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Advanced Yamaha Active Servo Technology II

In 1998, Yamaha brought to the marketplace 
speaker systems utilizing YST (Yamaha Active 
Servo Technology) to give powerful, high quality 
bass reproduction. This technique uses a direct 
connection between the amplifier and speaker, 
allowing accurate signal transmission and precise 
speaker control.

As this technology uses speaker units controlled by 
the negative impedance drive of the amplifier and 
resonance generated between the speaker cabinet 
volume and port, it creates more resonant energy 
(the “air woofer” concept) than the standard bass 

reflex method. This allows for bass reproduction 
from much smaller cabinets than was previously 
possible.
Yamaha’s newly developed Advanced YST II, with 
many refinements to Yamaha Active Servo 
Technology, allows better control of the forces 
driving the amplifier and speakers. From the 
amplifier’s point of view, the speaker impedance 
changes depending on the sound frequency. 
Yamaha developed a new circuit design combining 
negative-impedance and constant-current drives, 
which provides a more stable performance and 
clear bass reproduction with less murkiness.

High-amplitude 
bass sound

Cabinet

Port

Air woofer 
(Helmholtz resonator)

Advanced 
impedance 
Converter

Active Servo Processing 
Amplifier

Signals of low

amplitude

Signals