Yamaha YST-SW216 User Guide

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In 1988, 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 adds many 
refinements to Yamaha Active Servo Technology, 
allowing better control of the forces driving the amplifier 
and speaker. 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 without any murkiness.

High-amplitude 
bass sound

Port

Cabinet

Advanced impedance 
Converter

Active Servo 
Processing 
Amplifier

Signals of low amplitude

Air woofer 
(Helmholtz resonator)

Signals