balance their consumption by sending a different phase to
an entire neighborhood, and another to the next neighborhood down the
way. Seems costly to do it house by house by house as you say is the
case where you are.

Thanks for that enlightening post. It may well be that we don't have the
centre tapped supply arrangement. In the local substation Y arranged,
the centre is earth connected and referred to as the Neutral return and
each phase is then carried on a three conductor cable with the wire
armouring used as the neutral return and its also connected as the
safety earth.

as strange as that may seem;).....
 
J

Jim Lesurf

Jan 1, 1970
0
I am in the UK.
I would like to take a stereo signal from the line-out of my stereo (or
TV) to the line-in of my PC.
The equipment is in different rooms and the audio cable would be 10
metres. It will be this type:
http://www.maplin.co.uk/images/full/130i0.jpg
I don't understand the technical side but is 10 metres so long that it
might cause audio problems with things like frequency response or
voltage/current levels and so on?
Will I need to get some higher specification audio cable to cover that
distance? I want to keep cost down.


To be able to give a complete answer we would need to know the output
impedance values for your stereo and TV.

The output impedance (resistance) will tend to combine with the cable
capacitance to make an RC low-pass filter. This may or may not matter, but
to estimate the effect we'd need to relevant values.

Slainte,

Jim
 
A

Arny Krueger

Jan 1, 1970
0
tony sayer said:
Me too!, virtually every bit of wire string the
electricity grid together in the UK is Three phase. Only
in some remote places will you see overhead High voltage
in Single phase, and that only is likely to serve a
signal customer!..

Can't believe the USA is that different?. I know or hear
that they have split centre tapped supplies for 115 and
240 volt domestic supplies...

All true. It's called "legacy technology". Unlike Europe, the US missed
out on the cleansing benefits of being the site of a world war.
 
A

Arny Krueger

Jan 1, 1970
0
Why do you still use 115-120 supplies what with the extra
current demands, or is there still a perceived electric
shock issue?.....

Most US factories that use substantial amounts of power use 3 phase. It's
mostly just the residential areas and isolated light industrial areas that
lack it.

Electric power use per capita in the US is closer to being uniform or
decreasing, as opposed to there being extra current demands.

http://www.eia.doe.gov/emeu/25opec/sld020.htm

http://www.aceee.org/energy/effact.htm

"Total primary energy use per capita in the United States in 2000 was almost
identical to that in 1973. Over the same 27-year period economic output
(GDP) per capita increased 74 percent"

Since 2001 or so, there has been a major conversion of existing residential
lighting to compact fluorescent bulbs which produce about 3 times as much
light per watt.
 
J

Jim Lesurf

Jan 1, 1970
0
Palindrâ-»me said:
Jim Lesurf wrote:
A quick bit of mental arithmetic, with the sort of distances and cables
mentioned by the OP, gives me an estimate of around 5k output impedance,
or greater, before it should give a noticeable effect (OK, a bit lower
if you can hear and are concerned about getting >10kHz stuff).

Since I am reading (and replying) on an 'audio' newsgroup, my assumption is
that people may be concerned up to around 20kHz, and want to be aware of
any changes that might be considered worthy of note in the context of using
good audio equipment in a domestic situation.

If I assume a cable capacitance of around 100pF/m, then a 5kOhm source
would, I think, give a roll-off of the order of 1dB in the 15-20kHz region.
(Assuming I managed to push the right buttons on my guess-box. :) )

On this newsgroup I'd normally expect people to regard that as being large
enough to be worth at least being aware of it.

I haven't come across a line out with a higher impedance than 5k, so I
don't think it is going to be a problem.

You are probably correct. Alas, consumer equipment sometimes dissapoints
out expectations - particularly when the makers have failed to provide the
relevant data for the users. ;->
Even so, this can be compensated for by a tweak of the tone controls on
the PC.

Once one becomes aware of the problem, and knows what adjustment to make
to correct for it appropriately. Hence my initial comment. :)

Slainte,

Jim
 
J

Jim Lesurf

Jan 1, 1970
0
The output impedance of any modern IC-based audio equipment is probably
going to be quite low (under 100 ohms) because the output impedance of
an op-amp is inherently low. This includes line-level outputs that are
intended to drive high-impedance inputs.

I would agree that modern circuits that use IC amps with feedback should be
able to provide nominally low o/p impedances.

However, although in general you are probably correct, the snag is that
life may not always be that simple. :) For example, some outputs may
have a low nominal output impedance, but have a relatively low current
limit. Hence they may act like a low impedance source for low currents, but
show slew-rate limiting when asked to drive a high capacitance load (long
cable). For this reason the designer may even have included o/p series
resistors to protect against this and avoid the IC being overloaded.

As I indicated, all we have so far is that the source is a 'stereo or TV'
with no details of their actual outputs. Hence it will probably be OK, but
as I pointed out, we can't really be sure in the absence of the relevant
data.

