Yamaha YST-SW120/60 - Manual (Page 11)

11
ACTIVE SERVO TECHNOLOGY
The theory of Active Servo Technology is based upon two
major factors, the Helmholtz resonator and negative-
impedance drive. Active Servo Processing speakers
reproduce the bass frequencies through an “air woofer”,
which is a port or opening in the speaker’s cabinet. This
opening is used instead of, and performs the functions of, a
woofer in a conventionally designed speaker system.
Thus, signals of low amplitude within the cabinet can,
according to the Helmholtz resonance theory, be output
from this opening as waves of great amplitude if the design
is such that the size of the opening and the volume of the
cabinet are in the correct proportion to satisfy a certain
ratio.
In order to accomplish this, moreover, the amplitudes within
the cabinet must be both precise and of sufficient power
because these amplitudes must overcome the “load”
presented by the air that exists within the cabinet. Thus it is this problem that is resolved through the
employment of a design in which the amplifier functions to
supply special signals. If the electrical resistance of the voice
coil could be reduced to zero, the movement of the speaker
unit would become linear with respect to signal voltage, and,
to accomplish this, a special negative-impedance output-drive
amplifier for subtracting output impedance of the amplifier is
used.
By employing negative-impedance drive circuits, the amplifier
is able to generate precise, low-amplitude low-frequency
waves with superior damping characteristics, and these waves
are then radiated from the cabinet opening as high-amplitude
signals. The system can, therefore, by employing the
negative-impedance output drive amplifier and a speaker
cabinet with the Helmholtz resonator, reproduce an extremely
wide range of frequencies with amazing sound quality and
less distortion.
The features described above, then, are combined to be the
fundamental structure of the Active Servo Technology.
Air woofer
(Helmholtz resonator)
Negative-impedance
output drive
High-
amplitude
bass
sound
Active Servo
Processing
Amplifier Signals
Port
Cabinet
Signals of low amplitude