Advanced yamaha active servo technology – Yamaha YST-SW160/90 User Manual

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The theory of Yamaha Active Servo Technology has been 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
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.

bass sound



Air woofer
(Helmholtz resonator)



Signals of low amplitude

Advanced Negative-
Impedance Converter


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 conventional Yamaha Active Servo

Our new Active Servo Technology — Advanced Yamaha Active
Servo Technology — adopted Advanced Negative Impedance
Converter (ANIC) circuits, which allows the conventional
negative impedance converter to dynamically vary in order to
select an optimum value for speaker impedance variation. With
this new ANIC circuits, Advanced Yamaha Active Servo
Technology can provide more stable performance and improved
maximum sound pressure compared with the conventional
Yamaha Active Servo Technology, resulting in more natural and
energetic bass reproduction.