Encountering the Audion Silver Night PX25 Mk II amplifier proved something of a culture shock. There I was, happily living with Musical Fidelity’s kW-550 – a massive beast of an amplifier, with a mere 500W per channel output - and suddenly I go from this to something with just 8W. Talk about one extreme to the other…
As the name implies, this Audion amplifier is based around a pair of PX25 output tubes. An E5GN4 and 5687 complete the line-up – four tubes in total. With just 8W output, it goes without saying that your loudspeakers need to be very, very efficient if you’re hoping to get anything like respectable volume levels. My Impulse H-1 Horns are quite efficient (around the 93dB mark), but would this be enough? There was only one way to find out! Having just been using a 500W amp, I knew the PX25 Mk II would have no power to spare. But, of course, I hoped it would have enough for normal music at reasonable volume levels. However, as I discovered, this comes down to the chosen output impedance. I’ve long been an advocate of running valve amps on their 4 Ohm setting, but the limited output power of the PX25 simply couldn’t accommodate the resulting loss of level, clipping and all too audible distortion setting in alarmingly early.
Changing the output impedance of the amp from the 4 Ohms delivered to the required 8 Ohms means getting inside its entirely hard wired circuit, and should definitely be left to your dealer. The end result was exactly what I wanted, with more level, and much more headroom, a vital benefit as when the PX25 clips it does so very, very obviously.
Naturally, if your loudspeakers are 4 Ohms, the amplifier would produce its highest volume with its output transformer set to this value – changing it to 8 Ohms or higher would reduce the output. The ‘correct’ matching impedance is the one that gives the loudest sound – it indicates the most efficient energy transfer. Certainly, in my system I would not be able to accept the PX25 set to 4 Ohms. It simply hasn’t enough power to drive my speakers. But, with the output transformer set to 8 Ohms it has sufficient output to play most music as loudly as I want. It’s not got power to spare, but output is adequate.
Anyone choosing the PX25 has to realise this, and also needs to appreciate that its key strength is quality of sound – tonality, smoothness, naturalness – rather than sheer brute force. This is an amplifier for those who value purity and fluidity above impact or loudness. It’s unusually clean, and has a lovely sweet mid-band and upper treble. Detail is exquisite, and (within its power limits) the amplifier handles dynamic extremes without hardening tonally. The ideal speaker needs to have around 95dB (or greater) sensitivity. Something that sounds lively and fast, with good transient attack and crisp immediacy, would be perfect.
This amp is also something of an oddity, being somewhere between an integrated and a power amp. It offers a volume control, but only one input. Of course, you could use it with a pre-amp and out of curiosity, I did briefly try the PX25 with ASTINtrew’s At1000. It seemed to work quite well, giving a slightly more immediate and forwardly balanced sound. However, I felt that, overall, the PX25 sounded better on its own – it was sweeter and more refined, giving a subtler more delicate presentation. With an input sensitivity of 150mv it’s able to accept the output of most CD players, tuners, and phono stages without the need for a pre-amp. The only snag is – there’s just a single input! You have to plug/unplug each time you want to listen to a different source…
Sonically, the PX25 is one gorgeous utterly beguiling amplifier. Indeed, one of the ironies (given its limited power output) is the way its sheer smoothness and effortless ease actually invites you to increase volume levels. Tonality is very smooth and natural, creating a sound that is deceptively dynamic and rich in detail. You really notice this on simply-miked recordings of real acoustic instruments and un-amplified voices. It’s very good on massed violins, reproducing the woody mellowness of strings rather than their steely brilliance. Likewise, the human voice is reproduced without emphasis or exaggeration.
One disc that impressed was the coupling of Mozart’s piano concertos 9 and 11 on DG/Archiv with Malcolm Bilson and the English Baroque Soloists under John Eliot Gardiner. This 1984 release was the first in a complete cycle of the Mozart piano concertos using Period instruments. An early digital effort, it sounded tonally thin and ‘papery’ when I first heard it back in ’84, with edgy strings and a complete lack of body. It was one of those recordings that sounded muzzy and congested – as though being played by a worn stylus with a big ball of fluff clinging to it. And that was the CD… Yet via the PX25 it sounded beautifully clear and open. The sound had a light airy quality. The strings (once so acerbic) were brilliant and assertive - without the unpleasant edge that was once so apparent. You could sense the ‘scale’ of the forces used; small, yet clearly focused, creating a sound that was natural and transparent.
There’s a welcome absence of ‘glare’ with this amplifier. It presents the music with effortless ease and a deliciously fluid smoothness that sounds natural and right. I’d say it’s best suited to classical music, and acoustic jazz or folk – any music that demands the utmost refinement and truthfulness. It can handle rock too, but some listeners may prefer a less refined amplifier that provides more forwardness and aggression. On the other hand, it can be nice to listen to rock and pop via an amplifier that isn’t burning your ears all the time. Lack of power may be an issue for rock and pop, unless your speakers are very sensitive.
It was interesting to come to the Audion PX25 Mk II after the Musical Fidelity kW-550. The latter has a bigger more dimensional presentation, with greater scale and a wider dynamic range. The Audion is definitely smoother and more refined, with a silky ease and sweet mellow tonality the bigger amp cannot quite match. Conversely, the Audion doesn’t match the MF’s voluminous bottom end. With the PX25, bass is full and firm, but lacks the outstanding depth and breadth of the kW-550. Nor does the Audion project the music with quite the same sense of holographic 3D spaciousness as the MF. But the smaller amp definitely has a subtler tonality.
In use the PX25 runs surprisingly warm. Indeed, it gets pretty damn hot. While it sounds good when first switched on, there’s definitely a change after about fifteen to twenty minutes. The sound grows ever-more relaxed and spacious, reaching its optimum after about half an hour. Residual noise is very low; an important consideration for those intending to use the amplifier with sensitive speakers. With my H-1s I could hear nothing – no hiss, no hum, no buzz, even with my ear right up to the drive units, important given that it will inevitably find itself driving high-sensitivity loudspeakers.
Clearly, in view of the rather specialised nature of this amplifier, any recommendation has to be on a horses-for-courses basis. The Audion PX25 Mk II is not a universal product. In sympathetic situations it will sound wonderful. But, miscast, it may sound terrible. Using it with speakers under 90dB sensitivity is asking for trouble. I see it partnering ultra-efficient horn loudspeakers, perhaps with a vinyl front end. It’s perfect for that sort of system. It will also partner CD, of course, and its innate smoothness and refinement go a long way towards countering CD’s occasional forwardness and lack of subtlety. Its smoothness and natural, open tonality will also help to reduce the bright peaky forwardness that some high-efficiency speakers exhibit. Some Lowther based designs, for example, can shriek a bit (and sometimes more than a bit) with ‘normal’ amplifiers. But the Audion should be in its element here.
The only change I’d like to see is an extra couple of speaker terminals on the back, giving users the option to go quickly and easily between 4 Ohm and 8 Ohm taps on the output transformer. Other than that, I loved it. As indicated, the PX25 MkII is not a ‘universal’ amplifier. But playing “at home” its strengths will make it hard to beat.