Accuphase C 245 Brochure
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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