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.