Icon Audio Stereo 40 mkIII KT88

Launched in 2012 for the princely sum of £1,700, this was a great affordable audiophile valve amplifier. As chief designer David Shaw put it, “the original Stereo 40 was born in 2000, and was at the heart of what we do, good quality amplifiers at reasonable price. This is a mish-mash of ideas with several ‘nods’ to ideas the giants of the past like GEC, Blumlien, Williams, Philips, etc., worked on. This circuit has been refined over time to give the qualities that I like, especially with the transformers which we have spent many hours comparing performance of subtle changes here and there to get what I think is the correct sonic signature.”

Power is courtesy of the KT88 beam tetrode, as introduced by GEC in 1956, which runs at a sensibly low voltage yet gives out high current. Being a general purpose amp, the Stereo 40 runs these in full Class A push-pull mode, giving a claimed 33W per side. Four of these are fed by 6SN7 and 6SL7 tubes, which also appeared at roughly the same time, back when we all thought we’d be going to work in flying cars by the beginning of the twenty first century! Valve rectification is used, courtesy of a 274B, which is the central valve set into that lovely looking copper top plate.

Although glowing bottles are the headline news in any tube amp, it’s nothing without decent output transformers; they really do have a huge effect on the final sound. Icon Audio has fitted its own custom designed, Japanese hand-wound (99.9999%) pure copper wound tertiary output trannies. The other central transformer is of course to supply power, and to this end the Stereo 40/III sports choke regulation and eight large capacitors in a traditional ‘Pi’ configuration.

All wiring is hand fitted and soldered, and laid out ‘point to point’ for the shortest signal paths; a printed circuit board isn’t used, says Icon Audio, because it colours the sound. A Japanese sourced ALPS Blue volume potentiometer is fitted, as are Taiwanese “audiophile quality” metal film resistors and Japanese Rubicon and Nichichon power capacitors and SCR and Solen polypropylene audio caps. The company offered the option of Jensen ‘oil in paper’ capacitors (£320) beloved by enthusiasts, plus a range of premium and “new old stock” valves (costing up to £500 extra). The tubes supplied do the job, but valve aficionados have long since realised the sonic gains that can be had from fitting high quality glassware.

The 25kg, 390x390x230mm chassis itself is the usual Icon Audio fare, solid steel given a decent coat of sparkle black paint and the lavish copper top plate. The frontal 10mm aluminium alloy fascia plate sports polished dark chrome alloy knobs, and there’s a somewhat prosaic valve cover which should always be used if there is faintest possibility that the movements of children, pets or indeed adults might strike the valves – they run at very high voltages and aren’t anywhere near as domestically friendly as some folk expect.

Set up isn’t quite as simple as with a solid-state amp, but it isn’t exactly electronics PhD territory either. All eight valves need to be fitted into the right sockets; luckily they’re all numbered. This latest Mk III version of the Stereo 40 sports a natty retro look bias meter, which makes adjusting the output valve bias a breeze in conjunction with the bias setting control and the four trim pots on the top plate. But this shouldn’t need to be done for a good long while, as it’s properly set at the factory. Generously, Icon Audio has fitted a tape monitor loop (a classic 3-head open reel looks rather nice sat next to it), and there are four line inputs – something you wouldn’t have got with your original fifties tube preamp! The other key control on the front is the triode/ultralinear mode selector.

Good as the Stereo 40/III is, it hasn’t abolished the laws of physics, so you’ll need to partner it with a sympathetic pair of loudspeakers; ones which are reasonably easy to drive with a good sensitivity figure (over 90dB/1w/1m is a good general guide). This done, you’ll find the Icon Audio to be a big and ballsy performer, with – in ultralinear mode – an-upfront and engaging sound that veritably leaps out of the loudspeakers. The music is fluid, dynamic and vivid with an apparent openness that makes most solid-state amps seem rather grey and congealed.

For example, Steely Dan’s Ricki Don’t Lose That Number came over in a wonderfully carefree and insouciant way. Coming from the excellent Creek Destiny 2 solid-stater, the Icon Audio made even this sound a little stilted and prone to standing on ceremony. It has an easy, mellifluous sort of nature that shuffles along and makes any music it plays a breeze to listen to. It won’t deconstruct things, giving you lots to think about in a ‘hi-fi’ sense, rather it romps along in time with the music like a puppy that’s just got past the garden gate.

Still, with a damping factor of 5.0, it doesn’t have the same bottom end tenacity as a good transistor amp, so although I found the bass lines on the Steely Dan track to be satisfyingly quick and supple, there was that tube trademark ‘bloom’ for all to hear. It’s less than its Ming Da MC368 rival, but still advertises its thermionic origins just a touch too much for some tastes.

The Police’s Everything She Does is Magic was an interesting challenge for this amp, showcasing its strengths and weaknesses in equal measure. The soundstage was sublime, almost more vast than you’d expect from surround sound at its best. It is as if someone has pushed an almighty ‘stereo wide’ button and suddenly your listening room isn’t quite the same anymore. Inside this vast recorded acoustic, vocals came over softly and sweetly, and there was so much room between instruments that you could sit and languish in any one of the many tracks of the recording – my attention was drawn to the brilliance of Andy Summers’ guitar work, for example. I also loved the sugary cymbal sound of Stewart Copeland, but somehow he seemed to have lost his manic energy. This amp is great with microdynamics, but doesn’t quite have the guts to communicate one of his famed china boy cymbal crashes as it should be.

This epitomises the Stereo 40/III. Whatever you throw at it, it obligingly serves up a lovely sound back at you – as opposed of course to an accurate one. It’s never less than peachy to listen to, seemingly able to make even miserable music smile back at you. The truth is that this amplifier suits gentler music more than rockier stuff. With decent amounts of power by tube standards, a rich tonality and a nice, bouncy sound there’s nothing to object to about the Icon Audio Stereo 40/III on its own terms, but if you want explosive dynamics, vast tracts of bass power and forensic detailing then you’d never buy such a thing in the first place.

One comment

  1. Saidur Rahman

    sir, I need schematic diagram to restore my ST 40 MK III tube amp. How can I get the diagram.I am from Bangladesh thanks.

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