Over the past two decades, Unison Research has built itself an impressive reputation as a maker of fine quality valve amplification at realistic prices. Electronics lecturer Professor Gianni Sacchetti, a valve/hi-fi enthusiast who is still the main man behind the valve amp designs, originally started the Unison operation.

Subsequently, the business has been expanded by Giovanni Nasta, who is responsible for adding the Opera loudspeaker brand to the operation. Nasta also been busy extending the electronics activity with solid state and hybrid products under the Unico banner.

This £3,495 P70 integrated line-level stereo amplifier joins an already extensive range of more than a dozen different valve amps, but its addition to the collection is clearly justified on both styling and engineering grounds. For a power output of 70 watts per channel, it's a quite compact affair, though undeniably hefty thanks to the generously rated mains and output transformers.

Elegant Fascia

While the lower portion of the fascia is made from an elegant piece of hardwood in the Unison tradition, the top half is a thick piece of shaped and coloured semi-transparent Murano glass. It's a clever styling solution that not only looks very attractive but also emphasises the company's geographical proximity to Venice and its offshore island Murano, famous for high-class glassmaking since medieval times.

Very much a purist designer, Sacchetti has concentrated exclusively on single-ended designs for the past decade, but his latest creation, the P70, marks a break with that tradition by adopting the more popular push-pull approach.

The differences between the single-ended and push-pull approaches to valve amp design have long been the subject of debate and some controversy amongst valve amp cognoscenti. Without getting to deeply into the technicalities, although it's often claimed that the single-ended approach gives the sweeter sound, a push-pull design offers greater power output and superior efficiency from similar ingredients.

Compare this amp with Unison's single-ended Performance. The P70 delivers 2x 70 watts from four KT88 output valves, in a package weighing 35kg and costing £3,495, whereas the much larger Performance delivers just 40 watts per channel from six KT88s, weighs 50kg and costs £5,500.

Furthermore, as Sacchetti points out in his interview, the symmetry of a push-pull arrangement allows for fully balanced operation, which is not possible with a single-ended design. Whether that makes the P70 more desirable is debatable and beyond the scope of this review.

The short version is that balanced operation is almost universal in professional audio applications, because it avoids hum and noise problems with the long cable runs that are often found in complex studio applications. However, purist audio critics point out that balanced operation represents an unnecessary complication. While it has yet to become a popular mode of operation for hi-fi applications here in Britain, balanced connections are widely found and used in American high-end hi-fi equipment.

Reflecting the company's new enthusiasm for balanced operation, the four line-level inputs and record output are available with either balanced XLR sockets or single-ended phonos. The remaining feature count is rather limited, omitting potentially useful options like mono switching, muting and balance adjustment, or indeed any form of tone control.

Happily, however (in theory at least, though not for some reason on our pre-production sample), a remote control is available for volume and input selection via a classy wooden 'system' handset that an conveniently be stood upright on its 'heel' for easy one-hand access and operation. Happily too, the three metal knobs that adorn the front (which will be rather smaller in production models) cover these key basic functions.

A single pair of speaker output terminals serves each channel, optimised for six ohm loads. A special rear panel socket can supply the requisite power for Unison's matching PhonoOne or Simply Phono outboard vinyl stages (not supplied).

Internally constructed as a double-mono amplifier throughout, the input stage for each channel features two ECC83 double triodes and one ECC82 double triode, prior to the twin KT88 power output.

Unison's custom designed output transformers are claimed to be able to deliver full power at 30kHz, while proprietary circuitry holds harmonic distortion below 0.2 per cent. Output valve bias is automatically and accurately controlled and stabilised under all conditions, even from cold or if the valves are changed.

Sound quality

The most impressive thing about Unison's Performance amplifier was that it managed to provide the advantages of valve operation without the usual corresponding drawbacks. Much the same was true for this rather more powerful and compact but less costly P70.

The characteristic strength of any valve amplifier usually lies in the sweetness, transparency and dynamic expression of its midband, and that was immediately apparent here, adding previously unsuspected depth perspectives to familiar recordings, and emphasising the precision of the wide dynamic window through very superior rendition of low level ambience and instrumental decay and sustain.

Recorded voices sounded particularly persuasive and human, with notably delicate phrasing and inflexion, but poor quality sources were also laid bare. Unfamiliar coloration seemed audible at one point when listening to speech from Radio 4, but when the programme changed it quickly became apparent that the source of the coloration had been at the broadcast studio end of things, rather than in the reproduction chain.

If Radio 4 sometimes proved to be disappointing, Radio 3 was usually very satisfactory (in part because of the skill and sensitivity with which BBC engineers place microphones when recording orchestral music) and because a good quality valve amplifier is particularly well suited to replaying the natural instruments of a symphony orchestra - with its complex and subtle textures.

Since my Magnum Dynalab MD106T tuner is the only source I use with balanced outputs, this was an opportunity to check the balanced connection and while it worked perfectly well, it didn't seem in any way superior to the alternative single-ended connection.

Even when used in combination with B&W's highly analytical diamond tweeter and Rega's similarly transparent Apheta phono cartridge, both of which are on the bright, but extra clean side of neutral, the top end sounds notably sweet, delicate and open. There's no apparent roll off or loss of air and the sweetness of the source and the tweeters is fully maintained, despite the insertion of a solid state phono stage in the chain.

The bass end is often a weakness of valve amplifiers and the P70 does fall a little short of the best solid state models in terms of tautness, tightness and consistency. But across a wide range of material it's only caught out by the occasional track. For the most part the bass is clean, quick, lively and informative, just the way it ought to be, but in 'valve land' often isn't.

That's probably the most serious criticism of an otherwise impressively neutral and transparent valve amp. The styling is attractive and quirkily original, the unit is reasonably compact and there's enough power to satisfy the majority of users and systems.