Introduction to the Krell S-550i Integrated Amplifier

The S-550i is the larger of two new integrated amps from Krell. Both of these amps feature circuit boards that utilize surface mount components. This means they can pack more power into smaller chassis than ever before. The S-550i is rated at 275 watts per channel into 8 ohms, and that doubles to 550 wpc into a 4 ohm load. This being from a box that is less than 6 inches tall. So now you can get tons of Krell goodness in a small package and at a very affordable price.

For the uninitiated, Krell has been around for almost 30 years now, producing fine sounding equipment with a rugged, industrial look. Some of the all-time best systems I have ever heard were built around Krell amplification and digital source components.

In a very interesting move, Krell is getting into the car audio game and recently announced they are offering a premium sound system as an option to Acura’s flagship sedan, the 2014 RLX. This is a major move for Krell and I see it as quite the compliment because Acura (possibly more than any other car maker) has a long standing tradition of audio excellence.


  • Design: Fully Balanced Differential Class A/B Integrated Amplifier
  • Power Output: 275 wpc (8 Ohms); 550 wpc (4 Ohms)
  • THD: <0.025% at 1 kHz
  • Frequency Response: 20 Hz – 20 kHz +0, -0.08 dB
  • Inputs: 1 Balanced XLR, 3 Unbalanced RCA, 1 iPod 30-Pin Connector
  • Outputs: 1 Pair Pre Out RCA and 1 Pair Speaker
  • Input Impedance: Balanced – 95 kOhms; Single-Ended – 47.5 kOhms
  • Output Impedance: <0.073 ? at 20 Hz; <0.08 ?, 20 Hz to 20 kHz
  • Damping Factor: >110 at 20 Hz (8 Ohm Load); >75, 20 Hz to 20 kHz (8 Ohm Load)
  • Dimensions: 5.84″ H x 17.26″ W x 17.70″ D
  • Weight: 64 Pounds
  • MSRP: $5,000 USD
  • Krell Industries
  • SECRETS Tags: Krell, Integrated, Amplifiers, Audio, Stereo


The Design and Setup of the Krell S-550i Integrated Amplifier

The S-550i embodies many of Krell’s famous design features from their past amplifiers. Let’s first begin by taking a look at the preamp stage. The preamp circuit is pure Class A biased and fully balanced. The architecture of this circuit is Krell’s proprietary Current Mode topology which is a very high bandwidth design: Krell rates the preamp section as flat to 1 MHz!

The input stage is direct coupled with no capacitors in the signal path. The preamp section also features zero negative feedback. Krell says it still maintains very low distortion with a claimed THD of only 0.009% for the preamp section.

The all-important volume control is a resistor ladder network, widely regarded as the premium design choice for a volume control.

Moving on to the power amp section, we see that this unit preserves a fully balanced differential architecture all the way to the speaker outs. Krell didn’t skimp on the power supply either with a 1,750 VA transformer and 68,000?F of capacitance. As with other Krell amps, the circuitry is fully discrete with bipolar output transformers.

The substantial power rating for this integrated is nothing to sneeze at – 275 watts per channel into an 8 ohm load. This doubles to 550 watts per channel when driving a 4 ohm load. That is a lot of juice. I never found this amp’s limits during a long evaluation period despite that I am somebody who likes to listen to it pretty loud now and again.

So the S-550i is something of a classic Krell design. But there is one major advancement at hand – the use of Surface Mount components in the design and fabrication. Surface Mount Technology (SMT) involves components that mount to the surface of a circuit board rather than by way of a through-hole design. This has many advantages for audio applications. A good design can miniaturize the component, shorten the signal path and reduce costs through smaller components and quicker fabrication techniques. Krell has leveraged all these advantages to bring you a true high end product at an incredibly affordable price point.

The S-550i has four pairs of analog inputs. One is a balanced XLR and the other three are unbalanced RCA’s. There is no built-in phono stage or DAC in this integrated. There is one set of speaker outputs via high quality WBT binding posts. There is an unbalanced preamp out, but no amp in and no balanced preamp output.

An interesting accessory is an included iPod dock. This dock connects to the S-550i by way of a 30-pin connector. It taps the fully differential line level outputs of your iPod or iPhone. Once docked, your iPod can be controlled via the Krell’s remote control.

