Accuphase A 45 Brochure
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Bias stabilizer circuit Bias stabilizer circuit +INPUT –INPUT Bias stabilizer circuit Bias stabilizer circuit Bias stabilizer circuit Pure Class A power amplifi er uses power MOS-FET devices to deliver 45 watts per channel (8 ohms) – Fully balanced signal paths as found in high- quality instrumentation amplifi ers. Further refi ned MCS+ topology and current feedback result in superb sound quality and excellent ratings for S/N ratio, THD, and other parameters. Strong power supply and power MOS- FET devices in six-parallel push-pull confi guration sustain an amazing 360 watts per channel into ultra-low impedance 1-ohm loads (with music signals). Pure Class A power amplifi ers from Accuphase have long been blending the purity of class A operation with the superior performance of power MOS-FETs. Acclaimed by audiophiles the world over, these products refl ect a level of technical know-how that is second to none. Completing the A series lineup, the A-45 is ideal for systems that demand not quite the high power levels of the A-60 but require more power than the A-30 provides. The A-45 of course also demonstrates the same unwavering dedication to sound quality and outstanding design policy which are the hallmark of Accuphase. This pure class A stereo power amplifi er with its distinctive external heat sinks is built for the true enjoyment of music. The so-called instrumentation amplifi er principle is used throughout. The signal handling stages feature further improved MCS+ topology and the renowned current feedback approach. This results in electrical characteristics that surpass even the demanding standards set by its predecessors. Employing only highest grade materials and strictly selected parts, the A-45 realizes the two most important goals of an amplifi er: very low output impedance and constant drive voltage. The output stage features power MOS-FETs renowned for their excellent sound and supe- rior reliability. For each channel, six of these devices are arranged in a parallel push-pull arrangement. Because MOS-FETs have negative thermal characteristics, there is no danger of thermal runaway as exists with bipolar transistors. Operation remains totally stable even when the amplifi er is running hot. Driving these devices in pure class A assures high-defi nition sound that brings out the fi nest nuances in the music. The heavy-duty power supply features a mas- sive power transformer rated for a maximum of 600 VA, plus two smoothing capacitors of 47,000 μF each. This easily sustains output levels of as much as 360 watts per channel into 1 ohm (with music signals). If even higher power is required, bridged mode turns the A-45 into a high-grade monophonic power amplifi er. n Power modules with 6-parallel push-pull arrangement of power MOS-FETs deliver 360 watts per channel into 1 ohm (music signals only), 180 watts into 2 ohms, 90 watts into 4 ohms, or 45 watts into 8 ohms. n Amply dimensioned power supply with high-efficiency toroidal transformer and large fi ltering capacity. The high-effi ciency toroidal transformer is rated for 600 VA, and the two extra- large 47,000 μF aluminum electrolytic capacitors were selected for best sonic performance. n Bridged mode allows upgrading to monophonic amplifi er with 720 watts into 2 ohms (music signals only), 360 watts into 4 ohms, or 180 watts into 8 ohms. Bridged connection increases the output by a factor of 4, creating dramatic power reserves in a monophonic confi guration. n Four gain control settings minimize residual noise. The gain control switches gain in the fi rst stage of the instrumentation amplifi er. Four settings are available (MAX, –3 dB, –6 dB, –12 dB). n Fully balanced input stage shuts out external noise interference. n Mode selector for easy switching between dual mono/stereo/bridged operation. n PCB copper foil and all major signal path components are gold-plated. n Large analog power meters with off switch and sensitivity selector. n Oversize speaker terminals accept also Y lugs. Toroidal power transformer Filtering capacitors Gain selector Meter and input buttons Power MOS-FETs Large speaker terminals Parts selected for sound quality and reliability Assembly with meter and protection circuitryBalanced input connectors Gold-plated parts + 1-ohm operation possible with music signals only Output voltage (V) Fig. 2 Load impedance vs. output power (output voltage/output current) Fig. 1 Circuit diagram of amplifi er section (one channel) Output current (A) MCS+ (Multiple Circuit Summing-up) Instrumentation amplifi er Power MOS-FETs 6-parallel push-pull Gain control
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I-V converter Amplifie Current NFB network Trans-impedance amplifie Current adder – Input + Input Output Buffer Buffer Instrumentation amp confi guration allows fully balanced signal paths The newly adopted instrumentation amplifier principle ensures that all signal paths from the inputs to the power amp stage are fully bal- anced. This results in excellent CMRR (common Current feedback principle assures excel- lent phase characteristics in high range As shown in the illustration, the A-45 uses the output signal current rather than voltage for feedback. Since the impedance at the current feedback point is very low, there is almost no phase shift. A minimal amount of NFB therefore results in maximum improvement of circuit parameters. mode rejection ratio) and minimal distortion. Another significant advantage is that external noise and other external influences are virtually shut out. The result is a drastic improvement in operation stability and reliability. Further refi ned MCS+ topology for even lower noise Accuphases original MCS (Multiple Circuit Summing) principle uses a number of identical circuits connected in parallel to achieve superior performance characteristics. MCS+ is a further refined version of this approach. By extending parallel operation to the class-A drive stage of the current/voltage converter, the noise floor has been lowered further. n Power amplifi er assemblyPower amplifier assembly with six parallel push-pull power MOS-FET pairs per channel mounted directly to large heat sink, MCS+ cir- cuitry, and current feedback amplifi er. Instrumentation amplifi er confi guration Signal input stage Power amplifi er stage + INPUT – INPUT Instrumentation amplifi er and further refi ned MCS+ topology Principle of current feedback amplifi er