IIRC the old IHFA-707 specs required audio signal sources to be tested with
loads of 1000pF//10kOhms as the 'worst case'. I don't know what standards
(if any!) makers of domestic TVs routinely adopt these days, but 10 meters
of cable does seem to me to bring us into the area where the 1000pF limit
might be an issue.

Slainte,

Jim
 
S

Stacia

Jan 1, 1970
0
A "ground loop" is caused by having a common ground path for two
different signals. Current from each signal causes a voltage
drop, due to Ohmic losses, across the ground path for both
circuits. The effect is that each circuit affects the signal of
the other. Typically if one of those signals is 60 Hz power and
the other is something in the audio range, the audio signal will
then have a 60 Hz "buzz" on it.

I'm a complete newbie when it comes to this stuff, so I'm jumping in
with a question and hoping I'm in the right place.
We have always hooked up our computer (in one room) to our stereo (in
another) so we can play audio from the computer through the stereo
speakers. We recently moved and in this new house we're getting a hum
or buzz on the line when we play music from the computer. The cable
from computer to stereo is 30 feet and there was no hum in the audio
when we did this in our previous house.
What can we do to get rid of the hum? We've tried physically moving
the cable but that hasn't changed anything.

Thanks,
Stacia
 
T

tony sayer

Jan 1, 1970
0
Floyd L. Davidson said:
Virtually *every* outside plant telephone cable is wired up
exactly like that. There is a ground at both ends of each and
every section (3000 or 6000 feet), and the shields from each
coupled section are bonded to the other and to ground.

A three mile long section of cable might look just like this:

6000' 6000' 6000'

<-----------o----------o-----------> signal pair
<-----------o----------o----------->
+==//==+ +==//==+ +==//==+ shield
| | | | | |
| +-+-+ +-+-+ |
| | | |
----- ----- ----- -----
--- --- --- ---
- - - -

Yep, well thats balanced operation which as you say will go for miles
over telephone copper lines without humm..

Oddly enough in the UK they don't as a rule use screened cable, the
twisted balanced pair has very good rejection.

I seemed to think we were talking about domestic unbalanced lines
here?.....
 
G

GregS

Jan 1, 1970
0
I'm a complete newbie when it comes to this stuff, so I'm jumping in
with a question and hoping I'm in the right place.
We have always hooked up our computer (in one room) to our stereo (in
another) so we can play audio from the computer through the stereo
speakers. We recently moved and in this new house we're getting a hum
or buzz on the line when we play music from the computer. The cable
from computer to stereo is 30 feet and there was no hum in the audio
when we did this in our previous house.
What can we do to get rid of the hum? We've tried physically moving
the cable but that hasn't changed anything.


Ground loop isolator.

greg
 
T

tony sayer

Jan 1, 1970
0
Floyd L. Davidson said:
Regular telephone cable does not have a shield on each pair, but
does have a shield around the entire bundle of pairs. The above
diagram shows the reason!

Over here it seems to, well the half a dozen or so I've looked at!. We
we're involved in a short term radio broadcast some years ago and the
cable co supplied free of charge a few circuits about 3 odd miles to
link Two studios together, and apart from a small amount of HF loss..no
hum at all or other noise for that matter and all that cable was
unshielded....
The effects of shielding is almost useless at 50-60 Hz AC power
frequencies, which means that noise immunity would be only the
common mode rejection ratio if there was no shield or if a
shield is grounded at only one end. Instead the shield is
grounded at both ends, which allows any induction to not only
induce current into the cable pairs, but also into the shield.
The shield has is a larger conductor than the pairs, hence has
less resistance and therefore significantly more current flows.
That current flow in the shield causes an opposing current to be
induced into the signal pairs! And that reduces the amount of
noise in the signal pair significantly below what it would be if
common mode rejection was the only noise reduction mechanism.

Balanced working.. ever read up about it or used it in practice?...
The ground loop part is exactly the same in either case. The
example above is just a very convenient way to demonstrate
positively that cables *are* grounded at both ends, and that it
not only does not necessarily cause ground loop noise, but
actually is a way to reduce noise in the signal wires.
Really;-?....
 
K

Karl Uppiano

Jan 1, 1970
0
Virtually all power is generated as 3-phase...
Really? I'm not aware of any large scale generating facilities that generate
single phase, since you can always extract single phase from 3-phase.

No doubt 3-phase delivery is a problem in areas where they run just a single
phase. But using the original 3-phase rather than reconstructing 3-phase
from single phase has got to be more efficient. It also seems more reliable,
since the converter is a piece of rotating equipment (basically a
motor/generator set, IINM). Yuck. I bet the energy conversion efficiency is
less than 70%.
True for Detroit Edison, as well.