Speaking of the remote control, it is a heavy brick like affair. It is very substantial indeed, but it is not backlit and it is so very large that you can barely hold it in one hand and hit all the buttons. You even have to use a little Torx wrench to open the battery compartment. (The wrench is included, of course.) The remote has all the functions you could want – power toggle, direct source select, volume up/down, mute, iPod controls, balance control (very nice touch), setup menu controls and controls for other Krell products.

The amp’s menus and status information are displayed on the large two line TFT readout. You can do things in the menus like renaming the inputs, setting the input level, setting the display options and turning on Theater Throughput™ mode. This mode causes the amp to function as a simple power amp to power your main channels in a theater set up.

During the review period, I used this amp primarily to drive a pair of Sonus faber Venere 3.0 tower speakers. The amp was placed front and center in my rack. I set the display to time out when no commands were being received. I connected my Oppo Blu-ray player to the balanced inputs and connected my phono preamp to the S-1 input. I also connected the main left and right inputs from my Marantz surround processor to the S-550i’s S-2 inputs and set this input to Theater Throughput™ mode. At various times during the review, I connected the high level inputs of a REL T9 sub to the Krell’s binding posts and grounded this cable to the amp’s chassis as required due to its fully balanced design. This was the best method I had at my disposal to connect a sub to this system.


The Krell S-550i Integrated Amplifier In Use

Let me just say right off the bat that the Krell S-550i integrated amp displayed the chameleon-like character that is the signature of many of the great high end products on the market. It had a way of adapting to the source and allowing each performance to take on its own signature character. That is not to say that the Krell was lacking in its own innate character. The Krell had a number of very positive qualities that showed up in my notes over and over again.

The Krell S-550i’s most obvious qualities were a three dimensional layering, a vibrancy and a grain-free response with an unyielding grip on the speakers.

A good example where all these qualities can be heard is on the recent SACD re-release Pink Floyd’s Wish you Were Here. I listened to the original vinyl so much I wore out the grooves. This is not meant to be a cliché, I literally did. In this particular case though, I listened to the SACD’s stereo track over the Sonus faber Venere 3.0’s.

Tracks like “Shine on You Crazy Diamond” not only had the aforementioned 3d quality, but they also passed a lot of inner detail; details like being able to pick out the sound of the sax keys even in the louder passages. Elsewhere, I heard a solid foundation in the bass, great clarity in the mids and a transparent treble. There was no sense of strain and great dynamics throughout.

On the point of the treble, it at first may come across as a bit recessed or soft, but on further listening, I found it was just so very smooth and so very free of grain that it simply sounded recessed in comparison with amps that have an artificial edge in the treble.

After listening to the whole Pink Floyd album, I wrote “Man, if I’d have had this system when I was in college, I wouldn’t have graduated. Ever.”

Another good example of the vibrant and three dimensional sound was heard in the 180 gram reissue of the Steve Miller Band Greatest Hits 1974-78 album that I found on closeout at a Target store (?). This is such a great album with not a weak song on there.

I marveled at the vibrant and 3d presentation again. The amp’s vibrant quality was most revealed through the texture of the guitar strings. But it went further than that. I enjoyed cymbal sounds with a natural structure and the timing of each note was impeccable. Bringing to mind that the Krell S-550i does put you in a slightly closer than neutral perspective to the performers.

Another strength of the Krell amp started to come to the fore and that was in the vocals. They had an “in the room with you sound” with no chestiness, nasality or excessive sibilants. It was incredible.

I needed to get to the bottom of this vocal thing, so I put on Melody Gardot The Absence also on vinyl. This is a great test for voice reproduction and this is one record where the stage extends way outside the confines of the speakers. As a modern recording, the bass is fuller and better defined, helping reveal the Krell’s native character.

Ms. Gardot’s voice was reproduced with so pure a tone it was almost unbelievable for Hifi. But the sound was just slightly too hyper for the recording and I felt close to the stage. Don’t get the wrong idea as the sound was ethereal and had an understated elegance. I was thoroughly seduced and just sat back and simply enjoyed listening to the music.

I had yet another chance to be taken away by the music so much so that I had to keep reminding myself I was in the midst of a review. This time was while listening to Antonio Lysy at the Broad: Music from Argentina. I ripped this CD to my iPod and then played a compressed version through the Krell dock (320 kps). I do wish the iPod metadata would show up on the Krell’s display because I can’t read the info on an iPod from more than a few feet away and the Krell display was legible from my listening position.

