Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems.

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Default Noise on balanced line input

I have an interesting application which has a small problem. We
installed a security system and a background music system in a
building. The music is distributed via 70V lines at very low level
throughout the building. The speakers are three inch 45 ohm units
salvaged from an old apartment house intercom system. Each speaker is
connected to the line with a small 1 watt 70V to 45 ohm transformer. I
picked up a box of these transformers at an auction and the marriage
of the speakers and transformers seemed a good fit. The music works
very well but that's not the problem.

The alarm system when activated on an audible zone applies 12VDC to a
siren driver. The driver outputs a "whoop whoop" sound to a separate
outdoor 8 ohm speaker. In addition to the connection to the speaker I
also ran the output of the siren driver back to the background music
amp. I connected the primary, (70V side) of a typical speaker line
transformer to this siren output and the secondary, (speaker
connection) to a low impedance balanced mic input on the amplifier. So
when the alarm goes off, the sound of the siren is everywhere in the
building. The transformer gives me isolation between the alarm system
and the music system and as a bonus it just happens to step down the
voltage the right amount to feed a balanced low level mic. input with
the right amount of signal as well.

This scenario also works well however with one slight problem. The
cables for both systems are old CAT3's which were already in the
building. All these cables are bundled together between two buildings,
one where they're punched down onto 66blocks in one location to where
the equipment is in the other. On the siren driver line, audible
through the background music system speakers when the mic input is
advanced to the proper level, there is a slight "fluttering" sound.

I've heard this noise before when using my inductive probe to find a
wire with tone on it and so I'm sure the sound is the data stream from
the alarm system keypad getting into the siren driver circuit.I tried
hanging different size caps across the mic input but none of them
reduced the noise. I didn't have any chokes with me so I couldn't try
anything in that regard, however I plan to. But I was curious if
anyone else ever ran into a problem such as this and was ever able to
remedy it. Or perhaps someone may have some suggestions I might try.
Thanks for any assistance. Lenny.
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Default Noise on balanced line input

klem kedidelhopper wrote:
I connected the primary, (70V side) of a typical speaker line
transformer to this siren output and the secondary, (speaker
connection) to a low impedance balanced mic input on the amplifier.


Where does this transformer physically live? Close to the siren driver,
close to the music amp, somewhere else? If you think you're picking up
junk from the wires, it might help to move the transformer as close to
the music amp as you can get it. I think (and I'm sure I'll be
corrected if I'm wrong) that the mic inputs typically work with a lot
lower signal levels than, say, a line or aux input (for a CD player or
similar), so the mic input will be more sensitive to this kind of
pickup. If the smaller signal only travels a couple of inches, it will
be less likely to pick up junk.

If you have a line or aux input on the amp, you may be able to fix it
by moving the siren input to that. The suspected induced noise will
still be there, but the lower sensitivity of the line/aux input will
make it harder to hear.

On the siren driver line, audible through the background music system
speakers when the mic input is advanced to the proper level, there is
a slight "fluttering" sound.

I've heard this noise before when using my inductive probe to find a
wire with tone on it and so I'm sure the sound is the data stream from
the alarm system keypad getting into the siren driver circuit.


Are you sure? You might not be the first person to re-use existing
wiring in that installation, especially if it runs between two buildings
- grabbing a pair from the old telco wire is way easier than bringing a
shovel or a ladder. I have seen old Bell 25-pair 1A2 PBX cables
repurposed for *Ethernet*; it actually works sort of OK at 10 megabit,
100 megabit is iffy. My point is, there may be something besides the
alarm wiring that is radiating junk into the pair that you used for
the siren tone.

You might go to where the transformer is, disconnect it from the siren
driver, short the two primary wires together, and see if you still
hear the noise. If it goes away, it may be coming from the siren
driver itself; changing inputs on the music amp might be the fastest
way out of that situation. If it stays, then it's getting picked up on
the wiring somewhere.

