Posted at http://audioasylum.com by AbeCollins (A) on September 2004

      Krell model KSA-50S 50-W/Ch Power Amplifier
      

   The 'baby Krell' KSA-50S was Krell's entry model in the early 1990's but it was soon discontinued because most of Krell's customer market segment were demanding the larger upscale models like the Krell KSA-100S, KSA-200S, and KSA-300S.  The 'baby Krell' is still a fairly hefty amp at 52 Lbs and although 50 watts per channel may not seem like a lot of power, its a hefty 50wpc (at 8 ohms) doubling to 100wpc into 4 ohms, doubling again to 200wpc at 2 ohms, and 400wpc at 1 ohm.  Many amps aren't stable into such low impedence speaker loads but the Krell continues to pump out the power.  Some professional reviewers consider the 'baby Krell' to be one of the company's finest ever made.  With present audiophile class Krell models starting around $5000 to $20,000 or more, its nice to know that older models can still be found in great shape at reasonable prices.

    Well, I posted before about my itch to try the Krell KSA-50S in my system and I finally had my chance today. To summarize, I love the way this 'little' Krell sounds so far.... sweet, transparent, fast, with excellent bass depth, detail, and snap. More later.

    Someone commented that the KSA-50S is not 'true class A' biased like the older KSA-50. Well it is but it's done more intelligently with something Krell calls Sustained Plateau Biasing. And I like it because the unit doesn't draw huge amounts of power constantly like most basic full class A biased designs (like my old Pass Aleph that I once owned). Instead, it runs barely warm at idle but heats up and draws lots of power only while you're actually using the amp. When the signal goes away, the amp automatically goes back to a cool running standby mode.

    I took some measurements while obseriving the Sustained Plateau Bias feature in action. It's not a gimmick. It's practical and I like the way it works. There are three modes: Standby, Bias Level 1, and Bias Level 2. The LED indicators on the front panel tell you which Bias level you're operating at.

    The amp remains in standby mode with no signal present or with a very low-level signal. Unlike most standby modes, the amp will actually pass a signal and play music at low volume levels in this mode. I wouldn't normally listen at this level but might use it for very soft background music in the evenings while reading or working on the computer. In standby mode the Krell KSA-50S draws about 75-watts continuous from the AC mains as measured with my inline AC watt meter.

    At moderate to loud (normal) listening levels the KSA-50S steps up to Bias Level 1. At this point the amp is drawing around 625-watts continous with lots of effortless volume with excellent deep bass and transient attack. If I mute the source, the amp continues to draw 625-watts at Bias Level 1 for about 20 seconds before it goes back down to standby mode. 625-watts continuous is a lot of power consumption indicating a strong class A bias. It maintains this level for a 'sustained' period of time even with the signal removed eventually dropping down a notch.

    At high volume levels the KSA-50S will step up to Bias Level 2. At this point the amp is drawing about 850-watts continuous. When the signal is muted, the amp continues to draw 850-watts for about 20-seconds before it steps down.

    I'm no rocket scientist but it appears to me that a 50-wpc amp drawing 625-watts to 850-watts continuous while in normal operation must be heavily class A biased. It drops to 75-watts continous with no signal or very little signal present. Otherwise it draws a lot of power and runs very hot for a mere 50wpc amp. I like it... runs barely warm when in standby mode and runs true class A sizzling hot while enjoying music at realistic volume levels. I can leave the amp on 24/7 without worrying about the heat or the electic bill. And when the music starts to play, the amp automatically goes into its 'power hungry' class A mode.

Ok, enough of the techie stuff. How does this thing compare to the other amps I've owned? In short, very favorably.

    I love the sound of vacuum tube electronics.  You know, those glass bulbs that glow in the dark that your grandfather would test at the local drugstore.  If one didn't pass on the tube tester, he would buy a replacement.   Vacuum tubes have their own special magic but as hard as I try, I cannot get enough bass with great control in an affordable tube amp. Perhaps with a monster pair of tube monoblocks but not the 50 to 100wpc that I can afford.

    I own a very nice pair of AES/Cary SixPac tube monoblocks. They run 50-wpc in triode mode using 6 EL-34 tubes each. This is a very transparent sounding pair with good bass (better than my previous CJ Premier 11a). The SixPacs are wonderful especially with vocals and the shimmer and decay in the treble is outstanding without being bright or annoying. Cymbals sound very impressive here. There is also something special about the 3 dimensional soundstage from the AES/Cary SixPacs - better than any solid-state amp I have experienced so far and even better than the CJ Premier 11a or the ARC VT100mkII I once owned. I just wish the little SixPacs had more SLAM and SNAP.

    At the other extreme, I have the 'digital' HCA-2 stereo amp from PS Audio. This thing runs stone cold, draws very little AC power, and it packs more bottom end punch (slam and snap, remember?) than the SixPac tube monoblocks. But... it doesn't have near the 3 dimensional soundstage or the transparency. And although the bass is impressive from such a compact package, it's not quite a well defined or 'fast' as I like. The bass is there and it's good but it's not as detailed and dynamic as I would like. Yes, it is better than the SixPac tube monoblocks but at the cost of soundstage and overall transparency. It's also not as extended in the treble as the SixPacs. A different amp for a different application. I use the HCA-2 mostly for rock and home theater impact.

Ok now for the Krell KSA-50S. This amp is also a bit on the sweet side, not dry or clinical and for a mere 50wpc it really rocks! Thats what I said about the Pass Aleph 3 (at only 30wpc) but the Krell KSA-50S is better to my ears. My room isn't huge and my speakers are fairly modest with 8" drivers 6 Ohms nominal and 91db sensitivity.

    The KSA-50S is more transparent than the PS Audio HCA-2. It is more 'effortless' and less congested at higher volume levels and the bass region is fast and more distinct with excellent snap and slam. The Krell is also more transparent with better slam than the Pass Aleph 3. I find that I can play the KSA-50S at higher volume levels than any of the other amps I've owned while still maintaining great clarity with less congestion and it's like the amp has plenty of reserve or 'dyanmic headroom' if thats what its called. The treble is also more extended than the PS Audio HCA-2 or the Pass Labs Aleph 3.

    The shimmer and decay in the cymbals is still a bit better from the AES/Cary SixPac tube amps and the tube amps also present a better soundstage. But overall I consider the KSA-50S to be outstanding and so far the best solid-state I've ever owned. (I've also had several Classe including the fairly large CA-300, BAT VK200, Pass Labs Aleph 3, PS Audio HCA-2, and a couple integrated amps not in the same league).

    I'm not sure how the newer Krells sound but they're way out of my price range. And I've been told that the KSA-50S is one of the better older models. It's getting up there in years but still very impressive.

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