Compared to former versions of the MB-450, the Series III offers an unrestrained sense of openness, tighter and
more controlled bass, and an ease and musical neutrality that give you the most satisfying listening experience.
Circuit overview:
An all-tube amplifier, the MB-450 Series III is the culmination of VTL’s most advanced thinking in performance
engineering. A proprietary regulated power supply, high-grade parts, and an automatic tube bias system ensure
ultra high fidelity audio reproduction, while tube fault diagnostics and user feedback offer the ultimate in
usability and convenience.
The audio amplifier is comprised of a fully balanced differential input stage driving a differential phase splitter
and a lower impedance push-pull output stage with a dramatically improved, fully balanced, interleaved and
coupled output transformer. Furthermore, the new design in the Series III amplifier incorporates a shorter, faster
and fully balanced negative feedback loop, with zero global negative feedback. The revised negative feedback
loop completely eliminates ringing and requires no capacitor compensation to maintain critical phase integrity
and information.
Further engineering innovations for the MB450 Series III include a variable Damping Factor feedback control
that allows the user to precisely adjust the amplifier’s output impedance by setting the amount of negative
feedback to one of four possible levels. Impedance can now be varied to suit the listener’s taste, and to improve
control of the loudspeaker loads to deliver best performance.
Adjustable precision-regulated bias and screen supplies keep the operating point of the output tubes constant even
under AC and main power supply fluctuations, and stabilize the power supplies for greater tonal stability and sonic
integrity especially during complex, dynamic signal conditions.
All critical voltages in the power supply are precision regulated for consistent and predictable performance under
a wide variety of conditions. Large energy storage reservoirs mean that the amplifier is capable of delivering huge
peak current swings into demanding loads for powerful musical performance.
Optimized operating voltages and timed current inrush limiting dramatically extend tube life, with low idle
current draw on all stages. An ever-warm position further lowers idle current while keeping all voltages present
so the circuit remains warm when not in use.
The amplifier can run in one of two operating modes: Tetrode or Triode. The user can change mode either in
standby or in power-on mode. Users often prefer one mode to another, depending on the type of music. For
example, tetrode may offer a wider sound stage and greater dynamics for complex and demanding symphonic
music, while triode may work better for small ensembles or solo instruments.
Comprehensive fault sensing monitors the status of the output tubes, and alerts the user to any adverse
conditions. In the case of a problem in any of the output tubes, the amplifier will immediately shut down. The
tube fault is indicated on the front panel, as well as by a blinking LED on the top deck next to the faulty tube
making diagnosis and tube replacement easy.
The logic-controlled automatic bias system is designed to keep the tubes at peak performance and operates
completely outside of the signal path. The tube biasing circuit checks and adjusts the tube bias values
automatically during the power-on sequence, as well as when the amplifier is at idle (i.e. when no music signal is
being amplified.), and drops out of the circuit path when music signal is detected.
MB-450 Series III Signature Monoblock Owner’s Manual
2
