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Volume 8 No. 2

Krell KSA50 Amplifier Revisited
Martin Colloms tries out Krell's Classic 1983 Class A Reference Power Amplifier.
Review By Martin Colloms

Krell KSA50 Amplifier


  Experienced audio engineer Dan D'Agostino, along with his wife Rondi, started Krell back in 1980. His first product was the very substantial KSA100, which decorated a natural alloy case with massive handles and gold-plated flatheaded screws, and featured full Class A operation with constant output bias current over the power range. This was followed in 1982 by the smaller and more affordable KSA50, of comparable finish and build quality. I owned and enjoyed one of these fine transistor amplifiers in the 1980s, and its ability to drive a pair of Magneplanar MG3as with their delightful full height pure aluminium ribbon tweeters was especially memorable.

Some months ago Jon Honeyball lent HIFICRITIC a recently reconditioned KSA50, and for the first time I decided to measure the standing power draw from the mains. This came out at 339W (456VA), which is sufficient (with some to spare) to meet true Class A conditions for two 8ohms 50W channels (no lies; no pack drill).

An issue I wanted to confront during this reassessment concerns my sound quality scoring system. Back then it was limited to a 10 point scale: 4 was considered near useless; 9.9 was near perfect; and the good stuff tended to be found in a narrow band between 6.5 and 8.5. I now wanted to try and work out how that scaling could be related to my current open-ended arrangement which puts fine amplifiers at above 50 points. How would the old, if recently re-capacitored example rate on the present basis? It was certain to be above my historic, 'capped' 1983 score of a reference level 9.7.

A brief lab test was carried out to make sure the amplifier was in fine working order. I measured 95W plus 95W into 8 ohms, 170W plus 170W into 4 ohms; both channels showed fairly low distortion and noise at -76dB, and the distortion was desirably low harmonic order, as should be expected from Class A operation. The frequency response was flat and a very wide -0.05dB at 10Hz and -0.5dB by 73kHz, and channel balance was accurate. High frequency intermodulation was also very good at -82dB for 1W and -70dB at full power. These were very similar to my original results for a different sample.

After running in for a few days, the Avel Lindberg toroidal transformer was almost silent. The amplifier was well warmed up for critical comparisons, and was certainly no disgrace. The still quiet fan blew downwards so the hot part of the heatsink underneath the unit was at about 50ºC.

Krell KSA50 AmplifierFirst impressions were of generous and easy power delivery with a nicely weighted bass line and a smooth and well integrated treble. (Many older amps can sound a bit rough by today's standards.) Sibilants were clean and well focused, and transparency and image depth were both quite good. With time we became more aware of a faintly 'processed' treble quality, and some more general compression of dynamics and expression. When compared with more recent contenders, bass rhythms tended to be somewhat slower and more downbeat.

Conversely, the bass remains powerful and well extended, giving a fine sense of scale, and the sound quality remained stable with varying volume levels and different degrees of musical complexity, inspiring confidence. There was no significant roughness or related distortion.

All in all, the score on the current 'open' scale would be a creditable 25 marks, and the overall procedure felt a bit like reviewing an original Jaguar E-type. On this evidence at least, amplifiers have unquestionably become better over the last 30 years but this classic remains a worthy example of the art, and continues to demonstrate commendable longevity.

Thanks to Jon Honeyball for lending us his original Krell KSA50 .




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