Orfeo 60
Owner's Manual
Page 9
Technical Description
Circuit Description and Technical
Information
The philosophy behind the design of the
Orfeo 60
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
Orfeo 60
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
Orfeo 60
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
Orfeo
60
.
Single Ended Balanced Topology
The
Orfeo 60
develops its unusually high power
for a single-ended triode amplifier with a unique
Bel Canto circuit architecture which retains the
strengths of single-ended amplifiers and push-pull
designs. The input transformer is used to
generate perfectly balanced biphase signals
which drive separate 30 watt single-ended
amplifiers on each chassis which share a
common power supply. The outputs are then
combined in phase to generate double the power
output. This circuit maintains the advantages
inherent in single ended topologies and adds the
advantages of class A push-pull amplifiers
topologies.
The input/driver stage uses a professional grade
input transformer for generating the biphase
signals, optimizing EMI/RFI rejection and
monobloc grounding. Two 12AX7s in a modified
SRPP stage 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.