Yamaha AV-90PY - Manual (Page 6)

6
The theory of the 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 small 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 the signals. If the electrical resistance of the voice coil
is 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 (28 Hz – 20 kHz) with amazing
sound quality and less distortion.
The features described above, then, are combined to be the
fundamental structure of the Active Servo Technology.
YAMAHA 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