Krell S-550i Stereo Integrated Amplifier

The preamplifier section features circuit concepts from Krell’s flagship Phantom preamplifier. Discrete circuitry, in a balanced topology, is afforded its own power supply regulation. All signal gain is realized using surface mount technology featuring proprietary current mirrors with nearly 500 times the open loop linearity of other designs. Negative feedback is not used anywhere in the preamplifier, nor is it necessary. Open-loop distortion is typically less than 0.003%. The volume control is realized with a balanced resistor ladder — bandwidth and transient response of the preamp circuitry are virtually unaffected by the volume setting.

The S-550i packs a massive 1,750 watt transformer and 68,000µF of capacitance into an elegant 5″ high chassis. This power supply feeds classic Krell circuitry throughout the topology using only discrete circuitry. “Avoiding common place integrated circuits and op amps helps guarantee the S-550i deliver true audiophile performance.” adds McKiegan. As with all Krell amplifiers, the S-550i design is an ideal voltage source. That is, the S-550i doubles its power output when a speaker’s impedance is halved. For the S-550i, the results are 275 watts into 8 Ohms and 550 watts into 4 ohms. This capability is essential for realistic reproduction of a recording’s dynamic content. Most amplifiers can’t deliver this power doubling, leaving music sounding compressed and lifeless.

It was all a bit of shock back in the ‘90s when Krell suddenly introduced a neat little integrated amplifier in the form of the KAV-300i – such a contrast to their tradition of big, beefy power amps. But once past the scepticism we all realised, perhaps to our surprise, that this slim and unassuming device actually had a good measure of Krell’s unique signature: control, dynamics and an uncoloured presentation. The Connecticut firm have not looked back from their entry into a more affordable market, and since then the line-up has never been without an integrated amp. Music lovers on a budget, but who wanted to experience and enjoy the Krell sound, have been treated to the 300il, the 400xi, S-300i, the massive FBi – and now a newcomer in the form of the S-550i. Building on the success of the S-300i, which represented outstanding value for money with a significant uplift of performance over its predecessors – apart from the FBi, which is basically an integrated version of an FPB power amp – the S-550i delivers nearly 275 Watts per channel into 8 Ohms and 550 into 4 Ohms. With a hefty current delivery, this baby can take care of any speaker and keep it well under control. Standing only five inches high, it houses a massive 1,750 Watt transformer which feeds classic Krell circuitry. This attention to the power supply, which has been a Krell tradition right from the start, is essential in bringing music to life, giving a freedom from the compressed and lifeless sound which can so often be a penalty of inadequate power supply – especially when the music programme calls for a big dollop of instant juice to handle sudden transients and dynamics. The preamplifier section features circuitry based on Krell’s top-of-the-range Phantom, with dedicated power supply regulation. All signal gain components use surface mount technology, and have nearly 500 times the open loop linearity of other designs. No negative feedback is used, distortion is extremely low, and volume control is via a balanced resistor ladder. The result is another Krell preamplifier hallmark: virtually no alteration to transient response at different volume settings. The rear panel features the usual balanced and single-ended inputs, but there is also a docking cradle for an iPod. Krell realised some years ago that the humble iPod, derided by many hi-fi snobs, could actually produce more than acceptable music if everything was done properly – their dedicated KiD being a fine example.