Testing PA Amplification Outputs

[ewhiz]

When testing an amplifier that has a 8 ohm, 25v, & 70v output, should I be able to read either 25V AC to gnd. or 70v AC to gnd. using a multi-meter with no speakers attached? I am only reading 1-3 volts AC at the 25V pin and 3-7 volts at the 70v pin. Do the speakers with matching transformers have to be connected to correctly test this voltage?

 

[firechief]

The AC voltage on the output of any amp will depend on where the volume is set. Amps with these 70V outputs are usually used for long runs of speaker wire and losts of small speakers like in a office space. Its not a good idea to run the amp with no speakers on it. You could try a small speaker o'n the 8 ohm taps and measure the AC accross the speaker. The different outputs are probally just different taps on the output transformer so as long as you have some kind of load on one set of taps you wont break anything.
Is this a tube amp? What else can you tell us about it?

 

[llwhtt]

The 25V tap is approx. 3 Ohms, the 70V tap is approx. 24.5 Ohms. As firechief stated they are usually used with long runs and many speakers each with their own transformer mounted on the speaker.

Craig

 

[ewhiz]

This is a solid state amp.

http://archives.telex.com/archives/University Sound/Amplifiers/EDS/1790-6A EDS.pdf

I replaced both output transistors and several (6) signal diodes. The amp works perfect now...
However, I was trying to determine why it failed. This amp is being used in my church to power an 8 ohm speaker in the vestibule. The other speaker (70v) is located at the far end of the church in a room where they count the tithes. The end result being, the guys can hear the sermon while working.

While performing tests on the amp, I found that the speaker on the far end (70v) has no attenuator attached. It's just a plain 8 ohm speaker. No wonder the amp eventually failed! I believe once I attach a matching transformer to this 70v speaker, the impedance should match and the amplifier will receive no further damage.