Ashly Electronic Amplifier none - Manual (Page 21)

efficient use of expensive amplifier power. Also, the greater the range of
frequencies an amplifier is asked to handle, the more intermodulation
distortion products you can expect to see.
All of these problems have been answered pretty well by the use of active
crossovers
and mul ti-amplified systems, but of course those alternatives
present some new and unique problems as well. It remains true that, despite
their
limitations, passive crossovers still have value, as evidenced by their
continued use in the majority of consumer loudspeakers and low-power PA and
recording monitor applications.
ACTIVE CROSSOVERS IN MULTI-AMPLIFIED SYSTEMS
Active crossovers evolved as a solution to the previously mentioned problems
of
passive systems. In general, they are smaller, lighter, cheaper, more
flexible, more efficient, and have less distortion than passive types.
In an active system, the audio is frequency divided at line level before being
fed to the power amplifiers, as shown in figure 21 below.
Input Low Out Hi Out
MIXING CONSOLE
Output ^
ELECTRONIC
CROSSOVER
POWER AMPLIFIER
Input (HIGHS)
POWER AMPLIFIER
• • Input (LOWS)
•To tweeter
■To woofer
Figure 21 Biampl ification
This system has a number of important advantages. The active filters in the
electronic crossover eliminate the need for inductors, thereby reducing size,
weight, cost, distortion, and hum. Active filters can easily be made tunable,
which means that the crossover can be precisely modified to fit the speaker
system at any time, a prime requirement for touring PA systems and fixed
installations as well. The filters will respond in a stable and predictable
way because they are terminated by a constant impedance within the crossover
itsel f.
Since the amplifiers are asked to work within a more restricted bandwidth, IM
distortion will be lower, and since each speaker will be driven by its own
amplifier,
amp power can be distributed efficiently within each frequency
range. A given system may require a 250 watt amp for the woofer and only 40
watts for the horn.
One of the nicest benefits of an active system is an increase in apparent
loudness for a given amount of amplifier power. This is a result of the fact
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