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. It was the first pre-main amplifier in the 707 series. The design followed a philosophy of circuit purity, developed under the theme that "new sound is produced by a novel circuit that understands the essence of audio." The preamp and main amplifier sections are separated both electrically and structurally. It isn't the most powerful amp on paper, but its 85W per channel into 8Ω (20Hz-20kHz, 0.03% THD) comes with very low distortion and a wide bandwidth. Owners report a sense of control and clarity that feels more laboratory-grade than lounge-room cozy.

The AU-707 weighs reportedly 16.8kg, though some sources suggest up to 18kg. It measures 430mm wide, 168mm high and 389mm deep. That fits standard racks, but it needs room for ventilation, given the dense circuitry inside. The dual power transformer setup isolates left and right channels, which cuts crosstalk and power supply noise. That's a feature more commonly seen in separates. Four 15,000μF electrolytic capacitors in the power supply keep current stable under dynamic loads.

Specifications

Manufacturer	Sansui Electric Co., Ltd.
Year of Production	1976
Power Output	85W + 85W (8 Ω, 20 Hz to 20 kHz, 0.03% THD)
Total Harmonic Distortion	0.03% or Less (8 Ω, 20 Hz to 20 kHz at effective output)
Intermodulation Distortion	0.03% or Less (8 Ω, 70 Hz : 7 kHz = 4 : 1)
Output Bandwidth	5 Hz ~ 50 kHz (8 Ω, IHF, THD 0.03%)
Damping Factor	60 (8 Ω, IHF, 1 kHz)
Frequency Response	DC ~ 200 kHz +0, -3dB (1W)
Main Amplifier Input Sensitivity / Impedance	1V / 47k Ω (1 kHz)
Signal-to-Noise Ratio	115 dB or more (IHF, A-network, short circuit)
Channel Separation	75 dB or more (1 kHz, input short)
Phono Input Sensitivity / Impedance	2.5mV / 47k Ω (1 kHz)
Aux/Tuner/Tape Input Sensitivity / Impedance	150mV / 47k Ω (1 kHz)
Phono Maximum Allowable Input	320mV (1 kHz, THD 0.01%)
Tape Rec Output Level / Impedance	150mV / 47k Ω (1 kHz)
Pre-Out Level / Impedance	1V / 47k Ω (1 kHz)
Maximum Pre-Out	10V / 47k Ω (THD 0.05%)
Weight	16.8 kg (reportedly up to 18 kg)
Dimensions	430 mm (W) × 168 mm (H) × 389 mm (D)

Key Features

DC Amplifier Configuration in Power Section

The power amplifier section is a DC amplifier design with no output coupling capacitors. That preserves low-frequency transient response and phase accuracy, and it reduces phase shift near the bass extreme. It also contributes to the flat DC ~ 200 kHz frequency response at 1W. DC coupling adds complexity and the risk of offset voltage problems, so Sansui put a DC detection circuit at the output terminal to protect the speakers. If bias drifts, the failure can be catastrophic.

Dual FET First Stage with Matched Characteristics

A newly developed dual FET with matched characteristics is the first amplification stage in the power amp. The matched pair keeps the input differential stage symmetrical, which reduces even-order harmonic distortion and improves common-mode rejection. The FETs give high input impedance and low noise before the signal reaches the voltage amplification stage.

Differential Two-Stage Amplification with Low-Noise Transistors

The second stage is a differential two-stage amplification configuration built around low-noise transistors. It improves gain stability and suppresses thermal drift, which helps keep measured distortion low across the audible band. The cascaded differential setup also improves power supply rejection, so ripple or noise from the dual transformers has less effect.

Three-Stage Darlington Output Configuration

The output stage is a 3-stage Darlington configuration. It gives high current gain with relatively low drive current from the preceding stages. That allows tight control over reactive speaker loads and supports the high damping factor of 60 (8Ω, 1kHz). Darlington pairs can suffer from slower switching and thermal runaway risks. An ASO (safe operation area) detection current limiter circuit handles that by limiting output current under fault conditions.

Dual Power Transformers for Channel Isolation

Two independent power transformers are used, one for each channel. That gives complete electrical separation between the left and right amplification paths. It minimizes crosstalk and prevents shared power supply sag during dynamic transients, which was a rare feature in integrated amplifiers of this era. Each transformer feeds its own set of 15,000μF power supply capacitors (four total).

Separable Preamp and Main Amplifier Sections

The preamp and main amplifier sections can be separated physically and electrically, so the AU-707 can work as a preamp driving an external power amp. That suits system builders who want to upgrade power stages later or use the phono and line stages in a modular setup. The pre-out jacks support up to 10V output (THD 0.05%), enough to drive most external amplifiers to full output.

Subsonic and High Filters

The AU-707 has both a subsonic filter and a high filter. The subsonic filter attenuates infrasonic energy that can waste amplifier power and damage woofers. The high filter likely reduces ultrasonic noise or tape hiss, though exact cutoff frequencies are not documented.

Audio Muting Switch (-20dB)

A dedicated audio muting switch reduces volume by -20dB with a single press. It's handy for quick level checks or for avoiding loud transients during source changes. Unlike a full mute circuit that cuts the signal entirely, this attenuator just brings levels down to a safe range.

Center-Click Balance Control

The balance control has a center-click detent, so channel matching is precise and repeatable. It also gives tactile feedback when returning to a neutral setting, which helps when switching between stereo and mono sources or comparing speakers.

Loudness Function

The loudness contour boosts bass and treble at lower volume levels to compensate for the ear's reduced sensitivity in those ranges. It's often criticized as a crude fix. Because it's switchable, the amp's neutral default stays intact.

Headphone Jack and Spare AC Outlet

A front-panel headphone jack allows private listening without affecting main speaker output. A spare AC outlet on the rear panel can power a turntable or tuner directly from the amplifier, which cuts wall-wart clutter and may improve grounding consistency across the system.

Large Ground Jack

A large, dedicated ground jack helps eliminate hum and ground loops when connecting turntables or other grounded sources. That helps most in systems with multiple components, where grounding paths can get complicated.

Collectibility & Value

Owners report that the AU-707 will likely need a full recap, meaning replacement of the aging electrolytic capacitors, due to its age. That's a common need for amplifiers of this vintage. A noted issue is deteriorating glue on circuit boards, which can cause intermittent connections or component stress over time. Despite this, 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. No standardized pricing exists, and value depends heavily on condition and service history.

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