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