Accuphase A 30 Brochure
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Pure Class A power amplifier using power MOS-FET devices Fully balanced signal paths as found in high-quality instrumentation amplifiers. Further refined MCS+ topology and current feedback design result in even better S/N ratio, distortion, and other performance parameters. Robust power supply and power MOS-FET devices in triple parallel push-pull configuration sustain an amazing 150 watts per channel into ultra-low 1-ohm impedance loads (with music signals). Pure Class A power amplifiers from Accuphase have long been blending the purity of class A operation with the superior performance of power MOS-FETs. While building a string of outstanding amplifiers, Accuphase has accumulated a store of technical know-how that is second to none. The latest in this series, the A-30 is a pure class A stereo power amplifier based on the advanced technology of the model A-60. The A-30 employs the so-called instrumentation amplifier principle throughout. The signal handling stages feature further improved MCS+ topology and the renowned current feedback approach. This has resulted in electrical characteristics that surpass even the demanding standards set by its predecessors. Employing only highest grade materials and strictly selected parts, the A-30 pursues the two most important goals of an amplifier: very low output impedance (Note 1) and constant drive voltage (Note 2). The output stage of the A-30 features power MOS- FET devices renowned for their excellent sound and superior reliability. Because they have negative thermal characteristics, there is no danger of thermal runaway as exists with bipolar transistors. Operation remains totally stable even when theamplifier is running hot. Driving these devices in pure class A assures high-definition sound that brings out the finest nuances in the music. In pure class A operation, the power supply delivers a steady current, regardless of the presence or absence of a musical signal. This means that the amplifier remains unaffected by fluctuations in voltage and other external influences. On the other hand, it also means that the output stage generates considerable thermal energy. In the A-30, this is dissipated effectively by large heat sinks which provide ample capacity to remove the heat produced by the internal circuitry. The heavy-duty power supply easily sustains output levels of 120 watts into 2 ohms, 60 watts into 4 ohms, or 30 watts into 8 ohms (per channel). This linear progression demonstrates that the amplifier will be capable to drive even speakers with very low impedance ratings or with pronounced impedance fluctuations. Stability remains excellent at all times. The amplifier also has the necessary reserves to handle musical transients that require considerable power in an instant. If even higher power is required, bridged mode turns the A-30 into a high-output monophonic power amp. Output current (A) Output voltage (V) OUTPUT + B3 – B3 –INPUT +INPUT NFB NETWORK NFB NETWORK GAIN CONTROL CIRCUIT + – – + REGULATOR REGULATOR + B1 – B1 – B2 + B2 Q1 Q3 Q2 Q4 Q8 Q6 Q7 Q5 Q9 Q17 Q19 Q18 Q20 Q21 Q22 Q23 Q24 Q25 Q26 Q13 Q11 Q15 Q10 Q14 Q12 Q16 NFB NETWORK MCS+Multiple Circuit Summing( ) Bias stabilizer circuit Bias stabilizer circuit Bias stabilizer circuit Bias stabilizer circuitBias stabilizer circuit Fig. 1 Circuit diagram of amplifier section (one channel) Note 1: Low amplifier output impedanceThe 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 performance 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. This absorbs the counterelectromotive force generated by the voice coil and prevents the occurrence of intermodulation distortion. Note 2: Constant drive voltage principleEven in the presence of a load with wildly fluctuating impedance, the ideal power amplifier should deliver a constant voltage signal to the load. Figure 2 shows the results of actual output voltage/output current measurements at different load impedances for the A-30. It can be clearly seen that output voltage is virtually constant at various loads, which means that current increases in a linear fashion. Actual measurements of clipping power have yielded the following figures, which impressively demonstrate the more than ample performance of the A-30: 1 ohm: 175 watts, 2 ohms: 142 watts, 4 ohms: 94 watts, 8 ohms: 58 watts. ★1-ohm rating is for music signals only. Fig. 2 Load impedance vs. output power (Output voltage/output current) Instrumentation amp configuration allows fully balanced signal paths The A-30 features a new instrumentation amplifier principle whereby all signal paths from the inputs to the power amp stage are fully balanced. This results in excellent CMRR (Common 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 refined MCS+ topology for even lower noise Accuphase's original MCS (Multiple Circuit Summing) + – –INPUT NFB NETWORK GAIN CONTROL CIRCUIT NFB NETWORK OUTPUT +INPUT + – + – Signal input stage Power amplifier stage Instrumentation amplifier configuration principle uses a number of identical circuits connected in parallel to achieve superior performance characteristics. MCS+ is a fur ther refined version of this approach. Improvements in the bias circuitry of the input-stage buffer amplifier result in greater stability. This in turn makes it possible to extend the parallel operation approach to the class A drive stage of the current/voltage converter, thereby further lowering the noise floor. Power MOS-FET output stage with triple parallel push-pull power units delivers 120 watts into 2 ohms, 60 watts into 4 ohms, or 30 watts into 8 ohms with outstanding linearity The output stage (Figure 1) uses power MOS-FETs. Three pairs of these devices are arranged in a parallel push-pull configuration for each channel. The result is stable operation with ideal power linearity even down to ultra-low impedances. The maximum power dissipation of one MOS-FET is 130 watts, but the actual power load per pair is much
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lower, so that each device is driven only in its low- power range where linearity is excellent. A music signal consists of a continuous succession of pulse waveforms. To prevent clipping on occasional momentary high-level pulses, the maximum clipping level of the A-30 is set to 50 watts per channel into 8 ohms (sine wave output). Current feedback circuit topology assures excellent phase characteristics in high range In the A-30, the signal current rather than the more conventionally used voltage is used for feedback. Since the impedance at the current feedback point (current adder in Figure 3) is very low, there is almost BufferCurrent adder I-V converter Trans-impedance amplifier Amplifier Output Current NFB network – Input Buffer + Input Gain(Large) (High) Frequency no phase shift. Phase compensation therefore can be kept at a minimum. A minimal amount of NFB results in maximum improvement of circuit parameters. The result is excellent transient response and superb sonic transparency,coupled with utterly natural energy balance. 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. Robust power supply with large power transformer and high filtering capacity In any amplifier, the power supply plays a vital role since it acts as the original source for the output delivered to the speaker. The A-30 employs a large and highly efficient 400 VA power transformer. It ishoused in an enclosure filled with a material that transmits heat and absorbs vibrations. This completely prevents any adverse influences on other circuit parts. Two aluminum electrolytic capacitors rated for 47,000 µF each serve to smooth out the pulsating direct current from the rectifier, providing more than ample filtering capacity. Fig. 3 Principle of current feedback amplifier Fig. 4 Frequency response with current feedback(Response remains uniform also when gain changes) n Power amplifier assembly with three parallel push-pull power MOS-FET pairs per channel mounted directly to large heat sink, MCS+ circuitry, and current feedback amplifier