Of course, the music was dynamically compressed due to ripping the music as an mp3 but the sound was still quite enjoyable. So I was once again tempted to just enjoy and I had to fight that urge. I was in the middle of a review after all.

My favorite parts of this album are when Antonio’s cello solos are unaccompanied. But fuller arrangements could be emotive and the Krell did a bang up job with the piano in particular. There is an ease and naturalness to the sound. What I heard was clean, vibrant, and harmonically rich with extra smooth transitions between notes on the legato passages. Soft violin strings were refreshingly free of grain and noise.

In rounding out my listening impressions, I would like to share my experience with cinema. The new Warner Brothers release, Prisoners is just out on Blu-ray. This is a deeply troubling story starring Hugh Jackman and Jake Gyllenhaal.

I know Prisoners may not be everybody’s first choice for review material but it is for me. The picture and sound are reference quality. There is a mesmerizing score, deep bass pulses, tons of atmospheric effects and plenty of voices both screaming and whisper soft. There is one scene where I swear you can hear the sound of a tree growing. It is in many ways the perfect test track.

I put the Krell S-550i in Theater Throughput mode to drive the main speakers. The other three satellites were being driven by an Emotiva XPA5 multi channel amp. There was clearly less noise in the mains than in the center speaker. The S-550i was able to preserve every minute detail in the audio from the softest to the loudest. This was heard mostly in the score, the deep bass pulses to set the mood and the most delicate of environmental effects. The Krell really pulled me in to the film maker’s vision and I was entranced.


The Krell S-550i Integrated Amplifier On the Bench

I do not have the equipment to test maximum power output but I can address the S-550i’s power reserves in this way: I used the S-550i to bench test some speakers. I typically use a 200 wpc amp for testing speakers and I have found this amp may at times clip before I reach the speakers’ limits. Sine waves can be very demanding of a power amp. The Krell was capable of prodigious output. I can honestly say that I did not see any signs of it clipping even when running tests at very high output levels. It clearly bested my regular amplifier.

The below distortion measurements were run with a test signal through the balanced inputs and readings were taken at the left speaker out. Not shown is a frequency response plot which demonstrated a flat response curve from DC to 50 kHz. The amp started to roll off gently at this point and was down only 2 dB at 80 kHz then falling to 5 dB down at 90 kHz. This is a high bandwidth design.

At 1 kHz and 2 Volts RMS output into an 8 ? load, the THD+N I measured was 0.04215%.

At 1 kHz and 2 Volts RMS output into a 4 ? load, the THD+N I measured rose slightly to 0.05050%.

Raising the output level to 5 Volts into an 8 ? load, the THD+N at 1 kHz dropped as expected to 0.03019%.

The same test into 4 ? caused a slight rise in distortion and resulted in a THD+N measurement of 0.03451%.

In this test, the input signals are simultaneous 19 kHz and 20 kHz sine waves at 1:1. The B-A peak at 1 kHz is 86 dB below the test signal. The test was taken at 2V into 8 ?.

The S-550i’s Intermodulation Distortion (IMD) was only 0.0334% at 2V into an 8 ? load.

The same test at 5 volts shows a drop in the IMD to 0.0240%.


Conclusions about the Krell S-550i Integrated Amplifier

The Krell S-550i is a winner in just about every way. I’ll mention that up front that I did want a few extra features. It would be nice if the readout would show iPod metadata from the dock. And I would like to see a dedicated mono subwoofer line level out. Those are two very minor wishes. The real story is in the massive performance you get from this fine product. That’s what you’re paying for, right?

The S-550i is solid, rugged and reliable. Its performance is incredible in both the listening tests and on the bench tests. It has high bandwidth and low measured distortion especially considering it has no global negative feedback in the preamp stage.

On listening, the sound is vibrant, dynamic and dimensional. Pure Americana in so many ways . . . A high damping factor means it will grab your speakers by the cajones and never let go. Plus it puts out huge amounts of power with high current capability and the ability to double down when the load is halved. This is where the passion, drive and delicacy are unleashed from you favorite music. Don’t forget that the S-550i handles cinema well; so deeply quiet when required and so ruthlessly gripping when demanded. A very solid all-around performer that really is capable of driving just about any speaker I can dream of.