[mkiijam]

Can't seem to find the info anywhere quick. Need to test a 70v speaker amp. Don't have any transformer, but have coil winder and wire, bobbins etc...

I can spin up a test rig quick, but what's a good guess on turns count??? It doesn't need to last and I won't push much power through it... just need to test to see if the amp is working at all.

[moamps]

10:1 to 15:1, it depends of speaker impedance and power.
A power toroidal  transformer  110V/12V 10-20W would be just fine for testing purpose, IMO.

[mkiijam]

I'll just use an 8ohm speaker sitting around. I'll only do about 2-3 watts. Really just need to see if it's working.

[Audio1Man]

Hi mkiijam

A 70 volt audio power amplifier drives a 0-70 volts output signal, The 70 volts is where the amplifier produces full rated power. Test this amplifier as you would test any other audio amplifier.

Standard amplifiers are rated for 2-16 ohms loads. You would place a dummy load of the 2-16 ohm load on the amplifier and run the tests. The 70 volt amplifier need a load that matches rated power =70 volts.

R=E^2/P,  Example : 100 watt 70 volt amplifier needs 49 ohms, 200 watt needs 24.5 ohms. 

[JohnRoberts]

+1 to what Duke posted... This thread topic is confusing since you do not need a transformer to test a 70V amp. You can test the output like any other audio amplifier.

One important missing fact is the rated power (watts) of your 70V amp, that will determine how much load it can drive. 70V amps can come in sizes from 2W to 200W and more, while all make the exact same 70V output.

In 70V or "constant voltage" systems,  loudspeakers have step down transformers to deliver X watts to the individual speaker. This dramatically simplifies the installation math, as you just add up how many watts of speakers you have and just size the amp to deliver a comfortable margin more watts (say 10% safety margin).

You can probably confirm that it works with no load.

JR

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