A pre-main amplifier from Sansui's Classic Models lineup, with full tone controls and a protection circuit
Overview
The Sansui AU-5500 is a pre-main amplifier made by Sansui Electric Co., Ltd. It belongs to the brand's "Classic Models" lineup and was priced at ¥68,800 around 1975. It has a sophisticated phono stage, flexible signal routing and strong performance specifications.
Specifications
| Power Output | 30W + 30W (8 Ω, 20 Hz – 20 kHz) |
| Maximum Power Output | 38W + 38W (8 Ω, 1 kHz, single channel) |
| Music Power | 130W (4 Ω, 1 kHz) |
| Total Harmonic Distortion | Pre Amp: ≤0.1%, Main Amp: ≤0.1%, Overall: ≤0.15% |
| Intermodulation Distortion | Pre Amp: ≤0.1%, Main Amp: ≤0.1%, Overall: ≤0.2% (SMPTE) |
| Power Band Width (IHF) | 5 Hz – 30 kHz |
| Frequency Response (1W) | 10 Hz – 35 kHz ±0.5/-1 dB |
| RIAA Deviation | ±0.5 dB (30 Hz – 15 kHz) |
| Damping Factor | 30 (8 Ω) |
| Input Sensitivity / Impedance | Phono: 2.5mV/50kΩ, Tuner/AUX/Tape: 100mV/50kΩ, Main Amp: 800mV/50kΩ |
| Phono Maximum Input | 200mV (THD <0.2%) |
| Output Voltage (1 kHz) | Tape Rec (Pin/Din): 100mV/30mV, Pre Amp: 800mV |
| Channel Separation (1 kHz) | Phono: ≥50 dB, Tuner/AUX/Tape: ≥55 dB, Main Amp: ≥60 dB |
| Hum and Noise (IHF) | Phono: ≥70 dB, Tuner/AUX: ≥85 dB, Main Amp: ≥100 dB |
| Tone Control Range | Bass: ±13 dB (50 Hz), Midrange: ±5 dB (1 kHz), Treble: ±13 dB (15 kHz) |
| Loudness Compensation | +10 dB at 50 Hz, +8 dB at 10 kHz (Volume -30dB) |
| Low Filter | 70 Hz = -3dB (6dB/oct) |
| High Filter | 7 kHz = -3dB (6dB/oct) |
| Muting | -20 dB |
| Rated Power Consumption | 70W |
| Dimensions | 434 mm (W) × 130 mm (H) × 315 mm (D) |
| Weight | 10.4 kg |
Design
The power amplifier starts with a differential amplifier circuit built around a matched pair transistor at the first stage. The drive stage uses a 3-stage Darlington connection. The final output uses heteropolar power transistors with uniform characteristics. In the phono stage, a high-gain monolithic IC handles equalization. It is an NF-type circuit with a selected low-noise transistor at the input. Grounding avoids the chassis at the equalizer amplifier input. The tone section is a triple control system with adjustable midrange and a complete defeat. A large, regulated power transformer supplies power, and stabilized voltage goes to the equalizer and tone amplifiers. The dual tape monitor system allows bidirectional copying when two units are used. Other features are a loudness switch, an audio muting switch (-20dB), a stereo/monaural selector, speaker A/B/A+B switching, pre-main disconnect, DIN outputs, a grounding terminal and a spare AC outlet. The protective circuit combines electronic detection, a relay and a short-circuit fuse. It monitors temperature and voltage on the power transistors and suppresses pop noise at power-on. Low and high filters are included to reduce unwanted noise.
Context
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