Sansui AU-D33 Integrated Amplifier

18/01/2023

When the Sansui AU-D33 integrated amplifier was launched in 1982, it had a lot to live up to. Its predecessor, the AU-317II from 1980 [HFN Jun '15], delivered the sort of performance one would expect from a manufacturer of specialist hi-fi, thanks to its well engineered DC-coupled circuitry. The company's 'All hi-fi, everything hi-fi' slogan set out a clear manifesto - no transistor radios, no coffee machines, just quality audio products.

Forward Thinking

The turn of the decade had not been kind to Japanese manufacturers. The revaluation of the Yen had rendered their products more expensive in key export markets, such as the UK. It was therefore necessary for designers to find ways of making equipment that cut costs without sacrificing performance, and this ushered in the use of many new technologies.

It was distortion that Sansui chose to attack with its new range of models. This was a logical step as there was little that could be added in the way of useful extra features, while power levels were already more than adequate to meet the expectations of the domestic listener. Distortion was largely an academic issue in the late 1970s. The LP and tape sources then available contributed to it far more than did any amplifier with genuine hi-fi pretensions, while loudspeakers were in many ways the bottleneck they remain today. However, distortion was a useful figure for a 'number war' - something all those marketing technical products loved at the time. The AU-317II had a rated THD figure of 0.03%, which made it all but transparent in the context of a complete system of the day. Against this, though, the AU-D33 claimed an overall figure of 0.004%, which could be sold as a near tenfold improvement.

This figure did not arise from the careful revision of existing circuitry, however. Rather, a topology completely new to the Sansui range was introduced. As it added no extra controls or indicators to the amp in question, and was invisible to anyone not tempted to study circuit diagrams, a snappy name was needed to describe it. Enter the 'Super Feedforward System'.

Split The Difference

The first amplifiers to feature the Super Feedforward System were the AU-D9 and AU-D11 of 1981. They replaced the AU-D717 and AU-D919, which were well regarded but conventional designs. As with the majority of amplifier innovations of this era, the Super Feedforward System aimed to reduce distortion under dynamic conditions and not the steady-state modes of operation found only in the test lab. It worked in tandem with a normal negative feedback arrangement to ensure the amp's output stage could always drive the loudspeaker load as accurately as possible.

Negative feedback is achieved by comparing the output signal of an amplifier circuit to its input. Ignoring the effects of gain, any difference between the two can be assumed to be the result of non-linearity in the circuit - the root cause of distortion. The clever part is that by subtracting one signal from the other it is possible to largely cancel the effect, so improving the performance of the system as a whole. The problems with negative feedback come when the amplifier is asked to do too much. Large corrections, especially at high frequencies, absorb substantial amounts of power and headroom to accomplish. In a practical amp this limits the degree to which negative feedback can safely be applied.

Model Behaviour

Sansui's answer was to sample the input to the hard working power stages of the amp at a point where both the programme signal and additional corrections to counteract distortion were present. A model of the amplifier's distortion could be derived by subtracting the programme signal, and this model was then inverted and amplified by what Sansui called a 'correction amplifier'. By summing the output of this with that of the main amp via a network of passive components at the complete amplifier's output, the work of driving the speakers was shared.

Furthermore, since the overall result of the two circuits working together was sent back into the negative feedback system, the requirement to make corrections fell, improving the performance yet further. The system worked best at high frequencies, so the passive summing network was engineered to favour low frequencies from the main amplifier and higher frequencies from the correction amplifier. This meant the latter did not need to be as substantial as the former, making the system as a whole reasonably cost effective to implement.

A Nod To CD

The AU-D9 and AU-D11 were large and expensive models, designed as top-of-the-range flagships for the wealthier buyer. The AU-D33 was designed to have a wider popular appeal by being smaller, simpler and cheaper. At £139 it didn't cost much more than a NAD 3020 [HFN Nov '12]. The same concepts of the Super Feedforward System remained, but to aid circuit economy the tone controls were included in the feedback loop so that they altered the main amplifier's characteristics directly.

Power was also reduced, from 120W (AU-D11) to 50W (AU-D33). This did not mean that the AU-D33 was a stripped-out, poverty-spec model, for it still retained quality features such as a built-in MM/MC phono stage and a recording output selector, which operated independently from the main input switches. Some versions even had the auxiliary input labelled 'DA' (Digital Audio) as a nod to CD.

Sensible Load

The traditional Sansui livery is black, black and more black. The AU-D33 breaks this tradition with a restrained use of colours across its fascia. Two schemes were offered - natural aluminium with a blue line or matt black with red, with most bought in the latter format. Neat LED indicators in red, yellow and green complete the effect, with quality knobs used for the major controls.

Everything one would want is there, including the facility to defeat the tone controls and limit the range of the volume control by 20dB. Loudness compensation is also available, although it is unlikely to find much use in system of well-matched components.

Meanwhile, the turntable inputs have sensibly chosen fixed loading at the standard figures of 47kohm (MM) and 100ohm (MC). While not quite as solidly built as the AU-317II, the AU-D33 still feels both well made and sturdy, with a typically Japanese high standard of fit and finish.

A little bit of drama is added to the switching-on procedure as the power indicator (marked 'protector') flashes until the protection circuit is ready to close the relay which connects the speakers to the amplifier. Something else I appreciated was the sensibly limited range of the tone controls. They are ideal for making minor corrections to poorly produced material but won't generate a thumping bassline or searing treble where none is supposed to be.

Tim listens

Initial impressions of the AU-D33 revealed no obvious flaws. If this appears to be damning the amplifier with faint praise, previous experience with designs that place networks of passive components between output transistors and the outside world caused me to be wary, as these can sometimes sound overly laid-back. Yet the dynamic, lively 'D33 seemed free of this.

