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. Introduced in 1976, it represents a focused effort by Sansui to push the boundaries of integrated amplifier design during their peak era. Marketed as a "DC pre-main amplifier," it combines a fully discrete preamplifier section with a direct-coupled main amplifier, eliminating output capacitors to achieve a claimed frequency response extending from DC. Priced at ¥69,800 at launch, it targeted serious audiophiles who valued measured performance and circuit innovation over flashy cosmetics. Owners report it delivers on that promise with a clinical yet engaging sound, underpinned by robust build and thoughtful engineering—though its heft and appetite for power (185W consumption, resolved: 185W) demand respect from the outset.

This is no stripped-down design. The AU-607 integrates a full suite of features: 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. The preamplifier and main amplifier sections can be separated via internal links, allowing for future bi-amping or external amplification—a rare flexibility for an integrated unit of its time.

Specifications

Key Features

DC Amplifier Configuration with Direct-Coupled Output

The AU-607’s most defining trait is its DC amplifier design. By eliminating output coupling capacitors, Sansui achieved a frequency response that begins at true DC—reportedly 0 Hz—enabling exceptional transient response and low-frequency linearity. This wasn’t just theoretical; the circuit’s stability and lack of phase shift near the bass extreme gave it a taut, authoritative grip on speakers, especially noticeable with complex acoustic passages or tight electric bass. The trade-off, of course, 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 ensure channel independence, the AU-607 employs two separate power transformers—one for each channel—feeding a power supply with four 12,000μF capacitors, totaling 48,000μF. This dual-transformer setup minimizes crosstalk and prevents one channel from stealing current from the other during dynamic peaks. The sheer bulk of the power supply contributes significantly to 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 design multiplies current gain across cascaded transistor pairs, allowing precise control of high output currents with minimal driver-stage demand. In practice, this translated to excellent drive capability and control over difficult speaker loads, reflected in the 60 damping factor at 8Ω. The Darlington topology, while thermally sensitive, 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—ideal for amplifying delicate phono signals without loading the cartridge. 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. It features an ASO (Safe Operating Area) detection current limiter that guards the output transistors from thermal runaway under heavy loads, and a DC detection circuit that monitors the output terminals for dangerous offset voltages. If either fault is detected, the amplifier disconnects the speakers—a crucial safeguard that preserved both the unit and connected speakers. This dual-layer protection was advanced for an integrated amp in 1976.

32-Step Click-Stop Volume Control

The volume is managed by a 32-step click type potentiometer with a positive click action. This design ensures precise, repeatable level settings and eliminates the scratchy fade-in/fade-out common in worn variable resistors. While not infinitely variable, users appreciate the tactile feedback and the assurance that both channels are matched at every step. The volume control allows for smooth operation via the front panel knob.

Subsonic Filter and Loudness Compensation

The AU-607 includes a dedicated subsonic filter, useful for eliminating infrasonic noise from warped records or ported speakers without affecting the audible bass. Combined with the loudness switch—which applies a bass boost at lower listening levels—the amplifier offers practical tone-shaping tools. The tone control section itself 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. Positioned within the legendary “07 series,” it shared design philosophies with other high-end models of the era, emphasizing measured performance and circuit purity. While not the most powerful in the lineup, 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 commanding prices around $699 for units in very good used condition. Its collectibility stems from its technical pedigree, solid build, and reputation for a clean, dynamic sound. However, 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. Fully operational, well-maintained examples are the only ones worth the premium.

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