When a single-channel is driven into a 4Ω load, the Krell S-550i will deliver almost 1-kilowatt of power!

it’s every bit as high quality as the Krell amplifiers that are made in the US, so if Krell hadn’t put a ‘Made in China’ label on the rear panel, no-one would ever have been any the wiser.

In Use and LIstenIng sessIons

The first thing to be aware of before installing the Krell is that its 240V power plug is so large that you won’t be able to plug anything in alongside it if you’re using a standard double power point or a power distribution board of any kind. The reason is that the three wires in Krell’s power cable are so hugely thick that they won’t fit into a standard 15-amp 240V plug. I suspect it’s also a physical reminder of something Krell points out in its Set-Up Guide, where it says: ‘ Because of its powerful amplifier channels and high-capacity power supply, the S-550i will benefit from a dedicated a.c. circuit. Avoid connections through extension cords or multiple a.c. adapters. Use only the power cord provided with the S-550i to make the connection to AC power. Operation with a power cord other than the one supplied by Krell can induce noise, limit current, or otherwise impair the ability of the integrated amplifier to perform optimally.’

The second thing of which you’ll already be aware is that the Krell is a high-power amplifier and it uses a Class-AB output stage strongly biased towards Class-A, so unless it’s properly ventilated, it may become ‘overly warm’. My advice is to again follow the advice in Krell’s Set-Up Guide which specifies that the S-550i: ‘ requires at least 2.54cm of clearance on each side and at least 5cm of clearance above to provide adequate ventilation. Installations inside cabinetry may need extra ventilation.’ Although low-powered integrated Although Dan D’Agostino founded Krell in 1980, and acted as its chief designer for many years, he no longer works for Krell, although he reportedly still has a minority shareholding in the company. He reputedly named the company after an extinct race of advanced beings that lived on the planet Altair IV, as shown in the classic 1956 science fiction film Forbidden Planet. He did so because the first Krell amplifier, the KSA-100, was dubbed ‘the first high power, high-current, true Class-A biased stereo power amplifier available to audiophiles’ and, in the movie, the Krell had reached a stage of technological and scientific development so advanced that they were able to construct a machine that delivered ‘virtually unlimited’ power. amplifiers don’t often require you to be too particular about ventilation, amplifiers rated at 550-watts per channel demand that you do follow the ventilation guidelines.

For some reason, I didn’t get the detailed instruction manual that’s supposed to come with each amplifier, only the Set-Up Guide, and I couldn’t find an electronic one on the company’s website either… though I did find one for the S-300i. However, it’s a measure of how intuitive it is to program the Krell S-500i that I was able to do everything any typical user might want to do: adjust balance, input trim, label the inputs etc, without recourse to a manual. I was a little annoyed that you aren’t actually able to name each input, as I’d initially imagined when I heard you could program the inputs. Instead you only get to choose from a pre-programmed list of names Krell has stored in the S-500i’s memory… though if you can’t find one you want from the 78 that are in the amplifier’s memory, I’d be surprised. (Note to Krell’s design department: many audiophiles with high-end components like to label their inputs so that, for example, when they select the ‘Phono 1’ input, the front panel display on the amplifier shows ‘Linn’ or ‘LP12’ or the name of whatever other turntable they may own. In other words, it would be nice if S-550i owners were able to program their own alpha-numerics, rather than just pick the most appropriate one from a list… even if it is a long list.) The other thing I’d like Krell to eliminate is the turn-on thump. It’s not particularly aggressive, but it’s certainly noticeable.

If I were to use one word to describe the sound of the Krell, that word would certainly be ‘effortless’, (though I did spend some time agonising whether that word would be effortless or transparent), because no matter what type of music you’re listening to, or how loudly or softly you’re playing it—and certainly irrespective of the speakers you’re using!—the Krell S-550i never seemed to be doing any work at all. When I asked, it delivered. No fuss, no bother. It really was as if the amplifier were not in the chain of components at all (which is why I was sorely tempted to use the word transparent to describe it). This ability is certainly the mark of a superior amplifier: one where you can’t really tell it’s there at all, except that you know it must be, otherwise your speakers would not be producing such glorious sound.

