Testing a 20W PA amplifer with a 25V and a 70V output. What value of resistor would I connect across either the 25V or 70V line-out terminals to test the power/hum/distortion of the PA amp?

 

For a 20W output you can find the required resistance using R = (V^2) / P. For the 25V output this gives 31 ohms and for the 70V output this gives 245 ohms. In either case the resistor should be rated for at least 30W.

 

Are the 25V and 70V outputs designed to drive a certain load impedance, or are they a sort-of constant-voltage output?

 

Are the 25V and 70V outputs designed to drive a certain load impedance, or are they a sort-of constant-voltage output?

It's not a constant voltage output. The amp outputs are designed to deliver their maximum power at the rated voltage. Of course that voltage varies with the signal level and goes to 0Vac with no input signal.

The outputs are designed to drive transformers at each speaker that match the voltage (either 25V or 70V) to the speaker impedance. Often the transformer has taps for different watt levels (loudness) at the speaker (the watt level is based upon a given speaker impedance and at a 25V or 70V input, as appropriate). The amp is designed to drive a total watt rating for all the speaker transformer settings summed together to equal the amp rating (20W in this case). For example you could have 4 speakers on the line with each transformer set for a 5W output.

The amp is designed to drive a certain load impedance to get maximum power, per the calculations I did, but you aren't directly concerned about impedance levels when connecting up the amp to the speakers, as long as you use the proper matching transformer.

That make sense?

 

So if the amp is cranked to put out say 10Vp-p at the 25V tap (all taps open-circuited), I would expect the voltage at the 70V tap to be 10*70/25 = 28V?

 

So if the amp is cranked to put out say 10Vp-p at the 25V tap (all taps open-circuited), I would expect the voltage at the 70V tap to be 10*70/25 = 28V?

That is what I would expect (28Vpp), yes.