If it's getting picked up on the wiring, an idea: if the alarm "brain"
has enough 12 V drive to run two siren drivers, run the 12 volt siren
driver supply to where the music amp is. Put another siren driver
there, and put an 8 to 16 ohm power resistor (look at the siren driver
spec) across its output. Then put the ends of a pot (1K maybe?) across
that, and run the pot wiper and one end into the amp. That gives you
a local and hopefully "clean" source of siren noise for the music amp.
If the 12 V from the alarm is really too noisy, still run it to the
music amp, but use it to switch a little 12 V relay. Use a local
"clean" 12 V DC wall wart to power the local siren driver at the music
amp.

I didn't have any chokes with me so I couldn't try anything in that
regard, however I plan to.


If you have any ferrite beads (those lumps at the end of some computer
cords), looping the cable through one a few times right before the mic
input at the music amp might help a little. I think those tend to help
with higher frequencies than what you probably have, though.

Matt Roberds

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Default Noise on balanced line input

On Feb 16, 1:54*am, wrote:
klem kedidelhopper wrote:
I connected the primary, (70V side) of a typical speaker line
transformer to this siren output and the secondary, (speaker
connection) to a low impedance balanced mic input on the amplifier.


Where does this transformer physically live? *Close to the siren driver,
close to the music amp, somewhere else? *If you think you're picking up
junk from the wires, it might help to move the transformer as close to
the music amp as you can get it. *I think (and I'm sure I'll be
corrected if I'm wrong) that the mic inputs typically work with a lot
lower signal levels than, say, a line or aux input (for a CD player or
similar), so the mic input will be more sensitive to this kind of
pickup. *If the smaller signal only travels a couple of inches, it will
be less likely to pick up junk.

If you have a line or aux input on the amp, you may be able to fix it
by moving the siren input to that. *The suspected induced noise will
still be there, but the lower sensitivity of the line/aux input will
make it harder to hear.

On the siren driver line, audible through the background music system
speakers when the mic input is advanced to the proper level, there is
a slight "fluttering" sound.


I've heard this noise before when using my inductive probe to find a
wire with tone on it and so I'm sure the sound is the data stream from
the alarm system keypad getting into the siren driver circuit.


Are you sure? *You might not be the first person to re-use existing
wiring in that installation, especially if it runs between two buildings
- grabbing a pair from the old telco wire is way easier than bringing a
shovel or a ladder. * *I have seen old Bell 25-pair 1A2 PBX cables
repurposed for *Ethernet*; it actually works sort of OK at 10 megabit,
100 megabit is iffy. *My point is, there may be something besides the
alarm wiring that is radiating junk into the pair that you used for
the siren tone.

You might go to where the transformer is, disconnect it from the siren
driver, short the two primary wires together, and see if you still
hear the noise. *If it goes away, it may be coming from the siren
driver itself; changing inputs on the music amp might be the fastest
way out of that situation. *If it stays, then it's getting picked up on
the wiring somewhere.

If it's getting picked up on the wiring, an idea: if the alarm "brain"
has enough 12 V drive to run two siren drivers, run the 12 volt siren
driver supply to where the music amp is. *Put another siren driver
there, and put an 8 to 16 ohm power resistor (look at the siren driver
spec) across its output. *Then put the ends of a pot (1K maybe?) across
that, and run the pot wiper and one end into the amp. *That gives you
a local and hopefully "clean" source of siren noise for the music amp.
If the 12 V from the alarm is really too noisy, still run it to the
music amp, but use it to switch a little 12 V relay. *Use a local
"clean" 12 V DC wall wart to power the local siren driver at the music
amp.

I didn't have any chokes with me so I couldn't try anything in that
regard, however I plan to.


If you have any ferrite beads (those lumps at the end of some computer
cords), looping the cable through one a few times right before the mic
input at the music amp might help a little. *I think those tend to help
with higher frequencies than what you probably have, though.

Matt Roberds


I appreciate your ideas Matt. The transformer is mounted right on the
PA amp with a 6 inch secondary wire to the XLR connector. I'm certain
that the noise is being picked up by the cables. none are shielded and
the best there is is the CAT3. You also may be right about the chokes.
I've used them in the past for RF suppression, but this is at an
audible rate.

I made a service call once to a house very near an AM broadcast
transmitter. The station was just about as loud on the telephone as a
normal conversation would have been. I fed the CO line into two small
chokes, (value unknown), and then hung a cap, (again I don't remember
what I used. I wish I had written it down), across that. When I
connected the premises phone line up to my filter the line was clean.

