Sansui AU 6900 (1975–1977)

Sixty watts, a triple tone control with a midrange band, and a power supply split in two. It's the middle model of Sansui's 1975-77 integrated line, and it follows its big brother's playbook.

Overview

The AU 6900 is the middle of a three-model family - the AU 5900, AU 6900, and AU 7900 - that Sansui built from a shared design and a shared service manual. It's a pre-main amplifier, rated at 60W + 60W into both 4Ω and 8Ω from 20 Hz to 20 kHz. Total harmonic distortion is no more than 0.1% at rated output and 0.08% or less at half output. That looks conservative until you read the rest of the spec sheet. Power bandwidth is quoted at 5 Hz to 40 kHz into 8Ω, and frequency response at one watt runs 10 Hz to 50 kHz, +0/−1.5 dB. This is not a receiver and not a stripped-down integrated. It's a full preamp and a full power amp in one chassis, and a switch on the back panel separates the two sections.

Sansui's own literature describes it as "a pre-main amplifier developed based on the same concept as AU 7900," and the family resemblance runs deep. Russian service documentation notes that the AU 6900's circuit is "exactly the same as in the 5900 down to the ratings," with a 3×2 output stage. So the 6900 sits between the smaller 5900 and the larger 7900 in power and shares topology with both. The service manual is printed as a single document covering all three.

The original price was ¥65,000, around 1976. Production ran 1975-77. HiFi Engine called it a "medium-price/medium-power" amplifier, but the build includes a 15,000 µF × 2 filter capacitor bank, a dedicated power supply for the pre-drive stage, and a protection circuit that watches for DC at the output terminals, abnormal temperature, and safe operating area violations. That's a lot of work for the word "medium."

The AU 6900 is not the rarest Sansui, not the most powerful, and not the most sought-after. It is one of the more interesting mid-1970s integrateds from the company's golden era, though, and the design choices inside it are worth a careful look.

Key Features

Pure Complementary, Directly Coupled, All the Way Through

The power amplifier section uses a pure complementary circuit directly connected to all stages. There are no coupling capacitors in the signal path, which is why the response extends so far down (5 Hz power bandwidth, 10 Hz at 1W). The first stage uses dual transistors, and the drive stage is a Darlington three-stage configuration with an active load. The output stage is parallel push-pull, and the AU 6900 combines four power transistors per channel, matched for characteristics, to improve current linearity. That gives the quoted 0.1% THD at rated output and 0.08% at half output. It's not spectacular by late-1970s standards, but it's consistent across the power band.

Split Power Supply for the Pre-Drive Stage

One of the more consequential design choices in the AU 6900 is that the power supply for the pre-drive stage is separated from the supply for the output stage. The power supply section itself uses a large transformer, silicon diode rectification, and a 15,000 µF × 2 capacitor bank — but that big reservoir feeds the output stage, while a separate regulated supply handles everything else. Sansui's description notes that this "large power supply unit supplies a stabilized power supply to all block stages except for the power stage and drive stage." A ±2 supply with a total high voltage of 51V feeds the pre-drive stages with enough margin to keep them well clear of clipping.

NF-Type Phono Equalizer with Switchable Impedance

The phono stage is an NF-type equalizer circuit built around four low-noise transistors, with a first-stage differential amplifier chosen for good DC balance. RIAA deviation is quoted at ±0.3 dB from 30 Hz to 15 kHz. The Phono 1 input allows three-step impedance switching (30kΩ, 50kΩ, 100kΩ), which is unusual at this price point. It covers a wider range of cartridges than the fixed 50kΩ of most integrateds. Phono 2 is fixed at 2.5mV/50kΩ. Maximum allowable phono input is 250mV at 1 kHz with THD under 0.1%, which is enough headroom for most moving-magnet cartridges of the period.

Triple Tone Control with Selectable Turnover

Sansui's "triple tone control" adds a midrange band to the usual bass and treble: bass ±13 dB at 50 Hz, midrange ±5 dB at 1.5 kHz, treble ±13 dB at 15 kHz. The tone selector shifts the turnover points, bass between 600 Hz and 300 Hz and treble between 2 kHz and 4 kHz, so the controls can be voiced for different speakers or recordings. There's also a tone defeat function. When it's engaged, the tone control and filter circuits are bypassed entirely, and the signal from the buffer amplifier after the volume control goes straight to the main amplifier.

Protection Circuitry and Chassis-Less Construction

Three protection systems are on board: an output-terminal DC detection circuit combining an electronic circuit and a relay, an abnormal temperature detection circuit, and an ASO (safe operating area) current limiter. The chassis is a chassis-less structure. The power transformer is mounted independently, and the grounding is a one-point system. Both are intended to reduce the deterioration of sound quality from circulating ground currents. The input terminals on the back are mounted facing upward, which is unusual but practical for reaching them from above in a rack. A spare AC outlet is provided, along with loudness, low filter (−3 dB at 70 Hz, 6 dB/oct.), high filter (−3 dB at 7 kHz, 6 dB/oct.), muting (0 dB or −20 dB), and a stereo/mono mode selector.

Historical Context

The AU 6900 was built from 1975 to 1977 and sold for ¥65,000 around 1976. Sansui positioned it as a pre-main amplifier developed on the same concept as the AU 7900. The shared service manual, covering the AU 5900, AU 6900, and AU 7900 in one document, makes the family relationship explicit. Russian service documentation describes the 6900's circuit as identical to the 5900's down to the component ratings, with a 3×2 output stage. The 6900 simply runs at higher power. HiFi Engine frames the model as "medium-price/medium-power," with "the technical performance and tone quality you expect of Sansui equipment in this medium-price/medium-power" class. No AU 6900-specific history beyond this has been located in available documentation.

Collectibility & Value

Market data for the AU 6900 is thin but real. A 2025 Kijiji listing in Ontario offered a fully rebuilt unit, described as a "$1200 complete rebuild by real Sansui Tech in 2025," at a firm $925 CAD or $700 US. A StereoNet classified listed one at AUD $950 (now expired). A Russian Avito listing asked 37,000 ₽. On the parts side, a "Sansui AU-6900 restoration kit filter capacitor repair rebuild fix" sold for $167 in August 2022. The service manual is 20 pages, English, PDF. eBay listings group it with the AU 5900 and AU 7900 as a single document.

Owner reports are mixed, which fits a 45-to-50-year-old amplifier. One diyaudio.com owner rescued a unit from the trash, powered it up, and got a front-panel light but no sound. That's a common failure mode for any amplifier of this age that hasn't been serviced. Another owner on a Russian forum paired one with Pioneer HPM-70 speakers and a DAC and reported being happy with it. A cassettevintagee.com write-up calls it "a great option for music lovers who appreciate vintage design, rich sound quality, and durability," noting its "versatile input options and cost-effectiveness." No specific known-failure list from technicians has been published. Given the restoration kit and the age of the unit, a full recap and a check of the protection relay are the sensible starting point for any long-dormant example.

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