Sansui AU-666 (1970)

At 9.75 kg, it arrives like a promise kept — dense, deliberate, and wired for war.

Overview

The Sansui AU-666 isn’t a subtle piece of gear. Released in May 1970 by Sansui Electric Co., Ltd., it was priced at ¥52,800 — a serious investment at the time for a solid-state stereo integrated amplifier. It is a pre-main design, meaning the preamp and power amp sections can be separated via pre-out/main-in jacks, offering rare flexibility in an era when most integrateds were locked down. This isn’t some minimalist signal path; it’s a fully loaded command center with a microphone input, tape monitor, headphone jack, muting switch, and independent left/right tone controls. The build reflects Sansui’s push into high-end solid-state design, with a circuit topology that avoids coupling capacitors in the signal path wherever possible, relying instead on direct coupling to preserve phase coherence and extend frequency response. It’s not just engineered — it’s over-engineered, right down to the 31 transistors and five diodes that populate its chassis.

Specifications

Key Features

Fully Direct-Coupled, Capacitor-Less Output Stage

The AU-666’s amplifier topology is built around a fully direct-coupled design with a complementary Darlington output stage using PNP and NPN transistors. This eliminates coupling capacitors in the signal path, reducing phase shift and extending low-frequency response — a hallmark of high-end 1970s Japanese engineering. The result is a cleaner, more direct signal transfer from input to speaker, with fewer components to degrade over time. A two-stage differential amplifier with stabilizing circuit ensures precision at the front end, while the absence of output capacitors contributes to a tighter, more dynamic bass delivery.

Separate Ripple Filter Circuits for Preamp and Power Amp

Sansui didn’t skimp on power regulation. The AU-666 features separate ripple filter circuits for the preamplifier and the main amplifier’s positive and negative power supplies. This isolation prevents noise from the high-current power section from bleeding into the sensitive preamp stage, helping maintain a high signal-to-noise ratio of 100 dB or more (IHF). It’s a design choice typically found in separates, not integrated units, and speaks to the seriousness of the internal layout.

Triple Tone Control with Independent Left/Right Adjustment

Unlike most tone controls of the era, the AU-666 allows independent adjustment of bass (±16 dB at 30 Hz), midrange (±5 dB at 1500 Hz), and treble (±15 dB at 15,000 Hz) for each channel. This level of granular control is unusual in a 1970 integrated amplifier and suggests Sansui anticipated use in less-than-ideal acoustic environments or with mismatched speakers. The inclusion of both low and high filters (-10 dB at 20 Hz and 20 kHz respectively) adds further refinement, letting users tailor the frequency extremes without affecting the core tonal balance.

Pre-Out/Main-In Capability

One of the AU-666’s standout features is the ability to bypass the internal preamp and use the power amp section independently. The pre-out/main-in jacks allow owners to insert an external preamplifier or connect the AU-666’s preamp to another amplifier. This modularity was rare in integrated designs of the time and gives the unit a flexibility that modern equivalents often lack. It’s a nod to audiophiles who wanted to mix and match components without abandoning a trusted power stage.

Dual Speaker Outputs (A and B)

The amplifier includes two sets of speaker outputs, labeled A and B, allowing connection to two pairs of speakers. While not unusual in higher-end models, it’s a practical touch for those who want to switch between rooms or speaker types. The damping factor of 40 (at 8 Ω) suggests reasonable control over speaker cones,

Thermal Protection with Dedicated Transistor

Protection is taken seriously: six short-circuit fuses guard against catastrophic failure, and a dedicated transistor monitors thermal conditions. The thermal protection circuit helps prevent runaway current, though full cascade failures — where nearly every transistor in the chain goes down — can occur, especially in neglected units.

High-Grade Coupling Capacitors Where Needed

While the signal path is extensively direct-coupled, Sansui used high-grade electrolytic and Mylar capacitors in necessary locations. These were selected to minimize phase distortion and preserve high-frequency detail, a concession to practicality without sacrificing performance. The preamp section, in particular, relies on specially selected low-noise silicon transistors to maintain a clean signal from phono inputs upward.

Microphone Input with Flat NF Equalization

The inclusion of a microphone input with flat NF-type equalization is an odd but telling feature. It hints at the AU-666’s potential use in semi-professional or broadcast environments, or perhaps in large homes where paging or announcements were desired. The input sensitivity is set at 3mV / 50k Ω, making it compatible with dynamic mics, It’s a niche feature, but one that underscores the unit’s versatility.

Collectibility & Value

There is no documented current market value for the Sansui AU-666 in major collector databases, but one unit was listed for 32,000₹ (Indian Rupees) on Facebook, suggesting regional variation in pricing. The amplifier has a reputation among restorers for being prone to serious failures — one rebuild report noted “a complete cascade transistor failure all the way down,” requiring nearly every transistor to be replaced. Other owners have reported a weak right channel, humming on power-up, no sound at low volume, and general circuit instability after decades of inactivity. These issues imply that while the AU-666 is capable of excellent performance, it demands thorough restoration before being trusted in a system. Units that have been properly recapped and bias-adjusted are likely to perform as intended, but untested or unrestored examples carry significant risk.

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Related Models

• Sansui 2000A (1968-1970)
• Sansui 3000A (1968-1970)
• Sansui 5000X (1969-1971)
• Sansui 551 (1973-1975)
• Sansui 6060 (1975-1977)
• Quad 44/405-COMBINATION (1976)
• Braun AUDIO-1 (1965)
• Braun AUDIO-250 (1965)
• Braun AUDIO-310 (1965)
• Magnavox Component Systems (1972)