Sansui AU-D33
A 50-watt integrated amp with very low distortion, built to a budget.
Overview
Sansui's own service documentation dates the AU-D33 to November 1981. It carries the company's "Super Feedforward" circuit topology into the entry-level price bracket. Super Feedforward first appeared in the AU-D9, a 100W model that reviewers found competent but pricey. The AU-D33 is the same circuit technique pushed downmarket: 50 watts per channel into 8 ohms, rated from 20 to 20,000 Hz with total harmonic distortion held to no more than 0.004 percent. That distortion figure is the headline. At rated output, THD stays below 0.004 percent, and intermodulation distortion, measured by the SMPTE method with a 60 Hz to 7 kHz signal at a 4:1 ratio, comes in under 0.006 percent. For an amplifier at this price point those are low numbers. They're the kind of specs that show up in a service manual because technicians need them.
The frequency response looks good on paper too. From the AUX input, the amp runs DC to 300,000 Hz, +0 dB to -3.0 dB. That's not a typo. The AU-D33 passes DC. Whether your speakers or your sources can do anything useful with that bandwidth is another question, but it fits the DC-coupled design Sansui was pursuing at the time. The RIAA curve deviation for the phono stage, measured from 20 Hz to 100 kHz at the record output, stays within +0.2 dB and -0.5 dB. That's tight equalization.
The phono stage is unusually flexible for the era. There's a moving-magnet input at 2.5 mV sensitivity with 47 kilohms impedance, and a moving-coil input at 250 μV with 100 ohms impedance. The MM input can handle up to 170 mV before distortion exceeds 0.01 percent. The MC input tops out at 12 mV with distortion under 0.1 percent. Signal-to-noise ratios are 65 dB for MC, 84 dB for MM, and 105 dB for the line-level inputs. Channel separation measures 72 dB for MM and 90 dB for the line inputs, both at 1 kHz and rated power output. Damping factor is 40 into 8 ohms at 1 kHz with both channels driven. That's not spectacular, but it's adequate for most speakers of the period.
The controls are straightforward. Bass and treble each offer ±10 dB of adjustment, at 50 Hz and 10 kHz respectively. The loudness contour adds +8 dB at 100 Hz and +5 dB at 10 kHz. There's a high filter switch and a loudness switch on the accessory board, along with a 250 kΩ balance potentiometer. The service manual lists a subsonic filter and MC load switching as well, though the exact configuration varies by market.
Power consumption is listed at 210 watts, with a maximum rating of 300 watts. The amp measures 430 mm wide, 112 mm tall, and 334 mm deep, which is standard component width with a slim profile. Net weight is 7.3 kg, or 8.3 kg packed. It came in 100V, 120V, 220V, and 240V versions at 50/60 Hz, with North American units set for 120V at 60 Hz. The service manual includes bias and DC offset adjustment procedures. DC offset should be set to 0V ±20 mV at the speaker terminals. Bias current is adjusted to 4 mV between the emitters of the output transistors, measured at the test points on the F-3589 board. Any technician should check both on a unit this age.
Period reviews were positive. Audio-Collection (p.112) noted that the AU-D33 brought Super Feedforward to a lower price point, rated it at 50W per channel, and listed it at £140. The same publication (p.113) called the pickup equalization exceptionally accurate, within ±0.1 dB limits from 40 Hz to 20 kHz, which lines up with the manual's RIAA deviation spec. The reviewer also liked the build quality, noting it was a genuine "Made in Japan" model with a high-standard all-black finish. HiFI-Stereo (October 1982, p.50) published reference power measurements for the AU-D33 at 50W, confirming the rated output. The magazine's 1983 equipment listings (p.52) describe the control layout: concentric treble level and turnover controls, a record-out selector with tape-copy function, calibrated volume controls, and pushbutton switching for phono, tuner, AUX, and two tape loops.
The AU-D33 sits between Sansui's traditional amplifier designs and the more radical circuit approaches that would define the company's 1980s output. The Super Feedforward topology was meant to reduce distortion without the heavy negative feedback that conventional designs relied on. Whether it succeeded audibly is debatable. The measured distortion is low, but so was the distortion on plenty of well-designed conventional amps. The amp does offer moving-coil phono capability, DC-coupled response, and very low THD at a price that was competitive with the mid-fi competition.
For collectors, the AU-D33 is not the most sought-after Sansui integrated. It lacks the visual drama of the earlier AU series or the power of the AU-919. But it has a good phono stage. Units come up for sale regularly, often at modest prices. The main things to check are dirty switches, aging capacitors, and bias stability. The service manual's adjustment procedures are straightforward, and the amp is reasonably easy to work on.
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