It would be extremely expensive to retrofit large areas of legacy 1-phase
distribution with 3-phase today. However, I'll bet that the main substation
gets 3-phase feeds, which it parcels out as single phase in some
load-balanced arrangement from each of the 3 phases.
 
A

Arny Krueger

Jan 1, 1970
0
Karl Uppiano said:
Really? I'm not aware of any large scale generating
facilities that generate single phase, since you can
always extract single phase from 3-phase.

No doubt 3-phase delivery is a problem in areas where
they run just a single phase. But using the original
3-phase rather than reconstructing 3-phase from single
phase has got to be more efficient.

When the power company wants you to cough up $35,000 or so for running 3
phase from where it is, up to your building, you've got to be talking pretty
heavy use to cost-justify the new lines.
 
T

tony sayer

Jan 1, 1970
0
Floyd L. Davidson said:
I'm not sure what you are agreeing with there... that cables do or
don't! :)

Doesn't matter either way as long as its balanced working and in any
case telephone bandwidth isn't that responsive to 'ummmm...

Anyways these days in the UK the copper part isn't that long in new
cable co installations, the fibre to copper conversion is done very
locally to a subs premises and in the BT system the copper is longer but
doesn't humm..
Typically of course a customer never sees any part of such a telephone
cable. What you see is a "drop wire" run from that cable to your
location. That cable will not be shielded.

Yep but they don't use shielding on a lot of phone multicore in the UK
and it wouldn't matter anyway..
When done right, it works *extremely* well.


About 40 years of working with it every day in a huge variety of
situations.


Yup. I posted this URL in another message, but just in case...
here is a very interesting, if somewhat technical, article about
measured effects of grounded shielding. It is very interesting
in the context of this particular thread.

64.70.157.146/pdf/Bondingcableshields.pdf

Yes.. Thats got some good points but they don't seem to be very savvy on
some matters about EMC and RF and you can pick a few holes in that but
yes their correct in screening or shielding earthing at both ends
provided that the balance in the sending and receiving ends is what it
should be, injecting current into the shield won't affect what's carried
in the encased conductors. However in practice the final result is and
can be affected by transformer and electronic balanced inputs and how
"floating" they are.

I think we could all agree that balanced working isn't really a problem.

Now they mention unbalanced working, but haven't given it much
attention.

Now ASCII art permitting are we agreed that the following isn't going to
cause too much upset?..


--------------------------------------------------------------
A ________________________________________________________________ M

-------------------------------------------------------------

Poxy ASCII!. Now consider A is an amp input and M is a source microphone

The dotted lines are the shield on a lump of single cored microphone
cable. Now the amp is connected A to the centre conductor at the amp end
the screen to the earthed side of the amp input, at the other end the
microphone has say a phono type connector, and the mic is a dynamic
moving coil type with one end connected to the inner shielded conductor
of the cable, the other end is connected to the outer shielded
conductor, the mic is in a metal case and is connected to the shield of
the cable too.

The mic case is not connected to any earth, other than the outer shield
of the connecting cable, and lets say thats 10 meters long or 12
yards;) The mic is suspended in free space by a lump of nylon cord and
isn't connected to anything else at all...

Now are we agreed that that arrangement will or won't hum?......
 
T

tony sayer

Jan 1, 1970
0
Floyd L. Davidson said:
Ahem. That is absolutely false. Telecom engineering
necessarily goes to an extreme effort to reduce what is called
"power line influence". The reasons should be obvious:
telephone and power cables are often run side by side, on the
same poles, and in the same crawl spaces, sometimes for miles at
a stretch. It is not uncommon to see as much as 40 to 50 volts
of power line AC on a telecom cable. That requires an
astounding amount of noise immunity to allow a circuit to work.

Yes they do, in fact we've got a broadcast transmitter site which is fed
by a bit of BT, (British Telecom, the national Telco), overhead wire
for some miles and no hum at all!. And that is on the same pole set as
240 volt mains wiring and I've actually seen 11 kV lines with phone
lines near them. Not that advisable owing to the safety factor!.

Yes of course you can get leakage via induction and capacitance into the
telecom lines but this does not matter as it will inevitably be induced
in both conductors and cancelled out by common mode rejection. Doesn't
matter providing the insulation in the line and transformers will stand
it to have some kilovolts actually on the line as such...
Consider that the test tone level at a customer premise telephone
set is nominally targeted at -9 dBm, and the worst case acceptable
Signal-to-Noise ratio is 24 dB, which means that all noise should be
at least at -33 dBm, which is about 0.0000005 watts. But a 40 volts
hum across a 600 ohm impedance is 2.7 watts, and there is roughly
67 dB difference!

Can you explain how your measuring or have that configured please?..
Do you have any idea how many telephone lines actually have a 67
dB SNR?


So? "Very locally" can mean more than a *mile*...

