The M845
mono power amplifier is a single ended design based around the 845 power triode. Operating
in Class A and
giving 27 Watts of output power, it will
complement a wide range of medium to high efficiency loudspeakers. The
845 triode was originally
developed by RCA in 1931 as a radio
transmitting and audio amplification valve. The anode is machined from
solid graphite
to allow high power dissipation. It has an
extremely linear transfer characteristic. In recent years,
a
renewed interest in this kind of valve has
resulted in them being manufactured again by various factories in China. Implementing
the 845
in a sympathetic circuit which takes full
advantage of its capabilities is not straightforward. The filament draws
a significant
amount of power and also forms a part of the
signal path so energising it correctly is a critical part of the design.
The
valve also operates with very high voltages
across it so again, a careful choice of filtering components for that
high voltage
supply is essential such that adequate
current is available to the valve at all audio frequencies
instantaneously.
In the M845 this supply includes a hybrid Graetz bridge for the initial rectification and a substantial smoothing choke
as part of its CLC type filter. Although
all of the elements; power supply, circuit topologycomponent
choice have a profound effect on the overall
performance of a valve amplifier, one of the most important aspects is
the design
construction of the output transformer.
A high primary impedance, the resultant large step down ratio and the
need to
maintain a high level of insulation between
the windings all conspire against the transformer designer and the quest
for extended
bandwidth with efficient power transmission.
The M845 utilises an in house designed output transformer that underwent
extensive
trials using a variety of core materials and
winding techniques before the final configuration was chosen. The
other difficulty in using the
845 is in the design of a stage capable of
cleanly delivering the voltage swing necessary at its grid to take it to
full output.
The M845 incorporates a driver stage based on
the 6SN7 triode coupled via a custom wound, wideband, interstage
transformer.
The amplifiers input stage is directly
coupled to this driver meaning that there are no coupling capacitors
used in
the circuit at all. The circuit also operates without any negative feedback. The end result is an amplifier capable of reproducing
the full measure of instrument tone with extraordinary dynamic contrast, energy and colour. When
partnered
with appropriate loudspeakers the M845 will
deliver an unparalleled reproduction of all types of music.


Denumiri similare la Amplificator puresound M845: M 845

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