A-48S At A Glance

• Topology: Class A stereo power amplifier with 6-parallel push-pull MOS-FET output stage
• Output power (linear, 8 / 4 / 2 / 1 ohms): 50 W / 100 W / 200 W / 400 W per channel
• Amplification principle: Instrumentation amplifier with current-feedback amplification topology
• Input-stage low-noise circuitry: MCS+ configuration
• Feedback: Balanced remote sensing
• Damping factor: 1,000
• Filtering: 68,000 μF filtering capacitors
• Transformer: Large high-efficiency toroidal
• Operating modes: Stereo, bi-amping, and bridged-mono switchable
• Front-panel monitoring: Large high-sensitivity power meters ranging from –50 dB
• Construction details: Aluminum hairline-finish top plate; high-carbon cast-iron isolator feet with superior damping; protection circuitry via MOS-FET switches

The A-48S is the entry point into Accuphase's Class A stereo power amplifier line — the S-revision of the A-48 it succeeds, with a 6-parallel push-pull MOS-FET output stage in place of the original A-48's 4-parallel arrangement, and a 6% reduction in noise level. It delivers 50 watts per channel into 8 ohms in pure Class A, doubling cleanly down through 4 ohms and 2 ohms to 400 watts per channel into 1 ohm — the constant-voltage-source behavior that separates a serious Class A amplifier from one that merely operates in Class A at low power.

Class A, all the time. The output stage of the A-48S runs in Class A across its full rated output. Class A operation produces no crossover distortion — the dominant distortion mode of conventional Class A/B amplifiers and the source of much of the harshness associated with solid-state amplification — at the cost of substantially higher quiescent current and heat dissipation. The 6-parallel push-pull MOS-FET output stage distributes that current load across enough devices that each MOS-FET operates in its linear region with substantial headroom; the result is low distortion at the output throughout the amplifier's rated power range, not just at low levels.

Instrumentation amplifier topology with current feedback. Accuphase's amplifier topology of choice is the instrumentation amplifier — the same architecture used in laboratory measurement equipment where signal accuracy is paramount. The instrumentation-amplifier configuration provides true differential input handling and excellent common-mode rejection across the full audio band. Current-feedback amplification, applied at the gain stages, preserves phase accuracy at high frequencies where voltage-feedback amplifiers begin to show phase shift.

MCS+ low-noise input circuitry. The MCS+ topology pairs multiple input transistors in parallel, summing their outputs at the next stage. Per-transistor noise averages out across the parallel array; per-transistor variation randomizes. The net effect is a substantially lower-noise input stage than any single transistor could provide — a critical advantage in an amplifier whose noise floor sits below the residual noise of most source components.

Balanced remote sensing. Conventional amplifiers measure output voltage at the amplifier's own output terminals. The A-48S measures output voltage at the speaker terminals via dedicated sense leads, allowing the amplifier to compensate for voltage drop across the speaker cables. The result is a damping factor of 1,000 measured at the speakers themselves — not 1,000 at the amplifier's terminals degraded by cable resistance.

Power supply. A large high-efficiency toroidal transformer feeds 68,000 μF of filtering capacitance — the kind of supply reserve that holds rail voltage steady through the most demanding orchestral transients, where conventional supplies sag and modulate the signal against the supply rails. Protection circuitry uses MOS-FET switches in place of mechanical relays, eliminating the small contact-resistance variation that mechanical switches introduce over time.

Bi-amping and bridged-mono operation. Two A-48S units can be configured as a bi-amped pair — separate amplifiers for the high-pass and low-pass driver sections of a multi-driver speaker — or as a bridged-mono pair providing significantly higher output into appropriate impedance loads. Front-panel switching configures the mode without rear-panel re-wiring.

Front-panel meters. The A-48S carries Accuphase's signature high-sensitivity output meters, calibrated from –50 dB upward. At normal listening levels in a residential system, the meters provide a continuous visual readout of how much of the amplifier's output range is being used — useful diagnostic information about system gain structure and an aesthetic element that distinguishes a serious power amplifier from a black box.

Construction. Aluminum hairline-finish top plate, machined-aluminum chassis, high-carbon cast-iron isolator feet with superior damping characteristics, large speaker terminals connected directly to the protection circuitry.

Where the A-48S sits in the line. The A-48S is the entry to Class A stereo power amplification at Accuphase — the right choice for a customer pairing with a C-2300 or C-2900 line preamplifier, or as the power section in a complete Accuphase separates system at the entry tier of the Class A line. Customers stepping up gain the A-80 (10-parallel push-pull, 65 W Class A into 8 ohms) or the A-300 monoblock (20-parallel push-pull, 125 W Class A into 8 ohms per chassis).

Other Accuphase power amplifiers in the Alma catalog: A-80, A-300, P-4600, P-7500.

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