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Optimum match for P-370 power amplifier This state-of-the-art
preamplifier features current feedback technology, six fully separate line
amp units in triple-parallel dual MCS configuration, and fully dual-mono
construction with separate power supplies. Option board allows digital
signal input for top-quality CD reproduction.
Accuphase preamplifiers implement advanced
technology such as current feedback and
balanced signal transmission, in order to
achieve optimum quality reproduction that is
true to the spirit of the music. This is the reason
why our products continue to enjoy such a high
reputation. The C-245 is a stereo control center
that fully represents Accuphase technological
expertise. As the model name indicates, the
amplifier stands in the tradition of the C-200/
P300 series that has been the mainstay of
the Accuphase product lineup since its
inception. An ideally matched power amplifier
is found in the model P-370, capable of
delivering an amazing 400 watts per channel
into an ultra-low-impedance load of 1 ohm.
The line amplifier circuits of the C-245 are
based on the current feedback principle, which
has consistently received high praise for sonic
excellence and top-notch performance. Three
line amplifier modules are used for each
channel, resulting in a total of six line amp
modules, arranged in a triple parallel MCS
(Multiple Circuit Summing) configuration. This
arrangement provides drastic improvements
in S/N ratio, distortion, and other impor tant
performance parameters. All parts used in the
C-245 have been carefully selected on the
basis of extended listening tests. Features
such as tone controls and loudness
compensator allow precise fine tuning if
desired, and facilities for tape recorder
connection are also provided. In sound quality
and performance, the C-245 is a superbly
conceived top-notch product.
The possibility of installing option boards
further enhances flexibility. A Digital Input
Board using an MDS (Multiple Delta Sigma)
D/A converter can directly handle the digital
signal from a CD player or similar, for ultra-
pure music reproduction. A board for faithful
playback of analog records is also available. Current feedback circuit topology prevents
high-range phase shifts and ensures
superb sound quality
The C-245 uses the current feedback principle
for negative feedback. Figure 2 shows the
operating principle of this design.
employ a pure complementary push-pull
circuit topology. Hand-selected discrete par ts
are used in parallel for minimum noise.
Six line amplifiers are connected in a triple
parallel MCS (Multiple Circuit Summing)
configuration developed by Accuphase.
By arranging three line amp units for each
channel in a parallel connection, the
perfor mance advantages of each circuit
are added up, resulting in improved S/N
ratio, distortion and other vital
performance parameters. The overall
result is a stereo amplifier with superb
characteristics.
Balanced connection reliably blocks
induced noise
The longer the cable connections between
audio components, the higher is the
danger of external noise being introduced
into the signal path, which invariably leads
to sound quality degradation. The balanced
connection principle reliably prevents this
danger, by keeping the signal transfer
At the input point of the feedback loop, the
impedance is kept low for current detection.
A trans-impedance amplifier converts the cur-
rent 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 mini-
mum. A minimal amount of NFB results in
maximum improvement of circuit parameters.
The result is excellent transient response and
superb sonic transparency, coupled with ut-
terly natural
energy bal-
ance. Figure
3 shows fre-
quency re-
sponse for
different gain
settings of
the current
feedback am-
plifier. The graphs demonstrate that response
remains uniform over a wide range.
Six discrete line amplifiers in MCS
configuration designed for optimum sound
The line amp units of the C-245 (Figure 4)
Input
BufferCurrent adder
I-V
converter
Trans-impedance
amplifier Amplifier Output
Current NFB
network
Fig. 5 Line amplifier (MCS circuit)
operation principle
Fig. 1 C-245 block diagram
Fig. 3 Frequency response with current f eedback
(Response remains unifor m even when gain changes)
Fig. 4 Line amplifier unit circuit diagram
(3 units are used in parallel connection per channel)
Fig. 2 Current feedback amplifier principle diagram
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completely free from any kind of
interference.
Modular construction keeps amplifier
circuits separate
The six line amplifiers and balanced output
amplifiers are housed in a total of 10 separate
modules neatly arranged on a motherboard.
The units are firmly secured with a frame
construction to prevent external vibrations
from affecting the sound.
Complete mono construction with separate
transformers for left and right channels
The power supply of the C-245 employs a dual-
mono approach with separate power
transformers and filtering capacitors for the
two stereo channels. Each unit amplifier is
equipped with a wide-range low-impedance
voltage regulator to eliminate any possibility
of interference between stages.Tone controls use summing
active filters for highest sound quality
The tone control circuitry in the C-245 was
specially designed with summing active filters
such as
found in
high-quality
graphic
equalizers.
Figure 6 il-
lustrates
the opera-
tion prin-
ciple of this
circuit. The
flat signal is passed straight through, and only
when an adjustment is required, the charac-
teristics are created at F
1 and F 2 and added to
the signal, thereby producing the desired
change. This design provides efficient control
without degrading signal purity.
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-245 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 contact
resistance and outstanding long-term
reliability.
Input
Output
Tone control characteristics
Fig. 6 Tone control circuit diagram
(summing active filter type)
Supplied remote commander RC-20
Allows volume adjustment and source
switching.Line amplifiers and balanced input/
output amplifiers arranged on PCB
motherboard (total 10 modules)
Triple parallel unit amplifier
Sealed relays directly
connected to gold-plated input/output connectors