I did try the other high level AUX input but couldn't get it to work
with my 45 ohm transformer. I suspect the step down effect was
working against me at that point. Eliminating the transformer and
connecting the AUX input directly to the siren driver output didn't
work at all either. I'm not really sure why though. I might need a
transformer with a less aggressive turns ratio for that input to work.
Maybe I could try to capacitivly couple to the aux input but that
could be noisy too. Lenny
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Default Noise on balanced line input


"klem kedidelhopper"

I've heard this noise before when using my inductive probe to find a
wire with tone on it and so I'm sure the sound is the data stream from
the alarm system keypad getting into the siren driver circuit.I tried
hanging different size caps across the mic input but none of them
reduced the noise.



** If both the lines involved REALLY are *equal level & anti-phase
balanced * AND twisted - there should be no such problem.

So I bet that is not the real case.



.... Phil





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Default Noise on balanced line input

On Fri, 15 Feb 2013 16:37:47 -0800 (PST), klem kedidelhopper
wrote:

The music is distributed via 70V lines at very low level
throughout the building.


Methinks not. The voltage on a 70v constant voltage speaker system is
(insert drum roll...) 70.7 volts rms. No way is it "very low" unless
something is broken or you're running at drastically reduced volume.
The big advantage to 70v and 100v systems is that they can use cheap,
junk, small gauge wiring, such a CAT3 telephone wire for relatively
high power speakers. This can be a problem when someone actually does
use cheap, junk small gauge wire that sometimes simulates a fuse.

How it works:
http://rane.com/note136.html
http://www.crownaudio.com/media/pdf/amps/138905-1_10-05_constant_voltage.pdf

--
Jeff Liebermann
150 Felker St #D
http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558


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Default Noise on balanced line input


"Jeff Liebermann"
klem kedidelhopper

The music is distributed via 70V lines at very low level
throughout the building.


Methinks not. The voltage on a 70v constant voltage speaker system is
(insert drum roll...) 70.7 volts rms.


** Absolute nonsense.

70V is the maximum voltage output from the amplifier and there is nothing
constant about it.

No way is it "very low" unless
something is broken or you're running at drastically reduced volume.


** Volume pots allow the level to be set at the operators convenience.

It is very common to have a low, background music level and a much higher
speech level for announcements.


The big advantage to 70v and 100v systems is that they can use cheap,
junk, small gauge wiring, such a CAT3 telephone wire for relatively
high power speakers.


** More nonsense.

The real advantages include the use of long cable runs with low losses and
having many distributed speakers - which do not need to be all of the same
rating type or level.


..... Phil



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Default Noise on balanced line input

On Feb 17, 8:49*pm, "Phil Allison" wrote:
"Jeff Liebermann"
*klem kedidelhopper



The music is distributed via 70V lines at very low level
throughout the building.


Methinks not. *The voltage on a 70v constant voltage speaker system is
(insert drum roll...) 70.7 volts rms.


** Absolute nonsense.

70V is the maximum voltage output from the amplifier and there is nothing
constant about it.

No way is it "very low" unless
something is broken or you're running at drastically reduced volume.


** Volume pots allow the level to be set at the operators convenience.

It is very common to have a low, background music level and a much higher
speech level for announcements.

The big advantage to 70v and 100v systems is that they can use cheap,
junk, small gauge wiring, such a CAT3 telephone wire for relatively
high power speakers.


** More nonsense.

The real advantages include the use of long cable runs with low losses and
having many distributed speakers - which do not need to be all of the same
rating type or level.

.... * Phil


To further explain this, the amplifier's, (which is located in one
building), output is taken off the 70 V tap, however as Phil alluded
to the voltage on the line could be and in fact in this case is very
low, ,just a few volts. This is all I need because, again this is VERY
low background music and the level seems to be satisfactory throughout
the building. As far as the lines being balanced, I'm assuming that
they still are as I have done nothing anywhere that I can think of to
cause an unbalance. As I mentioned before this balanced line output
leaves the amp on two pairs of a CAT3 cable. I used two pairs because
they are the main feed for all the speakers. This "main" cable runs
from this building underground through PVC pipe to the 66 block in the
other building. At that location all the other CAT3 wires also appear
on that block as well. On each cable that was reused the brown pair is
connected to a speaker. in a different room. These brown pairs are all
placed in parallel and then connected to the 70V feed. So this is all
well. It is not however the problem. The problem is occurring when I
try to sample the siren driver output from the alarm and apply it to
the low level input of the music amplifier. That's when I hear the
data noise which I'm certain is being picked up by the "antenna" which
is the sample wire and fed into the mic input of the amplifier.
Lenny
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Default Noise on balanced line input


klem kedidelhopper wrote:

