Crown CTS1200 Reference Manual page 6

With CTs series amps, one channel can drive low-impedance loudspeakers, while another channel drives
loudspeakers with 70V transformers. This makes it easy to set up a system with large, low-Z speakers for
local coverage and distributed 70V speakers for distant rooms -- all with a single amplifier.
The Crown CTs 2000 is unusually adept at providing constant power levels into various loads. In dual mode,
it delivers 1000 watts into 2/4/8 ohms and into a 70V line. In bridge-mono mono, it delivers 2000 watts into
4/8/16 ohms, 2000 watts into a 140V line, and 2000 watts into a 200V line.
Crown's new Commercial Audio series of amplifiers and mixer-amps provide both low-Z and constant-
voltage operation. For example, the 180MA and 280MA mixer-amps offer 4-ohm, 70V and 100V outputs.
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-speaker P.A. systems. It also
minimizes power loss in the speaker 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 speakers.
By the late 1920s and early 1930s the "step-up, step-down" idea has been applied to loudspeaker lines in