This is to help a fellow with some office ceiling speakers not easy to get at-but the wires are in a closet. If you stick a voltmeter on there, what do you see? I'm kind of thinking some transient spike, and then a very low reading which would be the transformer resistance. But I've never tried it myself. I'm also wondering about 70V systems in general, I've never messed with those...
The impedance depends on the speaker power. Most 70v speakers have transformer taps that set the power the speaker will use at 70v input and this changes the impedance.
To find the impedance being presented to a public address amplifier requires the use of an impedance meter ( at 1 kHz) which will indicate the load presented to the amplifier. For example the maximum load that a 100 watt rated PA amplifier can handle would be 49 ohm for a 70 volt line system.
I think I should have specified using a voltmeter's ohmmeter, in other words what DC resistance would you see?
Yes but your dc coil resistance is usually close to the nominal impedance. The problem is that the nominal impedance of 70v speakers depends on there power setting.
The 70 or 100 volt speaker line distribution systems use transformers connected to each individual speaker, which allows a level adjustment to be made to suit the needs of the location that is covered by the speaker. Being transformer coupled and also connected in parallel across a line; a dc resistance reading is not helpful in establishing the load presented to the amplifier or even sorting out faults in the line.
Ugh. Sorry! I should have explained the problem better. The problem is to determine if a ceiling mounted speaker, which is not easy to physically inspect, is a normal speaker or has a transformer for 70V operation. If it's a normal speaker, an ohmmeter will read DC RESISTANCE of 3.6 or 6.2 or whatever. If it has a transformer, my inclination is to think the resistance will drop near zero (i.e. the resistance of one side of the transformer windings). But I've never actually tried it and don't have such a unit to test.
Primary windings of such transformers are in the kohm range, and you don't know how many of such are in parallel. You could inspect the amplifier having or not an output 70V transformer, within the enclosure or not. Or you could feed the amp with a 1kHz sine and read its output.
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