Back in the late nineteen nineties, tube amplification was regarded as an extreme niche in hi-fi – something akin to how most petrolheads think about kit cars (i.e. “interesting, but not for me”). So perhaps it’s a sign of the times that suddenly it feels like we’re getting more new valve products than transistor ones. Icon Audio was in the right place at the right time, which isn’t to say that its founder David Shaw was lucky. Rather, he identified the shift back to tubes – and took the plunge. Rather than producing esoteric products, such as AudioNote’s umpteen thousand pound Ongaku, David did precisely the reverse and came up with a surprisingly affordable range of kit.
Shaw makes no secret of the fact that his amplifier designs are based around classic circuits, because as any tube aficionado knows, there’s only a limited number of ways to do what are – unlike solid-state – largely straightforward, uncomplicated topologies. With valve amplifiers, it’s the implementation rather than the circuit itself (many of which are generic) that really matters. This means high quality wiring, passive component, power supplies and output transformers are needed to turn an average amp into an excellent one – and then of course there’s the quality of the actual tubes themselves.
In the case of the MB90 monoblocks launched in 2008, they’re a fairly traditional three stage design, with the front end based on the classic Leak Stereo 20, 50 and 60 series. Icon has used pairs of the older and bigger 6SL7 and 6SN7 driver valves from the early nineteen forties, which have a reputation for a richer sound allied to longevity. To this, designer David Shaw fits ceramic valve holders, silver Teflon audio cable, hard wired audio componentry including Polypropylene audio capacitors, 2W metal film and wire-wound resistors, plus custom low-oxygen copper wire wound Japanese EI transformers and a hefty choke regulated power supply.
The output stage of the amplifier uses the KT90, a more modern derivative of the GEC KT88 which is surely one of the most ubiquitous hi-fi power valves. It operates in a choice of modes – ultralinear (producing a claimed 100W) and triode (with a claimed 50W) – giving “two amplifiers in one”, as Icon Audio puts it. Close attention has been paid to the power supply, using eight large capacitors and a large choke in a traditional ‘Pi’ configuration. This is claimed to give a very low source impedance with a large reserve of power available for large transients without clipping. The MB90 is said to work in both high and low sensitivity modes, letting it work with both passive and conventional preamplifiers – I used an MF Audio Silver Passive.
The monoblocks themselves are very well finished, giving little away to Quad, for example in this respect. Each is sturdily made at around 25kg apiece, and there are no rough edges to the casing. The ‘sparkle’ paint finish is excellent, and the 20mm thick front panel confers an air of solidity. I cannot count myself as a fan of the supplied steel trimmed Perspex valve covers, which are a bit gauche – the tube amp equivalent of big wheels and blacked window glass on a car – but you can remove them.
In addition to the ultralinear/triode mode switch, the fascia has another rocker marked ‘warm up/standby’. This is claimed to prevent valve cathode stripping during start-up and enables the amplifier to remain in standby on low power without wasting electricity. In practice it proves a nice ‘halfway house’ between running the amp at full whack and not wearing out the valves by switching them on and off all the time. On standby, each MB90 eats 38W, which is considerably less than when on properly (88-200W apiece). I found it best to switch the amp from off to standby a while before the listening session, then fully power up the amps a minute or two before listening in anger, so to speak. Then, if had to answer the phone, break for dinner or answer the door, the standby mode was jolly useful.
Considering the sub-£1,700 retail price, I was highly surprised with how they rose to the challenge. Kicking off with Peter Gabriel’s Solsbury Hill, the MB90s running in ultralinear mode, and I could hear a clean and muscular modern tube power amplifier. With none of the softness, colouration or warmth that many still associate the breed with, the Icon Audios proved themselves thoroughly modern monoblocks. The opening guitar figure was crisply conveyed, and as soon as the percussion kicked in I found myself on the receiving end of a wide and detailed soundstage, bristling with energy. Bass wasn’t the tightest or tautest I’ve heard from tubes, but it was perfectly capable of playing a tune and extending my loudspeaker woofers by fairly sizeable amounts – it was certainly strong and insistent. Vocals proved clear too, this classic 1977 recording showing itself to be superb even by modern standards.
Moving to a more challenging recording in the shape of Stevie Nicks’ Room’s on Fire, and I could hear a touch of hardness to the upper midband, although the song had great dynamic impact. The MB90s seemed in their element with the closely miked 4/4 rock percussion work, pushing the song along with gusto. Snares were satisfyingly big and beefy, while cymbals glistened with detail, but had less atmosphere than I would have liked, making everything from the upper midband onwards appear processed. I suspected the MB90s were capable of better, so moved to triode mode. Whilst some of the relentless, adrenaline-fuelled gusto of the Icon Audio monoblocks had gone, in its place we had subtlety, transparency and finesse. Those coarse cymbals acquired a velveteen purity; at once smoother yet more atmospheric. The vocal track gained some finesse, making Nicks sound less like Rod Stewart and more like Judie Tzuke. The snares lost a little edge and bite, but timed in a far more natural way; suddenly it seemed we had a musician with feeling bashing the sticks instead of a session man following a click-track.
Indeed, the move to triode had quite a profound effect right across the board. As previously intimated, the midband and treble sweetened up and opened out, but the bass also took on a new life – becoming softer, looser and less muscular. I found myself turning the volume up on the preamp slightly to account for this, and it worked fine, bringing little apparent extra strain. Still, it was clear that the MB90s had lost a little ‘lung capacity’ in the name of a more musical sound. I actually far preferred things this way, feeling that what’s lost on the swings is more than gained on the roundabouts.
For example, Jean Michel Jarre’s Oxygene went from sounding like impressive hi-fi to a truly enrapturing experience. Yes, the bass was softer and looser, but it was orders of magnitude more tuneful, and this held right the way up the frequency spectrum. There was beautiful tonal detailing to JMJ’s vintage keyboards, and a deliciously wide and deep recorded acoustic. Best of all was how the music flowed rhythmically, showing ‘the magic of valves’ to full effect. Conversely, switching to ultralinear gave a far more brawny sound, with oodles of strong bass, more marked dynamic crescendos and an air of confidence that the MB90s lacked in triode mode.
After the first few hours, I found I did nearly all listening in triode mode and never went back. Suitably run in and warmed through, the Icon Audio MB90s became a truly enjoyable musical tool. Their real strength is that carefully set up, run in, warmed through and with the resonant valve covers off, they sound so very clean and open – especially when compared to the fuzzy Class AB solid-state designs that inhabit this price point.