Accuphase P 370 Brochure

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Accuphase P 370 Brochure

Extracted text from Accuphase P 370 Brochure (Ocr-read)


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The embodiment of musical power. Witness a stereo power amplifier capable of delivering 400 watts of linear power into 1-ohm loads. Current feedback topology guarantees stable operation up to ultra high frequencies. Massive power supply with 700 VA toroidal transformer, and wide-band high-power transistors in 3-parallel push-pull configuration ensure constant-voltage drive. Note 1: Low amplifier output impedance When forming the load of a power amplifier a loudspeaker generates a counterelectromotive force that can flow back into the amplifier via the NF loop. This phenomenon is influenced by fluctuations in speaker impedance, and interferes with the drive performance of the output circuitry. The internal impedance of a power amplifier should therefore be made as low as possible by using output devices with high current capability. Note 2: Constant drive voltage principle Even when the impedance of a load fluctuates drastically, the ideal power amplifier should deliver a constant voltage signal to the load. When the supplied voltage remains constant for any impedance, output power will be inversely propor tional to the impedance of the load. A conventional amplifier can be easily made to operate in this way down to a load impedance of about 4 ohms. However, at 2 ohms and below, much more substantial output reserves are needed. This can only be achieved by a thorough redesign of all basic amplifier aspects. Figure 1 Circuit diagram of amplifier section (one channel) Figure 2 Load impedance vs. output power (output voltage/output current) of P-370 In order for an amplifier to drive loudspeakers to optimum performance, two major goals must be realized: very low output impedance (Note 1), and constant drive voltage (Note 2). Low impedance not only ensures accurate speaker drive but also absorbs the counterelectromotive force generated by the voice coil, thereby eliminating a major source of intermodulation distortion. Accuphase amplifiers are capable of driving any kind of speaker load with optimum results, which is one of the reasons behind the high praise that these products invariably receive. The P-370 is a stereo power amplifier which fully implements these advanced circuit design principles. The output stage uses three pairs of high-power transistors in each channel, arranged in a parallel push-pull configuration. These devices are mounted to massive heat sinks, for efficient dissipation of thermal energy. Power linearity is maintained down to extremely low load impedances. This allows the amplifier to easily drive even speakers with very low impedance or uneven impedance curves. Using the P-370 in bridged mode creates a mono amplifier with even more impressive power reserves. The power supply section which acts as the energy source for the amplifier employs a high- efficiency "Super Ring" toroidal transformer in combination with large filtering capacitors. Current feedback topology combines total operation stability with excellent frequency response, while requiring only minimal amounts of negative feedback. Balanced inputs make the amplifier impervious to externally induced noise during signal transmission. The front panel features two large analog power meters which give the amplifier an elegant look. For use in an audiovisual system, the meter lights can be switched off so as not to interfere with the monitor image.Triple-parallel power units with push-pull configuration deliver ample linear power: 300 watts per channel into 2 ohms, 150 watts into 4 ohms, or 75 watts into 8 ohms The output stage uses high power transistors with excellent linearity and switching characteristics, rated for a collector dissipation of 150 watts and collector current of 15 amperes. These transistors are arranged in a 3-parallel push-pull configuration (Figure 1) and mounted on massive heat sinks, for efficient heat dissipation. This enables the P-370 to effortlessly drive even speakers with extremely low impedance or with reactive loads. Figure 2 shows the output/voltage characteristics at various load impedances. It can be seen that output voltage remains nearlyconstant regardless of load, which means that output current increases linearly. The actually measured clipping power is an impressive 400 watts into 1 ohm, 330 watts into 2 ohms, 220 watts into 4 ohms, or 133 watts into 8 ohms. Current feedback circuit topology prevents phase shifts The P-370 employs the so-called current feedback principle. Figure 3 shows the operating principle of this circuit. At the sensing point of the feedback loop, the impedance is kept low and current detection is performed. An impedance-converting amplifier then converts the current into a voltage to be used as the feedback signal. Since the impedance at the current feedback point (current adder in Figure 3) is very low, there is almost no phase shift. Phase compensation can be kept to a minimum, resulting in excellent transient response and superb sonic transparency. Figure 4 shows frequency response for different gain settings of the current feedback amplifier. The graphs demonstrate that response remains uniform over a wide range. BIAS STABILIZER CIRCUIT BIAS STABILIZER CIRCUIT BIAS STABILIZER CIRCUIT INPUT INPUT– +