Sansui AU-666

Thirty-three transistors, a capacitor-less output stage, and a microphone input - Sansui's compact control amplifier was built like a scaled-down broadcast console.

Overview

The AU-666 is a pre-main amplifier in the literal sense. Pull the connectors from the PRE OUTPUT and MAIN INPUT jacks and the preamp and power amp sections split apart, so the unit can work as two separate components in an electronic crossover system. Sansui's operating manual describes the procedure in detail. Feed the preamp output to an electronic crossover such as the CD-5, route the low-frequency band back into the AU-666's main input to drive woofers on the SYSTEM B terminals, and send the mid and high bands to outboard power amplifiers driving midrange and tweeters. A second pair of full-range speakers on SYSTEM A lets the owner A/B the crossover rig against a conventional stereo setup with a single switch.

The power section is a direct-coupled complementary design. The manual is explicit that the AU-666 uses direct-coupled complementary circuits in the power amplifier, with no output coupling capacitor in the signal path. Sansui rated it at 35 watts per channel continuous into 8 ohms and 45 watts per channel into 4 ohms, with music power (IHF) figures of 80W at 8 ohms and 100W at 4 ohms. Total harmonic distortion is quoted at less than 0.5% at rated output, and the power bandwidth spans 10 to 40,000 Hz at 8 ohms. Hum and noise is better than 100 dB by the IHF standard, and channel separation at 1 kHz rated output is better than 60 dB. A damping factor of 40 into 8 ohms suggests the output stage holds a reasonably firm grip on the speaker. Load impedance is specified from 4 to 16 ohms, so it will tolerate most speakers of its era.

The preamp side is just as serious. Phono 1 and 2 inputs are rated at 2mV into 50k ohms. The separate microphone input is rated at 3mV into 50k ohms, and the mic stage uses flat NF-type equalization rather than the RIAA curve, which hints at paging or semi-professional use. Tuner, AUX, and tape-play inputs sit at 180mV into 100k ohms, and the recording outputs are 180mV on pin jacks and 30mV on the DIN tape connector. Phono signal-to-noise is better than 70 dB, tuner and AUX better than 80 dB, and the preamp's own maximum output voltage is 5V with a rated output of 1V at less than 0.1% THD. Sansui's Triple Tone Controls adjust bass (±16 dB at 30 Hz), midrange (±5 dB at 1,500 Hz), and treble (±15 dB at 15,000 Hz). That was not common on integrated amplifiers of the period. Low and high filters cut 10 dB at 20 Hz and 20 kHz respectively, and the muting switch offers a -20 dB position.

Sansui's marketing for the AU series framed the whole line as a matter of matching the amplifier to the buyer's power needs and budget, with the flagship AU-999 at the top of the range. Studio-Sound (April 1972, p.11) ran a line-wide advertisement describing the AU-999 as a 180-watt control amplifier with direct-coupled circuitry, triple tone controls, separable pre- and power sections, high and low filters, and stereo balance check and muting circuits. The AU-666 sits well below that flagship in output, but it gets the same architecture - the separable sections, the triple tone stack, the filters, the muting - in a chassis that weighs 9.75 kg and measures 415 x 127 x 278 mm. Sansui's operating manual describes the AU-666 as featuring "the refined dull black panels common to all AU series professional control amplifiers."

The AU-666 is double-protected by quick-acting fuses at every power transistor stage and in the power circuit. The manual walks owners through the diagnostic sequence. If sound is distorted or absent, unplug the unit, check the speaker fuses on the rear panel, and replace them with the supplied 4-ampere fuses. If those are intact, pull the bonnet and check the internal fuses, replacing them with the supplied 3.5-ampere types. The manual is blunt that if fresh fuses blow the moment the power switch is turned on, the power circuit itself is suspect and the unit should go to a Sansui dealer or authorized service station. There are also three AC outlets on the rear panel - two switched (100VA total capacity) and one unswitched (150VA) - handy for a period system with a turntable and tape deck.

For anyone planning to run one today, the manual's voltage selector covers 100, 110, 117, 127, 220, 230, 240, and 250 volts. You set it by removing the nameplate on the rear panel and setting the arrow on the main voltage selector plug, with a sub-selector for red or white voltage markings. That helps with a unit that may have spent its life on a different mains standard. The manual also cautions against operating the amplifier in direct sun, near radiators, or in an airtight cabinet, and against placing anything on top of it. The case relies on the top and rear surfaces to diffuse heat, and transistors are, in Sansui's own words, extremely susceptible to heat.

The operating manual lists 33 transistors, 13 diodes, and 2 SCRs. Power consumption is rated at 240VA at maximum signal. The preamp and power amp sections are separated at the SYSTEM B and SYSTEM A+B speaker selector positions, which is the detail that makes the electronic-crossover trick work. Selecting SYSTEM B disconnects the internal preamp from the power amp so the external crossover chain can take over.

Collectors looking at an AU-666 today should go in with eyes open. It is a direct-coupled design with no output capacitor standing between a failing transistor and a set of speakers — the protection fuses are the line of defense, and they are only as good as the person who last checked them. The pre-main split, the triple tone controls, and the mic input make it a more interesting piece than a plain 35-watt integrated, but they also mean more switches, more jacks, and more places for decades of oxidation to accumulate. A unit that has been properly serviced and had its bias checked is likely to sound like what it is: a clean, low-distortion amplifier with a genuinely useful feature set. An untested one is a project.

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