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Power amp question

#21
Thanks for all the thorough replies, I’m learning a lot today. Since that Extron is useless it will be sent to the recycler.
That being said, I have another conference room renovation coming up and there is a Crown CT 475 which shows more promise. Keep in mind, I am only doing this for my own “fun”. If I can get it working it just keeps one more piece of equipment alive. I’m not concerned with being loud as I only play for my own enjoyment.
 
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#22
Troglodyte1911 said:
Thanks for all the thorough replies, I’m learning a lot today. Since that Extron is useless it will be sent to the recycler.
That being said, I have another conference room renovation coming up and there is a Crown CT 475 which shows more promise. Keep in mind, I am only doing this for my own “fun”. If I can get it working it just keeps one more piece of equipment alive. I’m not concerned with being loud as I only play for my own enjoyment.
Click to expand...
Yes, that will be a better choice.
 
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#23
A 70V amplifier has a constant 70VDC on the output terminals.
upload_2022-9-27_16-57-57.png


If you try to attach an 8 ohm speaker directly to this output, it will try to make (70^2)/8 = 612.5W.

It's likely the amp will not be able to sustain the voltage a 70V and you will probably damage both the amp and speaker.

When the amp is fed a signal, it superimposes the signal on top of the 70VDC:
upload_2022-9-27_17-0-47.png


Transformers are used to decode or extract the signal, so the speaker only sees the audio:
upload_2022-9-27_17-1-56.png


The intent of this type of amp is to distribute sound over long lengths of wire. The resistance of the wire will cause loses so the voltage will decrease. However, by elevating the AC on top of a DC carrier, AFAIK the losses effect primarily the DC portion of the signal

upload_2022-9-27_17-6-42.png


So at the end of the chain the DC may be a bit lower, but there will be minimal attenuation of the AC signal.

upload_2022-9-27_17-12-5.png

Ref: Constant-voltage speaker systems - Biamp Cornerstone

The transformers typically have multiple taps to match desired power level and also speaker impedance.
Capture1.JPG


The system may include step attenuators with on/off for local volume control.
 
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#24
No, it does not superimpose anything onto 70V.

The 70v standard means that the amp will deliver rated power at 70 volts RMS output, then you tap off this 70V line with an impedance matching transformer knowing the 70 volts RMS is rated power and the voltage gets stepped down to each speaker at each speaker by another smaller transformer.

The beauty of this system is that you don't have to calculate impedances, all the calculations are done with power only.

Say the amp is 100 watts, 70V. This means that you can connect speakers tapped at a total of 100 watts (for example 20 speakers tapped at 5 watts) and know that you will be within the amp's rated impedance. Speaker transformers generally have 4 or 5 power level taps.

This way, all of the squared factors go away, hidden actually, and it becomes simple math. Also, at 70V and higher impedance (lower current) speaker lines, the line loses fall dramatically. This is important when you hang 1000 feet of speaker cable off of a distributed speaker string.
 
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#28
agedhorse said:
There were a lot of crazy myths circulating in the "old school" installation world, which is exactly why 25V, 70V and 100V line standards were developed.
Click to expand...

And a lot of power amps/small PA amps (not for live music performance) weren't capable of high power output- I modded a Stromberg-Carlson PA amp from the '50s to work as a guitar amp and it had all of the voltages you showed, plus 50V and 4/8/16 Ohm taps. It was a push-pull design with 6L6 output tubes. OTOH, 45-ish Watts at that time was 'plenty of power' and since it had three mike inputs and a two screw terminal strip for Phono, it probably wasn't intended for high output SPL.
 

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