Originally printed in the July 1994 issue of Gramophone.

I'm sure you will need no reminding that one of the best regarded British amplifier manufacturers of the late-1960s and 1970s was J. E. Sudgen of Cleckheaton — not to be confused with A. R. Sugden of Brighouse, manufacturer of the Connoisseur turntables. It was Jim Sugden who, in 1967—"appalled at the sound the industry was offering as 'hi-fi", as his adverts put it — launched the first all-transistor integrated Class A amplifier to be seen on these shores, the A21. This he followed up with the C51/A51 pre/power combination and then, in 1972, the A48, a more powerful, Class AB, fully complementary development of the A2 I.

Their popularity notwithstanding, though, even by the mid-1970s the capacitor-coupled Sugdens were beginning to look old-hat. This was the era when split-rail, direct-coupled amplifiers were taking their first hesitant steps towards the ubiquity they enjoy today, and for capacitor coupling the writing was already on the wall. Although Sugden, which was acquired by Milbucon Holdings Limited in 1981 and is now based in Heckmondwike, still offers an A21 A—a rather more elaborate version of the original A21— alongside its modern designs, for most audiophiles the classic, capacitor-coupled, Sugdens are redolent of a bygone era.

All of which thin slice of audio history will seem utterly irrelevant to this review of a new British linelevel integrated amplifier from Audio Innovations until I tell you that, spiritually and electronically, it is being spoken of as a Sugden reincarnate. Single-rail, quasi-complementary, capacitor-coupled amplifiers are supposed to be museum pieces--yet here is one with a 1994 vintage.

That the Alto should have come from one of Britain's most energetic and successful promoters of valve amplification is, in the circumstances, doubly appropriate. Selling what some will dismiss as a product of the nostalgia industry is business as usual for Audio Innovations, while inserting a large electrolytic capacitor in the amplifier's output feed can hold no terrors for a company habituated to putting an output transformer in the same position.

Not that designers Guy Sergeant and Richard Stockley set out to make a retro-tech statement with the Alto. Its £300 target price precluded the use of valves, so for the company's first solid-state product they began with a clean sheet of paper and started the development cycle, naturally enough, using up-to-date design techniques. But none of the modern circuit topologies they tried provided the sound quality they were after. Only when, in frustration, they regressed to circuits of a previous era did they achieve a result they considered consistent with the Audio Innovations name.

For those unfamiliar with amplifier terminology, to whom terms like split-rail and quasi-complementary are just so much meaningless jargon, some brief words of technical explanation are in order. Unlike valves, which are inherently unipolar devices (able to conduct current in one direction only), transistors are by nature bipolar. According to the arrangement of semiconductor materials within it, Specification a transistor can be designed to conduct current in either direction. In the case of BJTs (bipolar junction transistors) the two different polarities are denoted NPN and PNP.

Designing NPN and PNP devices to have mirror-image operating characteristics generates a so-called complementary pair. Most modern solid-state amplifiers use complementary transistor pairs in their output stages, slung between equipotential positive and negative ('split') voltage rails. This allows the output point to be held at zero volts DC, permitting the loudspeaker to be directly coupled to the amplifier without the interposition of any DC-blocking component.

In the early years of transistor production, however, complementary pairs with sufficient current capability for driving loudspeakers were not available. Only NPN devices of suitable characteristics were to be had, forcing amplifier designers to contrive so-called quasi-complementary output stages in which two NPNs are used rather than an NPNIPNP pair.

With high-current complementary pairs now readily and cheaply available, quasi-complementary output symmetry is rarely seen today, although a few amplifier designers—notably Julian Vereker of Naim Audio— continue to prefer it on the basis that so-called complementary NPN and PNP devices have operating character statistics which are, in reality, only nominally equivalent.

With the Alto, Audio Innovations has also chosen the unfashionable quasi-complementary alternative, and then gone a further step back in time by employing a single voltage rail rather than split (positive and negative) supplies. This means that a large DC offset is present at the output point, which has to be prevented from reaching the loudspeakers by a high-value electrolytic capacitor. Hence the term capacitor-coupled, and the likening of the Alto to the old Jim Sugden designs.

One advantage of choosing this venerable electronic architecture is that it facilitates a circuit of rare simplicity. From input to output each channel of the Alto contains only seven transistors. Tone controls are eschewed as an unnecessary complication, as also is a balance control. However, there is a record-out selector which allows one source lobe taped while listening to another, or dubbing between two tape machines. Apart from that, the only front panel controls are the input selector, volume control and mains on/off switch (with a fetching blue 'telltale').

All six inputs are line-level, meaning there is no provision for direct connection of a record player. If you want to play vinyl discs it will be necessary to use a separate RIAA-equalized step-up device, of which there is an increasing selection becoming available.

Concentration on the Alto's innards means I have yet to mention what, on first acquaintance, is its most striking feature—the 'narrowed eyes' fascia and casework styling. For all its championing of bygone technologies, Audio Innovations has always wrapped its circuits in eye-catching, modern enclosures. Some of them, arguably, have been rather too self-consciously 'different', but the Alto's strikes me as a great success—arresting yet elegant. Admittedly the unusual form precludes the use of external heatsinks, but since the Alto operates in Class AB rather than Class A this is no problem. Internal heatsinks and vents in the casework provide all the heat dissipation necessary.

Performance

For the listening I partnered the Alto with Mission 760iSE loudspeakers and a Kenwood DP-X901 0/Audiolab 8000DAC two-box CD source. The loudspeakers were selected purely on the basis that I know and enjoy them, but the fact that many Audio Innovations dealers are selling the diminutive Missions in combination with the Alto made them a doubly apposite choice.

You don't have to listen long to the Alto to realise that it is an exceptional amplifier for the money. What distinguishes it is a rare ability—one which eludes many amplifiers, whatever their price tag—to project a wide, deep sound-stage and populate it with musicians who perform with both naturalness and vitality.

Nowhere are its strengths better appreciated than on simply miked recordings with well developed ambience portrayal. London Pro Musica's A Florentine Carnival (IMP CD PCD825, 2/87), for example—a sparkling collection of Fifteenth Century songs and instrumental pieces, masterfully recorded by Antony Howell—sprang into life via the Alto with a warmth and image scale which left even a highly regarded competitor sounding somewhat pinched and undernourished by comparison.

On pop and rock music the Alto has been criticised in some quarters for a lack of rhythmical coherence in the bass register—a characteristic it is tempting to ascribe to the capacitor coupling—but on classical programme it is mostly in its element. It conveys the character of both the musical performance and the recording venue with a simple honesty which—whatever the supposed superiority of their more modern circuit topologies—most competitors struggle to emulate.

Occasionally the Alto can sound a little less well resolved, a little fuzzier than its more overtly 'hi-fl' peers, but the longer you listen the less significant this shortfall seems. Across a broad selection of music it's the consistent warmth, weight and three dimensionality of the Alto's sound which set it apart, and which make it such a satisfying partner longterm.

In my personal affections the Alto now rates as the best amplifier I know at the price. If your musical diet includes pop and rock music then you should listen out for its bass limitations, but otherwise I can recommend it wholeheartedly. If you are looking for an amplifier at this price level, be very certain to sample it.

Keith Howard