Integrated Amplifiers

Krell S-550i Integrated Amplifier


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 68,000μF 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.