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Box "o" speaker line matching transformers

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
Reply to
klem kedidelhopper
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klem kedidelhopper wrote in news:5637a840- snipped-for-privacy@f4g2000yqh.googlegroups.com:

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.

Reply to
Sjouke Burry

klem kedidelhopper wrote in news: snipped-for-privacy@d4g2000vbw.googlegroups.com:

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.

Reply to
Sjouke Burry

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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

Reply to
klem kedidelhopper

Yes, if you are careful but you need a current limiting resistor in series to protect the transformer. They are not designed to be impedance protected. It's still not an ideal test, because the 25 or 70 volts is nominal, not an absolute upper limit. That's why I use the 1 volt to the secondary method. Then square the voltages you read on each primary tap and multiply by the speaker impedance to get the impedance for that tap. It only takes a minute or so per transformer, and is less likely to kill you if you're careless.

Reply to
Michael A. Terrell

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