Icon Audio Stereo 40 MkIV 'Plus' Integrated Amplifier

Icon Audio’s Canadian distributor, Warren Lavender, has been excited to share products with Audiophilia for review. I echoed his excitement for Icon’s remarkably good phono stage, the PS1 Mk.II. It was such good value at $2400, I purchased the unit for my mono setup (it has a rare ‘mono switch’). 

Lavender was equally excited about Icon amplification and preamp designs, including integrated amplifiers. As Icon owner/designer David Shaw calls them, ‘All Valve’. Brit-speak for tubes. As such, Lavender requested a review of the 50W (Ultralinear) 30W (Triode) Stereo 40 MkIV Integrated Amplifier. The MkIV is a KT88 based design. The 4 KT88s are a matched set.

I received the hot rodded version called ‘Plus’ (‘Signature’ in the UK). The standard MkIV retails for $3400, the Plus, $4350. The Plus adds upgraded Mundorf silver/gold capacitors and 4 white base ceramic David Shaw CV181 tubes. 

The original Stereo 40 design is almost twenty years old and has been constantly evolving to the sophisticated circuits used today. Shaw says the improvement in the MkIV is substantial. The new Stereo 40 is my first experience with a David Shaw amplification/preamp design. 

Features

Improvements and new features include a headphone socket, a redesigned driver circuit and an improved ‘choke regulated’ power supply. Even though the MkIV in Ultralinear mode gives a healthy 50W, Shaw suggests most of his clients will prefer Triode mode (30W). 

An ‘Easy Bias’ metre makes biasing easy (screwdriver included), the unit includes a Record Loop and comes with a simple metal remote control (Volume Up and Down, Mute). 

Specifications

• All hand wired point to point, no printed circuit board

• Class A, all Triode front end

• Signal to noise level -90db

• Freq response better than 20hz-20kHz +0 – 1db

• Power bandwidth 30Hz-30kHz -3 dB

• Total harmonic distortion 0.2% at full output

• Suitable for 4, 8, and 16ohm loudspeakers

• Supplied with valve cover (not used).

• Silver plated copper PTFE audio cable 

• Audiophile Gold plated Input & speaker terminals 

• Four line inputs: CD, Tuner, Aux plus Tape

• Tape monitor circuit with ‘Record Out’

• Input sensitivity 250mv.(high). 800mv (low).

• Power requirement 230-240volts (117v adjustable)

• 76 Watts standby, 140 W Min, 240 W max. 

• 39cmW, 41D, 23H, 23kg (allow for ventilation)

My Use

Tube power, especially in Triode, is always on a reviewer’s mind when the efficiency of speakers either helps or hinders the listening. My 87.5 dB, 4 Ohm, Alta Audio FRM-2M Celesta Loudspeakers ($15,000/pr.) offered few obstacles. If you want to play your latest punk recording at club levels, you better ensure efficiency numbers the same as mine or above. But for standard repertoire in a small to medium size room, the Stereo 40’s power is more than sufficient in either Ultralinear or Triode. 

Switching between output circuits is via a front plate toggle. Note: only switch modes when the MkIV is in ‘standby mode’ (also a front toggle). The remote is nicely built but has a small target point.

I cabled the MkIV with a loom of Nordost Frey 2 (review publishing Sept 11, 2020)—RCAs, speaker cable and power cord.

Visually, the MkIV is both utilitarian and stylish. Audiophiles will like the large, bright orange bias metre and the ever so functional toggle switches. All connections are high quality and general fit & finish is excellent. Make sure to use linen gloves as you wrestle the tubes into the sockets. We’re talking snug. Especially Shaw's own superb tubes, the four CV181s. Each tube is numbered. A tube cage is supplied (unused during the review).

The choke power supply ensures quiet operation—only the faintest sound was heard with volume at max and my ear next to the tweeter.

Beware lifting the integrated by yourself. The very heavy transformers on the rear of the chassis make moving it a plan ahead process. 

Sound

I enjoyed the break in period playing all sorts of repertoire in Ultralinear mode where the unit did not flinch during some pretty powerful musical explosions. After ten or so hours coaxing more and more power from the tubes, a thick curtain was lifted and the MkIV blossomed quite beautifully. Soundstages got deeper and wider and the varying acoustics of recordings began to develop and shine. It went from novice to professional. There’s a sigh of relief we reviewers exhale when this happens, especially with amplifiers. Closed and somewhat inhibited, and, then, all’s right with the world. 

Ultralinear mode was powerful and brawny without lots of tube warmth while retaining detail. A superior, unfussy circuit. The preamplifier section added much of the clarity I was hearing but friends and guests (and me) were surprised at the bass performance. Delicacy or power, bass is effective within the musical framework. Those explosions I mentioned, including the deepest bass and most demanding I know heard on the LA/Salonen/Rite (Part 1) on DG (FLAC file), sounded as effortless as some of the more powerful solid states I’ve reviewed. Gentle bass such as the bowings in the ‘Introduction’ of Berlioz’ Roméo et Juliette (Montreal Symphony/Dutoit/Decca) sounded wonderful. Perfectly placed and exactly as Berlioz intended—a solid but soft rhythmic impetus over scampering strings and woodwinds, and well underneath the stentorian brass ‘recitative’ that follows (please read Charles Brown’s magnificent Audiophilia post on this masterpiece). There’s even a grace note (acciaccatura) before each of the bass notes. They were deciphered very clearly in Ultralinear.

Well recorded piano music had a crystalline quality. Some of the felts’ softness was hardened somewhat, but returned when playing in Triode. Interestingly, the crystalline quality remained in Triode in addition to the tactile sensitivities. For example, my old mate John Bingham (a long time, beloved piano Prof at London’s Trinity College of Music) sounded as beautiful as ever via the Icon on his ‘70’s benchmark Schubert Songs, arr. Liszt on a Meridian LP (audiophiles, you should search for this record). His delicate touch sounded as effective as ever in Ultralinear. You’ll be one happy, tube-loving audiophile. But, in Triode, the sweetness on the trailing edge of notes was beguiling. 

Triode mode in general was magic. Musical, sweet, almost reach-out-and-touch-it experiences. Designer Shaw tells us of his customers’ predilections for Triode and I heard why. Interestingly, my wife, who has great ears and a ton of experiences living with a musician for 38 years, preferred the dynamism of Ultralinear. But, while Ultralinear was excellent, the Triode circuit connected me to the musicians in a more emotional way. My classic 1969/ASMF/Argo Rossini String Sonatas was stunning on the MkIV. I loved every note—the conductor-less Academy sounding as one, but the small string section about as sweet and uniform as one can. Audiophiles, you need this record, too. 

My guess is if you play your music really loudly, choose highly demanding repertoire and have a large room, you’ll be in Ultralinear 90% of the time. And that’s no bad thing. But for the audiophile who wants the utmost subtlety and connection to their favourites, especially small scale orchestral, chamber music, opera, lieder, jazz and acoustic pop, the toggle will be on Triode. What a lovely position Shaw places you in—and even late night listening when the family’s in bed or in that New York apartment, there’s the highly regarded headphone jack. Seems audiophiles can have it all. 

Summary

The Icon Audio Stereo 40 MkIV Plus is a star in the Stereo 40’s lineage and a proper and good advertisement for David Shaw’s superb design acumen. It’s easy to use, dead quiet, has power enough for most situations, and imbues music with sweetness and light. Such quality for $4350 is a remarkable value. Very highly recommended. 

Further information: Icon Audio