Sansui AU 607 (1976)

At 15.3 kg, it lands on the rack like a declaration: this is serious engineering, not just another vintage faceplate.

Overview

The Sansui AU-607 isn't a subtle machine. Sansui introduced it in 1976, during their peak era, as a push on integrated amplifier design. They marketed it as a "DC pre-main amplifier." It pairs a fully discrete preamplifier section with a direct-coupled main amplifier. There are no output capacitors, so the claimed frequency response extends from DC. It cost ¥69,800 at launch and went after serious audiophiles who cared about measured performance and circuit innovation more than flashy cosmetics. Owners report a clinical yet engaging sound and a solid build. It weighs a lot and draws 185W (resolved: 185W), so give it room and a decent outlet.

This is no stripped-down design. The AU-607 has an MM phono input with 2.5mV sensitivity, auxiliary inputs, dual tape loops, loudness compensation, subsonic filtering, a 32-step click-stop volume control, and a headphone output. Internal links let you separate the preamplifier and main amplifier sections for future bi-amping or external amplification. That was a rare flexibility for an integrated unit of its time.

Specifications

Key Features

DC Amplifier Configuration with Direct-Coupled Output

The DC amplifier design is the AU-607's defining trait. With no output coupling capacitors, Sansui got a frequency response that begins at true DC - reportedly 0 Hz - for exceptional transient response and low-frequency linearity. The circuit's stability and lack of phase shift near the bass extreme gave it a taut, authoritative grip on speakers, especially with complex acoustic passages or tight electric bass. The trade-off is vulnerability to DC offset at the output, which could damage speakers if a fault occurs.

Dual Power Transformers and 48,000μF Power Supply

To back the DC design and keep the channels independent, the AU-607 uses two separate power transformers - one for each channel. They feed a power supply with four 12,000μF capacitors, totaling 48,000μF. This dual-transformer setup minimizes crosstalk and keeps one channel from stealing current from the other during dynamic peaks. The bulk of the power supply accounts for much of the unit's 15.3 kg mass and underpins its 65W continuous output rating, which holds solid across the audible band with minimal sag.

3-Stage Darlington Output Stage

The main amplifier uses a 3-stage Darlington configuration for its output transistors. This multiplies current gain across cascaded transistor pairs and allows precise control of high output currents with minimal driver-stage demand. In practice that meant excellent drive capability and control over difficult speaker loads, reflected in the 60 damping factor at 8Ω. The Darlington topology is thermally sensitive, but it was a hallmark of high-current designs in the 1970s and contributed to the AU-607's muscular presentation.

First-Stage Dual FET Input

The preamplifier's input stage uses a newly developed dual FET (Field-Effect Transistor) matched for identical characteristics. FETs offer high input impedance and low noise, which suits delicate phono signals because the cartridge isn't loaded down. This choice helped achieve the 2.5mV sensitivity for MM cartridges and contributed to the exceptionally low noise floor, with a signal-to-noise ratio exceeding 115 dB. The dual configuration likely improved common-mode rejection and channel balance.

ASO and DC Protection Circuits

Given the risks of a DC-coupled output, Sansui included a comprehensive protection system. An ASO (Safe Operating Area) detection current limiter guards the output transistors from thermal runaway under heavy loads. A DC detection circuit monitors the output terminals for dangerous offset voltages. If either detects a fault, the amplifier disconnects the speakers, which preserved both the unit and the connected speakers. This dual-layer protection was advanced for an integrated amp in 1976.

32-Step Click-Stop Volume Control

A 32-step click type potentiometer with a positive click action manages the volume. It gives precise, repeatable level settings and avoids the scratchy fade-in/fade-out common in worn variable resistors. It isn't infinitely variable, but users appreciate the tactile feedback and the assurance that both channels are matched at every step. The front panel knob operates it smoothly.

Subsonic Filter and Loudness Compensation

The AU-607 has a dedicated subsonic filter, useful for eliminating infrasonic noise from warped records or ported speakers without affecting the audible bass. The loudness switch applies a bass boost at lower listening levels. The tone control section uses a differential amplification stage with a current source, contributing to low distortion and consistent performance across settings.

Historical Context

The AU-607 was developed under the theme of “creating new sound with a novel circuit that understands the essence of audio,” according to Audio Database. It sat within the “07 series” and shared design philosophies with other high-end models of the era, emphasizing measured performance and circuit purity. It was not the most powerful in the lineup, but its DC-coupled design and dual-transformer power supply placed it among the technically ambitious offerings of the mid-1970s Japanese hi-fi boom.

Collectibility & Value

Current listings show the AU-607 at around $699 for units in very good used condition. Its collectibility comes from its technical pedigree, solid build, and reputation for a clean, dynamic sound. Like all vintage electronics of this age, it is prone to aging components, particularly electrolytic capacitors in the power supply and signal path. Service kits for recapping are available and restoration guides exist, but a non-functional unit can quickly become a money pit. Only fully operational, well-maintained examples are worth the premium.

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