That said, the presentation was a little on the tame side when compared with that of the raucous AU-317II. Still, the '317 may be too up-front for some tastes, and the AU-D33 displays some welcome smoothness in its character. In motorcycle terms it is still more of a Kawasaki than a Honda (ie, visceral rather than considered) but it still has a sound I believe the majority of listeners could embrace and enjoy.

Whether or not this is down to the Super Feedforward System it is difficult to say, but the trade-off seemed to be a marginal reduction in bass 'punch' when the two models were compared directly, even when the AU-317II's extra 10W of claimed output was taken into consideration. Noise performance was excellent, there being none audible from the listening position at any setting of the volume control when using the line-level inputs.

New Order's Technique [FACD 275-2] revealed that the AU-D33's calling card is its busy, lively treble. Although well rounded by the standards of previous Sausui designs, it still dominates the listening experience. This, and plenty of power, make playing tracks such as 'Fine Time' and 'Vanishing Point' an invigorating experience, provided your neighbours are forgiving and your speakers are up to it.

This is, in many ways, the archetypal 'bedsit PA' amplifier; not too expensive but lots of go. The bass is powerful but just sometimes becomes poorly focused, a few combinations of sounds seeing it trip over itself and descend into brief passages of muddle. With sympathetic material, however, the low notes have a tight, stop-start quality about them which makes the AU-D33 sound clean and modern.

Love Affair

Gentler styles of music showed the AU-D33 to be able to separate out the layers in complex recordings. Sade's 'Frankie's First Affair', from her debut album Diamond Life [Epic 26044] was neatly dissected, with the percussion sounding especially vivid and the sax delivered with a delicious rasp. If there was a niggle it was that I've heard other amps do better with vocal textures. Adu's voice was full and rendered with a convincing centre presence, but the presentation was a little monochromatic and recessed compared with how it can sound via amps with best-in-class performance.

Nonetheless, the AU-D33 is a decent-sounding amplifier which does everything one could want of it. What's more, it looks good and is solidly made. For those in the market for an early '80s middleweight, it deserves to be on the shortlist.

Buying Secondhand

Despite the doubling up of some of the circuit functions that the amp's Super Feedforward System makes necessary, the AU-D33 has certainly proved reliable in service. Fault patterns are similar to those of any similar product, with the loudspeaker muting relay perhaps being the component to cause the most trouble.

Setting up the DC offset of the output stage can also be tricky since the present controls have an unusually abrupt action and this, combined with the hair-trigger sensitivity of the electronic protection system, is the usual cause of the sound dropping out suddenly at odd occasions. One can afford to be selective when buying as the AU-D33 was a highly popular model that lasted well and there are still plenty of them to be found.

Hi-Fi News Verdict

The Super Feedforward System used here means the AU-D33 offers something genuinely different to the typical Japanese amplifier of the period. Although toned down, the Sansui character is still present in spades, meaning the amp boasts a muscular and fast-paced sound. Easy to build a system around, attractive and well made, the AU-D33 should appeal to experienced and new listeners to vintage hi-fi alike.

Sansui AU-D33 Integrated Amplifier Lab Report

While Sansui was not making any particular assertions about the AU-D33's speaker-driving ability, its 'Super Feedforward' topology did make a claim for very low distortion. The latter was realised in practice with a midband THD of 0.0005-0.001% over the full 50W range specified for this integrated amplifier. There is some suggestion of crossover distortion at very low power - the minimum THD being achieved at ~10W/8ohm - but even at high frequencies the AU-D33 holds true to just ~0.004%/20kHz [see Graph 2, below]. CCIR intermodulation distortion is also a very low 0.0002% although stereo separation begins to 'close up' to around 40dB at 20kHz, possibly through capacitive coupling between tracks/wiring on the PCB. Overall gain is a sensibly modest +39.8dB - lower than the excessive 45dB+ of many integrateds from this era - requiring 205mV to raise the rated 50W/8ohm from the output. The A-wtd S/N ratio is bang-on 100.0dB at this output (and 83.0dB re. 0dBW).

The frequency response is very extended, so while the -0.25dB bass shelf (2Hz-200Hz) was unexpected, the treble rolls away to an imperceptible -0.2dB/20kHz and then to -4.7dB/100kHz. The moderate 0.15-0.20ohm output impedance will give rise to further shifts in balance when paired with low/variable-impedance loudspeakers - the latter are not especially recommended if only because the AU-D33 is current-limited to just 4.5A, achieving an underwhelming 32W/2ohm and 18W/1ohm. Into 'easier' loudspeaker loads the AU-D33 fares well enough, besting its rated 50W with 2x70W/8ohm and 2x55W/4ohm, and with sufficient headroom into these higher impedances to support 103W/8ohm [see Graph 1, below]. PM

Power output (<1% THD, 8/4ohm) 70W / 55W
Dynamic power (<1% THD, 8/4/2/1ohm) 103W / 57W / 32W / 18W
Output imp. (20Hz-20kHz/100kHz) 0.149-0.20ohm / 0.60ohm
Freq. resp. (20Hz-20kHz/100kHz, 0dBW) -0.26dB to -0.20dB/-4.7dB
Input sensitivity (for 0dBW/50W) 30mV / 210mV
A-wtd S/N ratio (re. 0dBW/50W) 83.0dB / 100.0dB
Distortion (20Hz-20kHz, 10W/8ohm) 0.0005-0.004%
Power consumption (Idle/rated output) 28W / 229W
Dimensions (WHD) / Weight 430x112x334mm / 7.3kg

Review: Tim Jarman, Lab: Paul Miller

Source: www.hifinews.com