I read that Stereophile review several times before I decided to take a chance on the 400xi because I too had concerns. According to John Atkinson's measurements taken of the Thiel 2.4: "Not only does its impedance drop to 2.73 ohms at 600Hz, but it stays significantly below 4 ohms from 100Hz to 50kHz, and there is a difficult combination of 4.5 ohms magnitude and 45° electrical phase angle at 80Hz."
After two weeks of serious listening in the evenings, sometimes at SPLs from 8 feet from the Thiels up around 85 to 90 dB, the 400xi gets hot, but not so hot that I cannot keep my hand on top of the amp. So it seems that even 2.73 Ohms minimum and a nominal less than 4 Ohms is OK for the Krell 400xi.
Also, according to the Krell's measurements and for other amps, Atkinson's test regimen puts amplifiers through some quite serious stress; music is a very different source signal than pink noise or 1kHz/10kHz square wave tones. And look what he did to thermally stress this amp so that it was cranking 500 Watts before he got the fuse to blow into 2 Ohms! Also, Atkinson points out that, "Concerned about the temperature-dependent nature of the amplifier's linearity, I measured the manner in which the KAV-400xi's THD+noise percentage varied with frequency at a moderately high level (16V) into 2, 4, and 8 ohms. The results are shown in fig.5: the audioband distortion is nicely below 0.1% into 8 ohms, with the right channel a little more linear than the left. A rise in THD above the audioband gets more severe into the lower impedances, but this is nothing to be concerned about."
The other thing too is that he thermally stresses the amp and does the waveform analysis when maximally thermally stessing the amp, something he does not routinely do for other products. That seems a bit unfair and I wonder if he has some chip about Krell that we don't know about.
He concludes that one should not partner the amp with speakers less than 4 Ohms (I conclude nominal impedance) but in my experience all is well.
Music does not stress an amplifier the same way as Atkinson does in the lab. And while I admire some of the punishing tests he puts SOME, NOT ALL amps through, readers will also remember that he admits that some of the tests he performs requires that he stand well away from any amp he puts them through some of these tortuous experiments. Gee I wonder why and what does that tell you?
I'll bet Atkinson has seen his share of dazzling fireworks.
The 400xi could probably use a bit more heat sinking, but under "normal" listening conditions (I mean are you guys risking your hearing in excess of 90 dB routinely???) and even a tougher load, this amp seems to have no issues. I'm not gonna lose sleep over this but instead enjoy how great it sounds. And great it does sound!
Cheers!
Steve
After two weeks of serious listening in the evenings, sometimes at SPLs from 8 feet from the Thiels up around 85 to 90 dB, the 400xi gets hot, but not so hot that I cannot keep my hand on top of the amp. So it seems that even 2.73 Ohms minimum and a nominal less than 4 Ohms is OK for the Krell 400xi.
Also, according to the Krell's measurements and for other amps, Atkinson's test regimen puts amplifiers through some quite serious stress; music is a very different source signal than pink noise or 1kHz/10kHz square wave tones. And look what he did to thermally stress this amp so that it was cranking 500 Watts before he got the fuse to blow into 2 Ohms! Also, Atkinson points out that, "Concerned about the temperature-dependent nature of the amplifier's linearity, I measured the manner in which the KAV-400xi's THD+noise percentage varied with frequency at a moderately high level (16V) into 2, 4, and 8 ohms. The results are shown in fig.5: the audioband distortion is nicely below 0.1% into 8 ohms, with the right channel a little more linear than the left. A rise in THD above the audioband gets more severe into the lower impedances, but this is nothing to be concerned about."
The other thing too is that he thermally stresses the amp and does the waveform analysis when maximally thermally stessing the amp, something he does not routinely do for other products. That seems a bit unfair and I wonder if he has some chip about Krell that we don't know about.
He concludes that one should not partner the amp with speakers less than 4 Ohms (I conclude nominal impedance) but in my experience all is well.
Music does not stress an amplifier the same way as Atkinson does in the lab. And while I admire some of the punishing tests he puts SOME, NOT ALL amps through, readers will also remember that he admits that some of the tests he performs requires that he stand well away from any amp he puts them through some of these tortuous experiments. Gee I wonder why and what does that tell you?
I'll bet Atkinson has seen his share of dazzling fireworks.
The 400xi could probably use a bit more heat sinking, but under "normal" listening conditions (I mean are you guys risking your hearing in excess of 90 dB routinely???) and even a tougher load, this amp seems to have no issues. I'm not gonna lose sleep over this but instead enjoy how great it sounds. And great it does sound!
Cheers!
Steve