Needless to say (though here I am saying it), you won’t ever need more amplifier power than is available from the Krell S-550i… I didn’t even begin to approach the maximum levels of which it’s capable before I had a neighbour knocking on the door to tell me to turn it down… and then coming around the back to wave at the window, because I didn’t hear him knocking at the door. Another neighbour phoned… and I didn’t hear that either. And when you’re not using the Krell’s maximum power (that is, you’re only playing back your music at ordinary to low levels), the S-550i is as sweet-sounding as a Class-A tube amp, with no harshness, no ‘rough’ edges, and with a perfectly silent background, so that notes decay into the silence of your own room, and not into the noise floor of the circuitry.

But even if you are playing at low levels, you’ll ‘hear’ the power of the Krell because of the incredible dynamics enabled by the power capability. For example, the sound of a kick drum, whose initial transient can be squashed by lower-powered amplifiers (those whose power output tops out at two figures), is delivered perfectly by the S-550i. There’s no impression of ‘loudness’ per se, because the transient is so short in duration: what you perceive… rather than hear… is sonic realism. It’s as if you’re hearing a kick drum in your room, and not some recorded facsimile of a kick drum.

Funnily enough, this manifests as a type of ‘softness’ to the bass, but I think this is because so many amps go into clipping in this situation, which creates a ‘hard’ sound so if you’re used to listening to recorded music, rather than live, and you have an amplifier with a fairly low power output, you might have become used to the ‘hard’ bass (caused by your amplifier clipping) and not realised that the ‘soft’ bass sound is actually more realistic.

The same is true of the midrange sound of the Krell: but here it’s not that the amplifier has a ‘sound’—it doesn’t. It’s just that the music and vocals are delivered so true-to-life that your brain is not having to build any sonic constructs… you’re hearing the music effortlessly, which means you can relax, which in turn means greatly improved enjoyment of what you’re listening to.

In addition to enjoying what you’re hearing more, you can also appreciate it more keenly, because the Krell is so accurate that you’re not missing out on anything. Even the most microscopic details of micro-dynamics are delivered precisely: you’ll hear every inflection in a vocal, every nuance in a melody line. In short, amazing detail.

ConCLUsIon

Your time has come. If you’ve always lusted after a Krell, now is the time to consummate your desires. It may not be the prettiest amplifier in town, but it’s certainly one of the most powerful. Its lineage and ancestry are impeccable and it will entertain your aural senses like no other amplifier. And best of all, the parents aren’t asking for much of a dowry!

Laboratory test resuLts

Power output has always been a strong point of Krell amplifiers, and you can see from the results of Newport Test Labs’ testing, as shown in the table that accompanies this review, that the S-550i maintains that tradition. When driving 8Ω loads, the S-550i delivered 315-watts per channel, both channels driven, at all three test frequencies: 20Hz, 1kHz and 20kHz. This is not only higher than specification, it’s also excellent performance. When driving 4Ω loads, and with both channels driven, Newport Test Labs measured the S-550i as delivering 524-watts at 20Hz and 20kHz, and 529-watts at 1kHz. This meant that into 4Ω loads, the S-550i’s output was a little lower than Krell’s specification of 550-watts, but the difference is minuscule (0.1dB). The performance into 2Ω loads speaks for itself: 800-watts per channel, at all frequencies across the audio band. This is more than sufficient to power even the most inefficient loudspeakers, even those with very low impedances. When a single-channel is driven into a 4Ω load, the Krell S-550i will deliver almost 1-kilowatt of power!

The Krell S-550i’s wideband frequency response was very flat and extraordinarily extended, being just 1dB down at 4Hz and 86kHz. The 3dB down-points were measured at 2.5Hz and 151kHz. Frequency response across the audio band is shown in Graph 5, where the black trace shows the response into a standard laboratory-grade non-inductive 8Ω load, which is exactly at reference level at 20Hz then rises to around +0.08dB at 100Hz, before falling to +0.01dB at 1kHz, to reference again at 8kHz, then rolling off above 10kHz to be about –0.05dB at 20kHz. Overall, normalised, this puts the measured frequency response at 20Hz to 20kHz ±0.06dB. The red trace on this graph plots the S-550i’s frequency response when driving a reactive load that simulates the impedance of a two-way bookshelf loudspeaker. This is a little more irregular than the response into a resistor, but overall the response is 20Hz to 20kHz ±0.09dB. This is excellent performance. Channel separation was not quite so excellent, but still far more than required to deliver pin-point imaging and proper stereo staging, so the figures are somewhat academic, particularly since the