The conrad-johnson Enhanced Triode (ET) Amplifiers are power
amplifiers designed to capture the best attributes of both fine vacuum-tube
designs and of the best solid-state designs. The ET250S is a 250-watt per
channel stereo power amplifier, the MET150 a five-channel amplifier rated at
150 watts per channel. These sophisticated zero-feedback amplifiers offer the
refinement of a vacuum-tube amplifier with the guts and control of a
solid-state amplifier.
Zero Feedback. Negative feedback
is an engineering technique used to reduce measured distortion. While feedback
does achieve this goal, it also introduces aberrations in the tran�sient response characteristics of the circuit.
We have found that eliminating loop negative feedback can result in much finer
gradations in dynamic response (micro-dynamics) and more accurate decay on
musical transients. But eliminating negative feedback requires much more care
in the design of the circuits - both in choosing devices and in power-supply design,
and places greater demands on the parts.
The Right Device for the Job. A power
amplifier has two distinctly different functions. First, a voltage gain stage
must increase the amplitude of the input signal by a factor of about thirty.
The distortion characteristics of the amplifier are dominated by this stage. No
device is better suited to accurately amplifying signals at high amplitude than
triode vacuum tubes. For our enhanced triode amplifiers, voltage gain is
provided by a 6922 vacuum tube, selected for its ability to track a high
amplitude signal accurately. Theory and 30 years of experience have taught us
that the voltage gain stage of a power amplifier dominates the sonic character
of the critical mid-range of a power amplifier, and indeed these amplifiers
exhibit the smooth, lucid midrange for which tube amplifiers are so highly
esteemed.
The high amplitude signal produced by the voltage gain stage
must then be efficiently coupled to the loudspeaker system through an output
stage to produce the power and control needed for a musically accurate response
- basically to provide current gain and a low output impedance. High-current
bi-polar devices have been specified for the output stages of these amplifiers,
based on their extremely low output impedance and their ability to handle a
great deal of current. This contributes the fast, weighty bass and seemingly
limitless power reserve that are notable in these amplifiers.
Solid Power Supplies. Rock solid power
supplies are needed to enable the audio circuits to reach their potential on
musical transients. The ET amplifiers feature oversized main power
transformers (so that the circuits are not starved for current), and massive
power storage reservoirs with high-speed capacitor shunts to deliver quick,
even startling transient response. Carefully designed DC voltage regulators
isolate the critical voltage gain stages to prevent demands of the output stage
from modulating the input signal.
Quality Parts. Conrad-Johnson
components are internationally known for the quality of their parts, and the ET
amplifiers are no exception. The audio circuits and their related power
supplies utilize precision low inductance laser-trimmed metal foil resistors.
The circuits employ polypropylene, polystyrene and Teflon capacitors
extensively, except in the main power supply storage, where high-speed
electrolytics are used because of the extremely high capacitance required.
Precision-machined input/output connectors have been carefully selected for optimal
sonic performance. Even the internal wiring is chosen on the basis of its sound
quality in its specific application.
Thoughtful engineering, specification of top quality component
parts, and painstaking care taken in production make the MET150 and ET250S
capable of extraordinary performance in music reproduction.
