Building on the success of the award winning S-300i integrated amplifier, the S-550i boasts the sound quality and power output capable of driving any loudspeaker to its maximum performance. Delivering 275 watts of stereo output, the S-550i is a power source for virtually any speaker on the market while occupying a fraction of the space typically required for a system delivering this sound quality. The S-550i is outfitted with balanced and single-ended inputs, iPod dock, and Theater Throughput™ mode. Developed by Krell, Theater Throughput™ simplifies integration into home theater systems. For custom integration applications, 12 volt triggers, RS-232 connectivity and rack mount ears are included. Operational parameters can be customized via the menu system that is accessible from the front panel or the included brushed Aluminum remote control.

Preamplifier Highlights

The preamplifier section features circuit concepts from Krell's flagship Phantom preamplifier. Krell Current Mode provides unequaled bandwidth for the most revealing playback of today's high resolution music sources. Direct coupled circuitry eliminates impact-robbing capacitors from the signal path and an elegant, custom-machined level control adjusts volume. 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.

Power Amplifier Highlights

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. 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.

1 pr. balanced via XLR connectors
3 pr. single-ended via RCA connectors
1 dedicated iPod input via 30-pin connector

1 pr. preamp outputs via RCA connectors
1 pr. speaker outputs via gold-plated binding posts

Control inputs
1 RS-232 input via 9-pin D-subminiature connector
1 remote IR detector input via 3-conductor 3.5 mm connector
1 12 VDC trigger input via 2-conductor 3.5 mm connector

Control output
1 programmable 12 VDC trigger output (30 mA maximum current) via 2-conductor 3.5 mm connector

Input impedance
Balanced: 95 kΩ
Single-ended: 47.5 kΩ

Frequency response
20 Hz to 20 kHz +0, -0.08 dB
<2 Hz to 140 kHz +0, -3 dB

Signal-to-noise ratio
>89 dB, wideband, unweighted, at maximum gain, referred to full power output
>96 dB, "A"-weighted

32.0 dB

Input sensitivity
Single-ended or balanced: 1.18 V RMS

Total harmonic distortion
<0.025% at 1 kHz, at 275 W, 8 Ω load
<0.20% at 20 kHz, at 275 W, 8 Ω load

Output power
275 W RMS per channel at 8 Ω
550 W RMS per channel at 4 Ω

Output voltage
133 V peak to peak
47 V RMS


Output current
20 A peak

Slew rate
80 V/μs

Output impedance
<0.073 Ω at 20 Hz
<0.08 Ω, 20 Hz to 20 kHz

Damping factor
>110 at 20 Hz, referred to 8 Ω
>75, 20 Hz to 20 kHz, referred to 8 Ω

Power consumption
Standby: 1 W
High-current standby: 40 W
Idle: 90 W
Maximum: 1800 W

Heat output
Standby: 3.4 BTU/hr
High-current standby: 140 BTU/hr
Idle: 310 BTU/hr
Maximum: 2400 BTU/hr

Specifications for preamp only:

6.3 dB (single-ended)

Volume control
Current-mode, 12-bit, discrete R-2R ladder

Input overload
Balanced: 12 V RMS
Single-ended: 6 V RMS

Output overload
8 V RMS (single-ended)

Frequency response
20 Hz to 20 kHz +0, -0.05 dB
<2 Hz to 1.0 MHz +0, -3 dB

Total harmonic distortion
<0.009%, 20 Hz to 20 kHz, at 2 V RMS

Signal-to-noise ratio
>87 dB, wideband, unweighted, at maximum gain, referred to 2 V RMS
>94 dB, "A"-weighted

17.26 in W x 5.84 in H x 17.70 in D
438 mm W x 148 mm H x 450 mm D

Unit only: 64 lb [29.0 kg]
As shipped: 76 lb [34.4 kg]


HiFi News
Sept - 2013 - Martin Colloms

"Was as satisfyingly real as one could hope to experience" .. more

  • S-550i Integrated Amplifier with iPod Dock
  • S-550i Integrated Amplifier

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