To further explain this, the amplifier's, (which is located in one
building), output is taken off the 70 V tap, however as Phil alluded
to the voltage on the line could be and in fact in this case is very
low, ,just a few volts. This is all I need because, again this is VERY
low background music and the level seems to be satisfactory throughout
the building. As far as the lines being balanced, I'm assuming that
they still are as I have done nothing anywhere that I can think of to
cause an unbalance.



Old cable can have a conductor shorted to ground somewhere. I've
found wire with badly abraded jacket laying on steel beams, or nicked on
the corner of an electrical box.


As I mentioned before this balanced line output
leaves the amp on two pairs of a CAT3 cable. I used two pairs because
they are the main feed for all the speakers. This "main" cable runs
from this building underground through PVC pipe to the 66 block in the
other building. At that location all the other CAT3 wires also appear
on that block as well. On each cable that was reused the brown pair is
connected to a speaker. in a different room. These brown pairs are all
placed in parallel and then connected to the 70V feed. So this is all
well. It is not however the problem. The problem is occurring when I
try to sample the siren driver output from the alarm and apply it to
the low level input of the music amplifier. That's when I hear the
data noise which I'm certain is being picked up by the "antenna" which
is the sample wire and fed into the mic input of the amplifier.



Is that "Siren driver output" the actual speaker connection, or DC to
power the sirens? Is the connection isolated by a transformer?
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On Feb 18, 12:10*pm, "Michael A. Terrell"
wrote:
klem kedidelhopper wrote:

To further explain this, the amplifier's, (which is located in one
building), output is taken off the 70 V tap, however as Phil alluded
to the voltage on the line could be and in fact in this case is very
low, ,just a few volts. This is all I need because, again this is VERY
low background music and the level seems to be satisfactory throughout
the building. As far as the lines being balanced, I'm assuming that
they still are as I have done nothing anywhere that I can think of to
cause an unbalance.


* *Old cable can have a conductor shorted to ground somewhere. I've
found wire with badly abraded jacket laying on steel beams, or nicked on
the corner of an electrical box.

As I mentioned before this balanced line output
leaves the amp on two pairs of a CAT3 cable. I used two pairs because
they are the main feed for all the speakers. This "main" cable runs
from this building underground through PVC pipe to the 66 block in the
other building. At that location all the other CAT3 wires also appear
on that block as well. On each cable that was reused the brown pair is
connected to a speaker. in a different room. These brown pairs are all
placed in parallel and then connected to the 70V feed. So this is all
well. It is not however the problem. The problem is occurring when I
try to sample the siren driver output from the alarm and apply it to
the low level input of the music amplifier. That's when I hear the
data noise which I'm certain is being picked up by the "antenna" which
is the sample wire and fed into the mic input of the amplifier.


* *Is that "Siren driver output" the actual speaker connection, or DC to
power the sirens? *Is the connection isolated by a transformer?


When the alarm is activated, 12VDC is applied to the "siren driver".
The driver outputs a "whoop whoop" sound to an 8 ohm speaker. The
speaker is located in a different building than the alarm system. The
CAT3 cable to the speaker is punched down onto a 66 block near the
alarm system control box. This cable serves two functions. It sends
the siren driver output to the speaker, and also by cross connecting
another of it's pairs on the 66 block also sends the siren driver
output back to the PA amp where I connected it to the 70V transformer
primary and then connected the 45 ohm secondary to the mic input.
That's when I heard the "ticking" or "fluttering" noise. I really do
think that the cables are physically in good shape. Lenny
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klem kedidelhopper wrote:

