Integrated Amplifiers

Krell S-550i Integrated Amplifier

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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 Ω load, the THD+N I measured was 0.04215%.

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

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

The same test into 4 Ω 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 Ω.

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

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