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The new joy of analog -“ realized in a superb high-end preamplifier.
Current feedback technology, Teflon-based printed circuit boards, four
separate unit amplifiers, fully dual-mono construction with separate
power supplies. Optional phono equalizer unit allows top-quality analog
disc reproduction.
All preamplifiers from Accuphase are based on
balanced signal transmission, taking a no-
compromise approach to quality. Furthermore, they
reflect the rich experience accumulated by
Accuphase during many years of building state-of-
the-art components. The C-275V is a fully
redesigned successor to the C-275, making
extensive use of sophisticated technology first
introduced in the highly renowned C-290V. As an
analog preamplifier, it has been honed to an even
higher degree of perfection. Every single part was
carefully selected through intensive listening tests.
It offers a full complement of sound tailoring features
such as tone controls and a loudness compensator.
The basic circuit design employs current feedback
topology developed by Accuphase, assuring
outstanding performance combined with excellent
sound quality.
As shown in Figure 1, in its standard configuration
the C-275V is a dedicated line amplifier, but by
installing an optional phono equalizer unit in a
special slot on the rear panel, it allows high-grade
analog disc reproduction as well.
In keeping with the aim of creating the ideal line
amplifier, the balanced output stage of the C-275V
employs symmetrically bridged feedback, resulting
in a floating design where the signal is kept entirely
separate from the ground line. This lavish approach
is highly desirable in a line amplifier and assures
the best possible performance. The printed circuit
boards are an important element of a preamplifier
in regards to electrical performance as well as sound
quality. In the C-275V, these are made from a Teflon
material (glass fluorocarbon resin) with low dielectric
constant and low loss, for optimum sound.
Less frequently used controls are located behind a
sub panel, and the elegant gold-colored panel face
has a stylish appeal. Even in the smallest detail,
the C-275V exudes an atmosphere of sheer class
which makes this analog preamplifier a joy to own
and rapture to listen to.
Current feedback topology prevents phase
shifts
The amplifying circuits in the C-275V use the current
feedback principle for negative feedback. Figure 2
shows the operating principle of this design. At the
input point of the feedback loop, the impedance is
Fig. 1 Block diagram of C-275V (one channel)(Option)
kept low for current detection. A trans-impedance
amplifier conver ts the current into a voltage to be
used as the feedback signal. Since the impedance
at the current feedback point (current adder in Figure
2) is very low, there is almost no phase shift. Phase
compensation therefore can be kept at a minimum.
A minimal amount of NFB results in maximum
improvement of circuit parameters. The result is
excellent transient response and superb sonic
transparency, coupled with utterly natural energy
balance.
Figure 3 shows
frequency
response for
different gain
settings of the
current feedback
amplifier. The
graphs demon-
strate that re-
sponse remains
uniform over a wide range.
Balanced output stage with bridged feedback
In balanced signal transmission, two identical
signals are transmitted simultaneously with inverted
phase and combined at the receiving end, thereby
canceling out common-mode noise and interference.
This principle is one of the requirements for truly
high-quality sound.
The principle of balanced sound transmission is
shown in Figure 4. The outputs of the two amplifiers
are connected to form a cross-feedback loop, which
sends the symmetrical (+) and (-“) signals with low
impedance to the next stage. The signals are
isolated from the ground line, resulting in an ideal
balanced circuit. Even if one side of the output is
grounded, both amplifiers continue to operate, and
the output voltage does not change.Discrete line amplifier for optimum sound
The line amplifier is a pure complementary push-
pull circuit. It is built from discrete components and
employs the superior current feedback principle.
Phase compensation can be kept to a minimum,
resulting in realistic ambience.
Printed circuit boards made from Teflon with low
dielectric constant and low loss
The printed circuit boards for the signal-carrying
circuits are made of Teflon, a glass fluorocarbon
resin material. Teflon has a stable, low dielectric
constant as well as superior heat resistance,
excellent high-frequency characteristics, and many
other desirable proper ties. As a result, signal purity
is enhanced and there is a drastic improvement in
perceived S/N ratio.
* Teflon is a registered trademark of DuPont USA.
Complete mono construction with separate
transformers for left and right channels
The power supply of the C-275V employs a dual-
mono approach with separate power transfor mers
and filtering capacitors for the two stereo channels.
Each unit amplifier is equipped with a wide-range
low-impedance voltage regulator to eliminate
possible interference between stages.
Tone controls use summing active filters for
highest sound quality
The tone control circuitry in the C-275V was
specially
designed with
summing
active filters
such as found
in high-quality
graphic
equalizers.
Figure 5
illustrates the
operation
principle of
(High)
(High)
Gain
Frequency
Input Output
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* Photo shows the amplifier with
optional phono equalizer unit
AD-275V installednRemote commander
RC-26
allows volume
control and input
source selectionnMother PCB with gold-plated
copper paths supports the four
unit amplifiers (line and
balanced amplifiers), as well
as filtering capacitors and
peripheral circuitry.
Unit amplifier in thick aluminum housing
this circuit. The flat signal is passed straight through,
and only when an adjustment is required, the
characteristics are created at F1 and F2 and added
to the signal, thereby producing the desired change.
This design provides efficient control without
degrading signal purity.
Vibration-free design with unit amplifiers in
aluminum enclosures fastened to 8-mm
hardened aluminum chassis
The C-275V has four unit amplifiers, two each for
the line input and line output in each channel. Each
unit is powered by its own local voltage regulator
circuitry, and is housed in a strong aluminum
enclosure, to prevent interference between units.
Hermetically sealed relays connected
directly to gold-plated input/output jacks
Logic-controlled relays for signal switching
assure high sound quality and long-term
reliability
The use of logic-controlled relays at strategic
locations makes it possible to keep signal paths
extremely short. The relays used in the C-275V are
high-performance hermetically sealed types as used
in professional communication applications. The
contacts are twin crossbar types plated with gold
and silver palladium alloy, for minimum contactresistance and outstanding long-term reliability.
Other features
nVolume control designed for sonic purity
nSupplied remote commander for volume
adjustment and program source switching
nDedicated headphone amplifier for superior
sound
nVersatile array of functions
mTape recorder facilities with monitor switch and
copy switch
mOutput phase selector button
mLoudness compensator with three selectable
characteristics
mSubsonic filter removes ultra low frequency
noise