When the alarm is activated, 12VDC is applied to the "siren driver".
The driver outputs a "whoop whoop" sound to an 8 ohm speaker. The
speaker is located in a different building than the alarm system. The
CAT3 cable to the speaker is punched down onto a 66 block near the
alarm system control box. This cable serves two functions. It sends
the siren driver output to the speaker, and also by cross connecting
another of it's pairs on the 66 block also sends the siren driver
output back to the PA amp where I connected it to the 70V transformer
primary and then connected the 45 ohm secondary to the mic input.
That's when I heard the "ticking" or "fluttering" noise. I really do
think that the cables are physically in good shape. Lenny



If the level is right, I would add a 12 volt relay to connect the audio
to the amp when the siren is on. It sounds like the level to that mic
input is way too hot. You say, 70 volt, but not the wattage so I can't
calculate the turns ratio. You also don't state the output level from
that siren driver. From working with alarms back 40 years ago, the
drivers were a minimum of 5 watts, and some were close to 45 watts. You
could be feeding several volts to that mic input when the siren goes,
and it may destroy the transistor or IC for that input. I always used a
pot after the transformer to set the maximum level. I used to make promo
tapes for Cinemax, HBO and other CATV channels and mail them out to
radio stations each month. They were recorded of C-band and the only
place to get the audio was from the external speaker jack on the
monitor. I put a 10 ohm resistor across the input, and used a 10K pot to
set level. No one ever complained about the audio quality.


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Default Noise on balanced line input

klem kedidelhopper wrote:


This scenario also works well however with one slight problem. The
cables for both systems are old CAT3's which were already in the
building. All these cables are bundled together between two buildings,
one where they're punched down onto 66blocks in one location to where
the equipment is in the other. On the siren driver line, audible
through the background music system speakers when the mic input is
advanced to the proper level, there is a slight "fluttering" sound.

You need to use a shielded audio transformer. You can probably get a
1:1 mike isolation transformer, or possibly a mike single-ended to
balanced transformer to fix this. My guess is the speaker line transformer
is not at all balanced or shielded, and so some unbalanced signals can
get into the long bundled cables and then go through the transformer
and into the balanced input. A shielded transformer has a shield
between primary and secondary, and should prevent unbalanced signals
from getting across, only the balanced part could get through. The shield
of the transformer needs to be grounded to the PA amp.

Jon
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On Mon, 18 Feb 2013 12:49:51 +1100, "Phil Allison"
wrote:


"Jeff Liebermann"
klem kedidelhopper

The music is distributed via 70V lines at very low level
throughout the building.


Methinks not. The voltage on a 70v constant voltage speaker system is
(insert drum roll...) 70.7 volts rms.


** Absolute nonsense.

70V is the maximum voltage output from the amplifier and there is nothing
constant about it.


Please tell that to the Wikipedia authors and system vendors that
insist on calling it a "Constant voltage speaker system".
http://en.wikipedia.org/wiki/Constant_voltage_speaker_system
However, you're correct, the voltage is not constant, but that's what
it's called, and what I've been calling it for many years (without
really thinking much about the implications).

No way is it "very low" unless
something is broken or you're running at drastically reduced volume.


** Volume pots allow the level to be set at the operators convenience.

It is very common to have a low, background music level and a much higher
speech level for announcements.


From the original question:
"Each speaker is connected to the line with a small 1 watt 70V to 45
ohm transformer"
So, that means that at maximum volume setting, and at maximum
amplifier power output, this speaker will be delivering a fabulous 1
watt rms. That's about right for background music, or in-room
intercom, but not enough to shake the hallways for an announcement.

The big advantage to 70v and 100v systems is that they can use cheap,
junk, small gauge wiring, such a CAT3 telephone wire for relatively
high power speakers.


** More nonsense.

The real advantages include the use of long cable runs with low losses and
having many distributed speakers - which do not need to be all of the same
rating type or level.


Agreed. I didn't want to itemize the benefits and limitations in
detail. That's why I included the links with the details.

From my experience, the principal advantage to the service company was
the ability to sneak the wiring past the inspectors disguised as telco
wire, alarm wire, doorbell wire, "thermocouple" wire, barbed wire, or
whatever else could be found or scrounged for the occasion. It kinda
balances the added cost of the xformers.





--
Jeff Liebermann
150 Felker St #D
http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
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