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