Audio Note OTO

This article written by Malcolm Steward was first published in Audiophile magazine (UK) 1994
Audio Note oto

This is less of a review, more a look at an alternative way of life. Many of us are using muscular amplifiers to drive speakers that are reluctant to stir without a truckload of Watts encouraging them. Audio Note’s Peter Qvortrup thinks this is not a sensible proposition. He says we should use efficient loudspeakers – 90dB or more – driven with lower powered, valve amplifiers, such as the Audio Note Oto.

There are two versions of the Oto integrated valve amplifier. One has a regular push-pull output stage, the other is single-ended. Neither is priced like an average integrated amplifier: the standard Oto is £1250, the SE version £1499.

Qvortrup countered my question why anyone would spend the sort of money that would buy them a respectable 100-Watt pre/power combination on a 12-Watt integrated valve amplifier by explaining that the valve and transistor amplifier camps view things very differently: valve aficionados see nothing unusual in using such an amplifier. Qvortrup is committed to valves because he says he has yet to hear a transistor amplifier that presents natural instrumentation, especially strings and woodwind, in a manner that he considers harmonically correct. He believes that transistors are simply inherently inferior in this vital – to him – respect. He justifies integrated amplifiers by saying that, all else being equal, an integrated amplifier should outperform a similarly-priced two-boxer because the operation and interaction of the various amplifier stages is wholly predictable and controlled in an integrated design. I don’t think that’s the whole story but I’ll admit that it’s a powerful argument against mixed marriages of pre-amplifiers and power amplifiers from different manufacturers.

There are two versions of the Oto, says Qvortrup, simply to prove a point: to teach the market that single ended operation, which was discarded in the ‘thirties, is a worthwhile technology. His experience, contrary to received wisdom, suggests that the Oto SE drives a wider range of speakers satisfactorily than the push-pull version. It measures worse and is – on paper – less powerful, but it sounds bigger and better.

Both Otos use the same chassis, controls, wiring, valves, phono stage and circuit boards, with the exception of the output board. The main differences between them are their output and line pre-amplifier stages. In the SE, the latter is independent and powered by the pre-amplifier power supply unit. In the push-pull Oto the line pre-amp is integrated into the power amplifier stage and is powered by the power amp psu. The push-pull’s stage incorporates phase splitting but this circuit is reconfigured in the SE amplifier as a straightforward gain stage. The amplifiers’ power supplies differ, too, reflecting the demands placed upon them by the different circuit topologies: single-ended working loads the power supply more heavily than push-pull operation.

The power amplifier output stages are radically different: in the push-pull version separate amplifiers handle the positive and negative halves of the signal while in the SE one amplifier handles both. The Oto SE’s output transformer is about three times the size of the push-pull Oto’s. This is to prevent it saturating as it blocks the standing direct current that exists at the single-ended amplifier’s output.

Having compared the Otos (see separate panel) and emerged in complete agreement with Qvortrup’s preference for the SE version, I compared it to a similarly priced transistorised pre-power, the Heybrook C3/P3 combination. I used both with a Naim CDS CD player and a variety of non-stressful loudspeakers, including the 90dB efficient Pro Monitor TB1s also reviewed in this issue, to see if Qvortrup’s promotion of low powered integrateds was valid.

I’d venture that it is, but not under all circumstances. For example, I enjoyed the Oto SE’s rendition of The Pogues predominantly acoustic Sitting On Top Of The World, which sounded particularly realistic in terms of vocal and timbral quality. However, at the levels I thought appropriate for the music the Heybrook was the more persuasive amplifier. The Oto reached the point where it began to sound less graceful far sooner than the Heybrook. The latter also had a tighter grip on music’s lower octaves, which I found better conveyed rock music’s timing and rhythms.

The Heybrook was definitely preferred with Rage Against The Machine’s Bullet In The Head; the Oto sounded uncomfortable, as though it was working much harder than it wanted to. It was, however, distinctly more persuasive, and cosmetically more alluring than the Heybrook, with the Concert Under The Dome recording of Boccherini’s Sonata in A. Storming heavy metal rap was not its forte: piano and cello were.

Ultimately I agree with Qvortrup over loudspeaker efficiency but I still think the all-round solution is efficient speakers used with reasonably big power amplifiers. The Oto strikes me as an excellent niche product but not one for the mainstream.

OTO -vs- OTO

I started comparing the Otos by listening to Tim Simenon’s remix of Björk and David Arnold’s Play Dead, a tasteful, powerful melange of synthesized and orchestral music underpinned with ferocious bass lines. The standard, push-pull Oto was enjoyable to audition but musically not quite as persuasive as the single ended version. The standard amplifier captured the depth of the track’s bass line but didn’t convey its impact or timbre especially well. The SE opened up the lowest octave more successfully, revealing how low the line plummeted, the weight behind it and the fabric of the individual notes it contained. The SE’s bass line was properly visceral and made the standard amplifier’s treatment sound like a fair but pale imitation. On the twelve-inch remix of the song, where the bass line is punchier and funkier, the Oto SE was clearly superior. It better separated the line from the orchestral playing, which made its contribution to the track’s rhythmic flow far more effective. It also opened up the treble extreme more explicitly. Percussion strikes and horn stabs had more dynamism and greater timbral realism.

The Oto SE’s warm, full-blooded presentation of Björk’s voice displayed more authority and solidity. Although the SE lost none of the standard amp’s feeling of delicacy or spaciousness, its presentation was markedly more powerful and inviting. The way in which it gradated orchestral dynamics, for example, both major changes in overall amplitude and minor level differences in the strings, was more convincing. Both amplifiers gave good hi-fi performances but the SE was more musically persuasive; its facility for clearly separating different strands within the music while knitting them tightly into a cogent whole gave better insight into dense mixes.

The SE’s more scrupulous demeanour also made better sense of intricately arranged jazz. It sorted out a six-piece arrangement – more if you count each of the midi’d keyboard patches – giving a more plausible view of the music’s harmonic and temporal structure. It better related different instrumental lines to each other, sounding more cohesive and adept than the standard Oto, which wasn’t exactly a slouch in this respect. The SE simply made events more telling, locking them into a stricter temporal and tuneful regimen than the standard amp, where the music sounded looser and less expertly ordered.

Switching from the Audio Note (Snell-U-Like) Type Js I’d been using to my regular Naim SBLs, the Oto SE demonstrated its superiority with less efficient speakers. Here it sounded generally more powerful and decidedly more in control. It showed more grip and finesse, especially with potent bass lines. All instruments, though, appeared more solid – the flute on Orphy Robinson’s Jigsaw, for instance, having palpable substance supporting notes that were predominantly breath on the standard amplifier.

For me, there was no doubt that the Oto SE was the Oto to go for.

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