Box "o" speaker line matching transformers

[klem kedidelhopper]

I have a job to run about 35 speakers including cameras and security
through a building. The building is a sprawling 200 year old Behemoth
of a place that I don't want to try to fish wire through. The building
was part of a now defunct college and as luck would have it almost
every room is home run wired back to a closet with at least 4 CAT5
wires. This closet seems like a good place to install our equipment
and so we plan to run the alarm system as well as the cameras off
these CAT 5's. In addition there will be a background music system.
The music will be very soft and so I would like to go with 70V lines
with speakers tapped at .25 to .50 W each. The runs would be no more
than say 100 to 125 feet and so I don't think that the 24 gauge wire
should be a problem with each CAT5 cable handling one or even two
speakers apiece. So anyhow that's the job and I'd appreciate any
comments on that but my other question is this: I have boxes of
unidentified line to voice transformers. I don't know if they're 70 or
25 volt units. Is there an easy way to determine this? I don't have an
impedance bridge. Thanks, Lenny

 

[Sjouke Burry]

I have a job to run about 35 speakers including cameras and security
through a building. The building is a sprawling 200 year old Behemoth
of a place that I don't want to try to fish wire through. The building
was part of a now defunct college and as luck would have it almost
every room is home run wired back to a closet with at least 4 CAT5
wires. This closet seems like a good place to install our equipment
and so we plan to run the alarm system as well as the cameras off
these CAT 5's. In addition there will be a background music system.
The music will be very soft and so I would like to go with 70V lines
with speakers tapped at .25 to .50 W each. The runs would be no more
than say 100 to 125 feet and so I don't think that the 24 gauge wire
should be a problem with each CAT5 cable handling one or even two
speakers apiece. So anyhow that's the job and I'd appreciate any
comments on that but my other question is this: I have boxes of
unidentified line to voice transformers. I don't know if they're 70 or
25 volt units. Is there an easy way to determine this? I don't have an
impedance bridge. Thanks, Lenny

Try to find the saturation voltage of the trannies.
Put a variable 50(or 60) hz voltage on one, measuring the
current.
When that current suddenly starts to increase, you are over
the saturation voltage.
Subtract about 30 % to grade the trannie.

 

[Sjouke Burry]

So then assuming the UUT is in fact an unknown 70V transformer, I
might have the supply cranked up to almost 100V before I see this
increase in current? Can I feed this transformer off my bench variac?
Will this current increase be a sudden or very gradual increase? Lenny

A small lightbulb in series(5-15W) should act as a suitable series
limit resistor.
And the current will go sharply up with only a few volt increase.
The reason is, that the core becomes saturated, and the inductance
of the primary will drop to almost zero, leaving only copper
wire resistance as limiter.
With the right bulb in series, you might not even need a
current measurement, the bulb will uddenly start shining.
Most transformers will be designed such, that at the design
voltage, and low frequency, the core is almost saturated.

 

[klem kedidelhopper]

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Try to find the saturation voltage of the trannies.
Put a variable 50(or 60) hz voltage on one, measuring the
current.
When that current suddenly starts to increase, you are over
the saturation voltage.
Subtract about 30 % to grade the trannie.

So then assuming the UUT is in fact an unknown 70V transformer, I
might have the supply cranked up to almost 100V before I see this
increase in current? Can I feed this transformer off my bench variac?
Will this current increase be a sudden or very gradual increase? Lenny