Specifications

M&K Sales Training Outline
page 14
May 98
http://www.mksound.com
Servo and other types
of feedback
Many customers ask about negative feedback in powered subwoof-
ers. M&K does not currently manufacture subwoofers using servo
feedback, although the original Volkswoofer
did have a servo circuit.
Here is some discussion on the subject.
Feedback uses a sensor mounted on the speaker that detects the
movement of the cone. The signal generated by this movement is
sent from the speaker voice coil area back to the electrical input of the
power amplifier and compared to the subwoofer's input signal. Any
difference between the two represents distortion. The feedback cir-
cuit takes that difference (distortion) signal, inverts its phase, and ap-
plies it back to the amplifier’s input, theoretically canceling the distor-
tion.
When an input signal is steady-state, like a sinewave, feedback can
measure extremely well. Unfortunately, music
always changes. And
if a musical signal has changed by the time the "correction" signal has
been applied, the sound is not improved—it is made worse!
This is why designers of high-end amplifiers strive for mini-
mum
feedback. Even though greater feedback produces lower mea-
sured
distortion, excessive feedback produces poorer sound due to a
phenomenon known as Transient Intermodulation Distortion.
In loudspeakers, the same principle applies, but to a much greater
degree, because the delay in applying the correction signal is due to
the motion of the speaker instead of the speed of electrons moving
through an amplifier (at nearly the speed of light).
In speakers, feedback also affects transient performance and dy-
namic range. Feedback systems can become unstable with sharp
transients and high output levels, so designers must put limit-
ing circuitry on the amplifier. This changes the sound quality by
dulling transients and compressing dynamics.
M&K's philosophy is to strive for excellent reproduction of transients,
not sine waves. Compare the sound of an M&K Powered Subwoofer
to any subwoofer using servo feedback. The dynamic and transient
performance of the M&K will result in an
audibly cleaner and more
detailed musical sound.
Powered Subwoofers