Technical Description - Bel Canto SETi 40 User's Manual And Operating Information

Technical Description

Circuit Description and Technical
Information
The philosophy behind the design of the SETi 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 SETi 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 SETi
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 that 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 that
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.
SETi 40
Owner's Manual
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 SETi 40.
The SETi 40 bandwidth is similar to many amplifiers
which use feedback and push-pull operation to
achieve good performance. The SETi 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 SETi 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.
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