The biggest difference is how high-output voltage is created. Some power amps require a step-up transformer to achieve the proper output voltage. Any professional power amp can be outfitted with a step-up transformer to make it a distributed amp. But first there must be proper understanding of why and how distributed audio is created.
Many distributed systems require only 100-watts at a 70.7-volt system operating voltage. (A 100-watt amplifier only has a voltage swing of 28 volts RMS at 8 ohms.) Thus a step-up transformer needs to be attached to achieve the required distributed voltage. There are two types of step-up transformers, isolation and autoformer, and both have pros and cons.
The autoformer is the easier to engineer, and the cheaper option. As a result, it’s the step-up transformer found in most power amps. Autoformers also supply good frequency response, but the downside is lack of protection between the loudspeaker and the power amp. Therefore, odds are pretty good that a system using an autoformer will have a short circuit on a line of 10 loudspeakers running on 300 feet of wiring. And if a short occurs, the power amp will shut down or even fail completely.
A step-up isolation transformer is a more solid approach in terms of power amp protection. With this approach, the power amp always sees a constant load impedance. If a short occurs anywhere in the chain, most likely the power amp will continue to be able to drive the system. However, frequency response suffers, limited to about 50 Hz to 8 kHz. A good bandwidth for an isolation step-up transformer would be 40 Hz to 18 kHz (+/- 1 dB).
So prior to purchasing a power amp with a step-up transformer, find out what type of transformer it is as well as its bandwidth. Be sure to check (or ask for) specifications of both frequency response and dB.
THE REQUIRED “SWING”
Increasingly, power amps don’t require a step-up transformer to deliver high-output voltage. A decade ago, Crown, for example, began offering a line without output transformers, instead outfitted with high enough internal DC rails to produce the required output “swing” for the load. These power amps also load protect against lower distributed impedances. It’s a very good solution for eliminating bandwidth limitations.
And in fact, many of the latest hybrid of high-power amps now on the market deliver enough voltage to drive a 70- volt system without need of step-up transformers. With higher power amps comes a larger voltage swing. For a 600-watt power amp running at eight ohms, voltage output is 69 volts. The impedance of a 600-watt 70.7-volt distributed system is 8.33 ohms. (This can be calculated using Ohm’s law.) What this means is that you don’t necessarily need to use all of the power provided by the amp. If you only need 300 watts, the 600-watt power amp used in my example will deliver the necessary power along with the required voltage swing.