Accuphase P-650 Brochure

m Powerful 6-parallel push-pull output stage deliv ers linear

power into ultra-low impedance loads m Current feedback guarantees great sound and stable operation m Bridged connection mode allows upgrading to true monophonic amplifier m Printed circuit boards made from T eflon material m Massive Super Ring toroidal transformer rated for 1 kV A

Astounding energy emerging from perfect poise – witness a stereo po wer amplifier capable of delivering 720 watts (actual measurement) into 1ohm loads. Massive power supply with 1000 VA toroidal transformer and wide-band high-power transistors in 6-parallel push-pull configuration ensure constant-voltage drive. Teflon printed circuit boards feature low dielectric constant and low loss. Current feedback topology guarantees stable operation up to ultra high frequencies.

Accuphase power amplifiers are designed to realize two major goals: very low output impedance (Note 1), and constant drive voltage (Note 2). As a result, 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 low impedance not only ensures accurate speaker driv e b ut also absorbs the counterelectromotive force generated by the voice coil, thereby eliminating a major source of intermodulation distortion. The ove rall result is a significant improvement in sound quality.

The P-650 is a stereo power amplifier which fully implements these advanced circuit design principles. Using only strictly selected high-quality parts, this product was designed with a full mastery of all aspects of amplifier performance. The output uses six pairs of high-power transistors in each channel, arranged in a parallel push-pull configuration. These devices are mounted to massive heat sinks located on both sides of the unit, 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 . By using the P-650 in bridged mode, you can create a mono amplifier with even more impressive power reserves.

Current feedback topology combines total operation stability with excellent frequency response, while requiring only minimal amounts of negative feedback. The printed circuit boards of the P-650 are made of a Teflon material with extremely low dielectric constant and low loss, resulting in more transparent sound. The front panel in traditional champagne gold features two large analog power meters. The elegant and sophisticated appearance of the amplifier will enhance every listening room.

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Figure 1 Circuit diagram of amplifier section (one channel)

Note 1: Low amplifier output impedance

The load of a power amplifier, namely the 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 perfo rmance of the amplifier. The output 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 in the presence of a load with wildly fluctuating impedance, 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 proportional 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. Howe ver , at 2 ohms and below , much more substantial output reserves are needed, which can only be sustained by an extremely well designed and capable output stage and a highly robust and powerful power supply section. To build such an amplifier is a task that requires not only considerable experience and resources but also a thorough reappraisal of basic tenets.

Power units with 6-parallel push-pull configuration deliver ample linear power: 400 watts into 2 ohms, 200 watts into 4 ohms, or 100 watts into 8 ohms

The output stage uses high power transistors with excellent linearity and switching characteristics, rated for a collector dissipation of 130 watts and collector current of 15 amperes. These transistors are arranged in a 6-parallel push-pull configuration (Figure 1) and mounted on massive heat sinks made from diecast aluminum, for efficient heat dissipation. This enables the P-650 to eff ortlessly drive even speakers with e xtremely 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 nearly constant regardless

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Figure 2 Load impedance vs. output power

(output voltage/output current) of P-650

of load, which means that output current increases linearly. The actually measured clipping power is an impressive 720 watts into 1 ohm, 518 watts into 2 ohms, 320 watts into 4 ohms, or 178 watts into 8 ohms.

Current feedback circuit topology prevents phase shifts

The P-650 employs the so-called current feedbac k principle. Figure 3 shows the operating principle of this circuit. At the sensing point of the f eedback loop, the impedance is kept low and current

detection is performed. An impedance-conv erting 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.

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