Mystery transformer

[Axtman]

Does anybody know what this transformer goes to.

[glebert]

I'm guessing it is to drive a constant voltage distribution network. I never can figure those out.

[Greg Robinson]

It's a 70V line transformer, looks to be about 50W, which lines up nicely with the 14.3V primary input, that's 4ohms at 50W. The 70/28.3/25/20V secondaries connect to a matching line transformer that steps it down on the other side for the speaker, total network impedance/power levels should match up with a properly designed system.

[Enzo]

Imagine you want to install 20 ceiling speakers in a restaurant. Now imagine the whacko series/parallel wiring arrangement it would take to get a reasonable 8 ohm load for the amp. Sheesh.

SO we connect a transformer on the output that translates the output up to 70.7v at full power. Yous says 14.3v input, so I am guessing it was for a 25w @ 8 ohm or 50w @ 4 ohm. PA amp. Now you can just send a pair of wires - like zip cord - from speaker to speaker, all in parallel. But at each speaker we add a small transformer for just that speaker. It takes the 70v and steps it back down for the speaker. The small transformer has secondary taps Typical taps might be 10-5-2.5-1.25-0.75 watts. You select how loud each speaker should be relative to the rest. And all you need calculate is the total of all the taps you selected. SO ten speakers on 2.5 watt taps is25 watts total.

70v is about the most common, but there are also 25 volt systems, and for that matter 100v systems. The other taps on yours? I am not familiar with them.

Look up constant voltage speaker systems.

[glebert]

Imagine you want to install 20 ceiling speakers in a restaurant. Now imagine the whacko series/parallel wiring arrangement it would take to get a reasonable 8 ohm load for the amp. Sheesh.

SO we connect a transformer on the output that translates the output up to 70.7v at full power. Yous says 14.3v input, so I am guessing it was for a 25w @ 8 ohm or 50w @ 4 ohm. PA amp. Now you can just send a pair of wires - like zip cord - from speaker to speaker, all in parallel. But at each speaker we add a small transformer for just that speaker. It takes the 70v and steps it back down for the speaker. The small transformer has secondary taps Typical taps might be 10-5-2.5-1.25-0.75 watts. You select how loud each speaker should be relative to the rest. And all you need calculate is the total of all the taps you selected. SO ten speakers on 2.5 watt taps is25 watts total.

70v is about the most common, but there are also 25 volt systems, and for that matter 100v systems. The other taps on yours? I am not familiar with them.

Look up constant voltage speaker systems.

As usual the Enzo explanation makes more sense than the 10 page whitepapers.

[Axtman]

Thanks everyone! The transformer was $1 at a local garage sale, so I thought I would take a chance on it. I doubt constanI will ever use it. BTW, Googling "P81938" comes up with nothing.

[Leo_Gnardo]

A perfect one dollar paperweight, kool!

[glebert]

I wonder if there is some way to make a totally Rube Goldberg attenuator using a constant voltage system. Obviously you would need to use the speaker side transformer as well, and dissipate the extra watts that aren't going to speakers. 1/4 watt tap sounds like a fun bedroom amp setting!

[J M Fahey]

Imagine you want to install 20 ceiling speakers in a restaurant. Now imagine the whacko series/parallel wiring arrangement it would take to get a reasonable 8 ohm load for the amp. Sheesh.

SO we connect a transformer on the output that translates the output up to 70.7v at full power. Yous says 14.3v input, so I am guessing it was for a 25w @ 8 ohm or 50w @ 4 ohm. PA amp. Now you can just send a pair of wires - like zip cord - from speaker to speaker, all in parallel. But at each speaker we add a small transformer for just that speaker. It takes the 70v and steps it back down for the speaker. The small transformer has secondary taps Typical taps might be 10-5-2.5-1.25-0.75 watts. You select how loud each speaker should be relative to the rest. And all you need calculate is the total of all the taps you selected. SO ten speakers on 2.5 watt taps is25 watts total.

70v is about the most common, but there are also 25 volt systems, and for that matter 100v systems. The other taps on yours? I am not familiar with them.

Look up constant voltage speaker systems.

Agree and add:
25V and 70V are very popular and widely used constant voltage PA distribution systems in US, so they are present, 100V lines is more of a UK thing (many old Marshall, VOX, etc. OTs offered a 100V taps, maybe because they originally bought generic over the counter ones which included those, not that they ever used them), 28.3V is 100W@8 ohm or 50W@16 ohm, 20V is 50W@8 ohm, 100W@ 4 ohm, both clearly meant for nearby regular 8 and 4 ohm speakers.

So a very useful output menu.

This transformer is used after a "regular" SS amplifier, 14.3V meaning 50W@4 ohm or 100W @2 ohm.

And why the lowish voltages and impedances?

A very popular PA amp configuration is one fed from internal +36V Mains fed supply, which allows in emergency use (tsunami, hurricane, flooding, whatever) feeding it from a 24V truck battery or 2 x 12V car batteries in series, still providing reasonable power out.

Not bad at all for providing emergency instructions to public over a wide area, from a Hospital or large factory to train stations or outdoors.