Bel Canto Design SET 40 - Manual (Page 9)

SET 40 Owner's Manual Page 9
Technical Description
Circuit Description and Technical
Information
The philosophy behind the design of the SET 40
is based on the observation that simplicity of the
circuit and the reduction of the number of active
devices in an audio amplifier will permit the
greatest quantity of information to get through the
amplifier. This allows the illusion of music to be
most powerful and results in the amplifier
'disappearing' effectively from the playback chain.
This chameleon-like performance is achieved
through an elegantly simple circuit.
The primary design goal of the SET 40 was to
take the idea of the single ended (SE) triode
amplifier to its ultimate level, using only two triode
stages to keep distortion low and reduce the
number of active devices to a strict minimum, while
delivering enough power so that a wide range of
loudspeaker choices could be made for use in a
home audio system. Unlike other high power SE
triode amplifiers which use several stages of gain
and buffering, the SET 40 takes advantage of the
distortion cancellation and reduction which occurs
when using just two triode stages. The low
distortion figures at typical playback levels of 1-5
watts are achieved with no local or loop feedback.
This total absence of feedback also maximizes
the amplifier's ability to transmit the information
content in the music signal.
Care in the choice of active and passive parts
results in an audio amplifier which will remain valid
in the future as fashion and taste changes. In
other words, it is likely to become a 'classic,'
retaining its musically satisfying performance for
many years to come. This thinking begins at the
power supply which uses two high value PI filters
with 2-10 Henry inductors and over 150 Joules of
energy storage. This provides a very quiet and
well regulated supply for the output stage. The
capacitors used for all high tension supplies and
decoupling are the highest quality audio grade
polypropylene film caps. The single coupling
capacitors are custom Bel Canto film and foil units.
The output stage uses the 845 triode (a tube
which has seen many, many years of use as an
audio amplifier in high power radio station
modulators) to drive a custom output transformer
which provides superb power handling and
bandwidth. The 845 tube is run at maximum
power in radio stations for over 8000 hours, you
should get similar performance life in the SET 40.
The SET 40 bandwidth is similar to many amplifiers
which use feedback and push-pull operation to
achieve good performance. The SET 40 achieves
this without resorting to these information robbing
circuit techniques.
The use of self bias for the 845 tube allows a
higher grid resistance to be used, permitting a
single high gain, relatively low power, input/driver
stage. This type of bias scheme is more difficult to
implement than grid bias schemes, requiring high
quality, high value decoupling capacitors, large
power resistors and a higher voltage on the main
supply. (This voltage is near 1200 V DC on the
SET 40!)
The input/driver stage uses two 12AX7s in a
modified SRPP stage which provide total gain of
30 dB and 1 K ohm of output impedance. This
input stage can supply over 100 Vrms at low
distortion into the 400 K ohm grid resistor used on
the 845. The SRPP type load provides power
supply rejection and reduces the sensitivity of the
input stage to power supply variations. It also
reduces power supply modulation and avoids the
problems inherent in the use of high power/high
voltage resistor load devices.