Often less than in ntl or telewest installations but longer in BT ones.
Ntl care the cableco in the UK but that name is to disappear and their
to be called Virgin!...
What do you mean by "BT system the copper is longer but doesn't
humm.."? They have hum resistance copper??? ;-)

Nope;!, just a way of putting that, see above,...
Virtually *all* "multicore" telecom cable is shielded. (Some
customer premise cable is not. But you won't find anything
within a telephone central office that isn't, and you won't find
any outside plant distribution cable that isn't.)

In a central office most all of it here is twisted pair. I think some
terminology things betwixt the UK and USA are showing up here. All the
cable co Telco multicores I've seen, though not all, are unshielded.

What do you define shielding as, just a wrap of aluminium foil with a
drain wire or a fully woven copper mesh?..
Where are you coming up with these ideas? Have you ever even
seen the specs for any of this?


Heh heh, lets see you try picking any holes in it!

OK then, part 2 "On the other hand cable shields which are bonded at one
end etc". Read that thorough carefully, doesn't make sense. Then take a
lump of Andrews 4-50 Heliax and see what a good radiator that is even
greater number of wavelengths . They didn't even state if it were open
circuit or terminated on a load...

Actually we've had a lot of EMC experience over the years in radio,
audio and automotive environments and what's made by far and away the
biggest effect is bypassing of transistor junctions at RF
frequencies....
You didn't read it, did you? It *does* affect the signal pairs.
It reduces the noise on them, significantly.

Were is this noise coming from then?...
In practice, what they showed was that it improves noise
immunity.

"Floating" makes no difference at all.

Well think about that, Say we have a cable the inner pairs are wrapped
around one of the power lines that you describe, and there are a LOT of
volts induced on that wiring. OK now into a transformer there will be
galvanic isolation i.e. the ends or centre tap of that transformer isn't
connected to anything. Now take a electronically balanced input. At some
point that will be connected to say an input IC which will have supply
rails etc, and that IC will be coupled through to the output of that
line receiving amplifier now don't you think that if there were some
matter of kilovolts on said line, then that will break down the
transistor junctions ?..
Longitudinal balance is
the most significant factor. Magnetic shielding is ineffective
below about 10 kHz, and reverse induction via the shield (by
grounding it at both ends) is much more significant for power
line frequencies and their harmonics (which commonly exist up to
2 or 3 kHz).

I think you have that wrong. Provided that the rejection is what it
should be then whatever is induced on the pairs will cancel out.
We could all agree that common mode rejection is not always
sufficient, and that reverse induction is virtually *always*
applied to outside plant communications cables because of that.

Exactly what you mean by "balanced working", I'm not sure.

What we've been discussing. Take a signal source and connect a
transformer thereto and connect that to a pair of wires twisted together
and then connect that to another transformer and the out put winding of
that to a load. That do?..
It is rarely used for critical circuits where induction
interference from power lines would be important. (For obvious
reasons...)
Yes..


Nothing you have said suggests it could possibly hum, given that
you have not mentioned the presence of any power line related
equipment at all. If this thing is located out in the ocean, on
a floating barge that has no AC electric power, it won't hum.

On the other hand, if you place a fluorescent light fixture close
to it, it might well hum!
Why?.

Regardless, that is one of the worst possible ways to wire 10
meters of cable to a microphone.

Yes agreed and you wouldn't do that, well not in a pro environment
anyway.

Now if say you ground that to the local mains earth at one end, and say
10 meters away at the microphone case end earth that to a driven rod
earth, will it or wont it hummmmmmmmmmm?.....
 
L

Laurence Payne

Jan 1, 1970
0
All true. It's called "legacy technology". Unlike Europe, the US missed
out on the cleansing benefits of being the site of a world war.

Ah. THAT'S why America is picking fights with everyone! Fair
enough. Where would you like bombed first? :)
 
A

Arny Krueger

Jan 1, 1970
0
Ah. THAT'S why America is picking fights with everyone!

Oh come on, we're very selective with the fights we pick. Our targets are
always very weak.
Fair enough. Where would you like bombed first? :)

Someone already did a big number on downtown Manhattan, but it was not
sufficient to get a new electrical code written.
 
G

Glenn Richards

Jan 1, 1970
0
Laurence said:
Ah. THAT'S why America is picking fights with everyone! Fair
enough. Where would you like bombed first? :)

World Trade Center?

Oh, wait a minute... :)
 
G

Glenn Richards

Jan 1, 1970
0
Roy said:
His diagrams didn't kill the thread, and no, you do NOT quote
everything over and over again, dolt boy.

You don't ("you" being the generic "Usenet user"). "He" on the other
hand does. On many occasions he's quoted back several pages and added
one or two lines to the bottom... the word "trim" doesn't exist in his
vocabulary. One wonders if he uses AOL.

Still, it could be worse. At least it's not top-posted HTML.
 
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