Sansui AU-707 (1976)

At 16.8kg, it lands on the rack like a declaration: this is serious engineering, not just another receiver.

Overview

The Sansui AU-707 is an integrated stereo amplifier released in 1976 by Sansui Electric Co., Ltd., positioned as the first pre-main amplifier in the 707 series. it was built around a philosophy of circuit purity—developed under the theme that "new sound is produced by a novel circuit that understands the essence of audio." This wasn’t marketing fluff; the design reflects a deliberate pursuit of signal integrity, separating the preamp and main amplifier sections both electrically and structurally. While not the most powerful amp on paper, its 85W per channel output into 8Ω (20Hz–20kHz, 0.03% THD) is backed by exceptionally low distortion and a wide bandwidth that suggests restraint and precision over brute force. Owners report a sense of control and clarity that feels more laboratory-grade than lounge-room cozy.

Weighing in at reportedly 16.8kg—though some sources suggest up to 18kg—the AU-707 conveys solidity before it’s even powered on. Its dimensions (430mm wide, 168mm high, 389mm deep) fit standard racks but demand space for ventilation, especially given the dense circuitry within. The dual power transformer setup isolates left and right channels, minimizing crosstalk and power supply noise, a feature more commonly seen in separates. Combined with four 15,000μF electrolytic capacitors in the power supply, the amp delivers stable current under dynamic loads. This is not a unit designed for background listening; it’s for those who want to see how deep the rabbit hole goes.

Specifications

Key Features

DC Amplifier Configuration in Power Section

The power amplifier section employs a DC amplifier design, eliminating output coupling capacitors to preserve low-frequency transient response and phase accuracy. This approach reduces phase shift near the bass extreme and contributes to the amplifier’s flat DC ~ 200 kHz frequency response at 1W. While DC coupling increases complexity and risk of offset voltage issues, Sansui implemented a DC detection circuit at the output terminal to protect speakers—a necessary safeguard given the potential for catastrophic failure if bias drifts.

Dual FET First Stage with Matched Characteristics

A newly developed dual FET with matched characteristics serves as the first amplification stage in the power amp. This pairing ensures symmetry in the input differential stage, reducing even-order harmonic distortion and improving common-mode rejection. The use of FETs here provides high input impedance and low noise, critical for preserving signal integrity before the signal reaches the voltage amplification stage.

Differential Two-Stage Amplification with Low-Noise Transistors

The second stage uses a differential two-stage amplification configuration built around low-noise transistors. This design enhances gain stability and suppresses thermal drift, contributing to the amp’s low measured distortion across the audible band. The cascaded differential setup also improves power supply rejection, minimizing the impact of ripple or noise from the dual transformers.

Three-Stage Darlington Output Configuration

The output stage utilizes a 3-stage Darlington configuration, allowing for high current gain with relatively low drive current from the preceding stages. This topology enables tight control over reactive speaker loads and supports the amp’s high damping factor of 60 (8Ω, 1kHz). While Darlington pairs can suffer from slower switching and thermal runaway risks, the inclusion of an ASO (safe operation area) detection current limiter circuit mitigates these dangers by dynamically limiting output current under fault conditions.

Dual Power Transformers for Channel Isolation

Two independent power transformers are used—one for each channel—ensuring complete electrical separation between left and right amplification paths. This architecture minimizes crosstalk and prevents shared power supply sag during dynamic transients, a rare feature in integrated amplifiers of this era. Each transformer feeds its own set of 15,000μF power supply capacitors (four total), delivering robust energy reserves.

Separable Preamp and Main Amplifier Sections

The preamp and main amplifier sections can be physically and electrically separated, allowing the AU-707 to function as a preamp driving an external power amp. This flexibility appeals to system builders who want to upgrade power stages later or integrate the Sansui’s refined phono and line stages into a modular setup. The pre-out jacks support up to 10V output (THD 0.05%), sufficient to drive most external amplifiers to full output.

Subsonic and High Filters

Equipped with both a subsonic filter and a high filter, the AU-707 offers practical tools for system tuning. The subsonic filter attenuates infrasonic energy that can waste amplifier power and damage woofers, while the high filter likely reduces ultrasonic noise or tape hiss—though exact cutoff frequencies are not documented. These switches reflect Sansui’s attention to real-world usability beyond just specs.

Audio Muting Switch (-20dB)

A dedicated audio muting switch reduces volume by -20dB with a single press, useful for quick level checks or avoiding loud transients during source changes. Unlike full mute circuits that cut signal entirely, this attenuator maintains system engagement while bringing levels to a safe range. It’s a subtle but thoughtful inclusion for critical listening environments.

Center-Click Balance Control

The balance control features a center-click detent, ensuring precise and repeatable channel matching. This mechanical refinement prevents accidental imbalance and gives tactile feedback when returning to a neutral setting—especially valuable when switching between stereo and mono sources or comparing speaker performance.

Loudness Function

The loudness contour boosts bass and treble at lower volume levels to compensate for the ear’s reduced sensitivity in those ranges. While often criticized as a crude fix, its inclusion acknowledges typical listening habits. Whether used or defeated, the fact that it’s switchable preserves the amp’s neutral default character.

Headphone Jack and Spare AC Outlet

A front-panel headphone jack allows private listening without affecting main speaker output. The presence of a spare AC outlet on the rear panel enables powering a turntable or tuner directly from the amplifier, reducing wall-wart clutter and potentially improving grounding consistency across the system.

Large Ground Jack

A dedicated, large ground jack helps eliminate hum and ground loops when connecting turntables or other grounded sources. This is particularly useful in systems with multiple components where grounding paths can become complex.

Collectibility & Value

Owners report that the AU-707 will likely require a full recap—replacement of aging electrolytic capacitors—due to its age, a common need for amplifiers of this vintage. A noted issue involves deteriorating glue on circuit boards, which can cause intermittent connections or component stress over time. Despite these maintenance considerations, the amplifier is held in high regard; one owner described it as “a nice sounding amp, and very powerful,” while another claimed “the sound quality is amazing and this amp will drive speakers to their knees!” Market examples vary: a listing for an AU-707G Extra appeared at approximately Ks450,000 (Myanmar Kyat), and an AU-α707MR variant sold for $1,300, suggesting strong demand for well-maintained units. However, no standardized pricing exists, and value depends heavily on condition and service history.

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