Pros and cons of transformerless systems
The high-voltage, transformerless approach eliminates the drawbacks of amplifier transformers:
• cost
• weight
• limited bandwidth
• distortion
• core saturation at low frequencies.
On the other hand, transformers are useful to prevent ground loops, ultrasonic oscillations and RFI. Some local ordinances require transformer-isolated systems.
Let’s look at the core-saturation problem in more detail. Sound systems can generate unwanted low frequencies, due to, say, a dropped microphone or a phantom-powered mic pulled out of its connector.
Low frequencies at high power tend to saturate the core of a transformer. The less the amount of iron in the transformer, the more likely it is to saturate.
Saturation reduces the impedance of the transformer, which in turn may cause the amplifier to go into current limiting. When this occurs, negative voltage spikes are generated in the transformer that travel back to the amplifier—a phenomenon called flyback. The spikes cause a raspy, distorted sound. In addition, the extreme low-impedance load might cause the power amplifier to fail.
Some Crown amplifiers are designed with high-current capability to tolerate these low-frequency stresses.
Production amplifiers are given a “torture test.” Each amplifier must deliver a 15-Hz signal at full power into a saturated power transformer for 1 second without developing a hernia!
Many transformers are reactive, so their impedance varies with frequency. Some 8-ohm transformers measure as low as 1 ohm at low frequencies. That’s another reason for specifying an amplifier with high current capability.
CONCLUSION
Using a high-voltage system greatly simplifies the installation of multiple-loudspeaker PA systems. It also minimizes power loss in the loudspeaker cables. If you take care that your load does not exceed the power and impedance limits of your power amplifier, you’ll be rewarded with a safe, efficient system.
APPENDIX: HISTORY OF CONSTANT-VOLTAGE SYSTEMS
In early industrial sound systems, multiple loudspeakers were carefully configured to provide a matching impedance load to the amplifier. But as these systems grew in size, several problems arose: how to connect multiple loudspeakers to the same amplifier without loading it down, how to individually control the sound power level fed to those loudspeakers, and how to overcome the power loss associated with the typically long lines that ran between the power amp and loudspeakers.
By the late 1920s and early 1930s the “step-up, step-down” idea has been applied to loudspeaker lines in what has become known as “constant voltage” distributed systems. (Radio Physics Course 2nd Ed., Radio Technical Publishing co., N.Y., 1931).
Various voltages have been tried such as 25, 35, 50, 70, 100, 140, and 200 volts, but the 70V system has become the most widespread.
After World War II, we find constant-voltage systems depicted in such reference works as Radio Engineering 3rd Ed. (McGraw-Hill, N.Y., 1947). By the end of that decade, several standards had evolved to regulate 70V specifications for amplifiers and transformers. (Radio Manufacturer’s Association, SE-101-A And SE-106, both from July 1949). In the 1950’s we find the use of 70V systems very well established as evidenced by Radiotron Designer’s Handbook 4th Ed. (RCA, N.J., 1953 and Radio Engineering Handbook 5th Ed. (McGraw-Hill, N.Y., 1959).