Krell KST-100

KRELL KST-100 AMPLIFIER

Up for sale is my personal Krell KST-100 Class A/B stereo power amplifier. It is in pristine condition and was given a factory refreshing and a new output transistors and OEM toroidal transformer (over $400 alone) in June 2018, less than a year ago. Normally, interior photos of equipment are less than illuminating for most audiophiles who are not engineers. But Class A and Class A/B amps can run very hot and cause damage to the interior components if not properly ventilated. Some evidence of this can be evidenced by the purpling of heat sinks but more telling and important are the interior boards themselves which should appear bright green. Heat damaged boards are brown on the edges. Look at the attached photos and you will see none of that whatsoever. In fact, contact Krell and speak with their support and repair staff and they will confirm what is said here. This amp will be good for 15 to 20 years at least and can power easily the most demanding of electrostatics and planar speakers. It is conservatively rated at 100 wpc into 8Ω, 200 wpc into 4Ω, 400 wpc into 2Ω and will again double its output into loads as low as 1Ω. Also understand half of each of those ratings is delivered in Class A power. Even with my Magneplanar 3.6R high current and high power hungry planar speakers (4Ω) ever drove this amazing tank of an amp past the Class A power production capabilities – in short I do not believe I have ever heard this amp in anything other than Class A!

It is filled with the legendary Krell over-build quality and design. It is virtually wireless inside using a stacked method of construction around the massive near softball sized toroidal transformer. It weighs in at 64 pounds but can be relatively easily handled by one person as it has two (2) sets of handles, one on the front panel finished in the industrial Krell style and anthracite grey color (indicating it is a newer model, original KSTs were all black) and another set at the rear of the unit. The manual and bench testing sheets before the factory refreshing, toroidal transformer and bench testing after completion are included.  Set forth below is a more technical review and analysis of this high current high output silent industrial strength monster from Krell Industries.

The KST-100 is quite compact by Krell standards, its heatsinks are contained within the overall width of the alloy front panel. Described as class-A to half power (more on this later), it is quite powerful at 100Wpc into 8 ohms (20dBW); ie, 50W 8 ohms class-A, with the specified power doubling into successive halvings of that impedance; with 800W quoted into 1 ohm. Simple removal of the top cover provides access to a mono switch which allows the amplifier to perform in unbalanced bridge mode, working as a monoblock delivering 400W into 8 ohms and 800W into 4 ohms (both equivalent to 26dBW, footnote 1). Given the past history of Krell products being able to drive low-level impedances, these figures are believable, if not understated.

Power connection to the KST is via an IEC AC plug, while both phono and XLR input terminals are provided. In unbalanced mode, small shorting pins are provided for the unused XLR signal polarity. Five-way binding posts are provided for speaker connection, their hex heads allowing the use of a wrench for good contact. Two sets are present, to aid bi-wiring arrangements. The only front-panel control is the on/off rocker, a circuit-breaker type included in the amplifier protection system.

Design Notes
The KST uses a single central toroidal transformer feeding separate rectifiers and reservoir capacitors for each channel. By normal standards, these are generously rated, this necessary in view of the 1-ohm load, 800Wpc rating. Eight high-current, TO3-can power transistors are used per output channel, arranged as complementary pairs, with two more employed as burst-proof emitter-follower drivers.

The design uses a fully complementary balanced circuit, symmetrical throughout, DC-coupled and DC–servo-controlled for output offset. Effective over-current, short-circuit, and overheating protection are provided.

At the input, cascades of differential FET and bipolar stages are used to provide a true balanced input of beneficially high-input impedance. Following the practice of the latest series, the KST-100 is essentially "wireless," with the amplifier cleverly integrated on one full-sized printed circuit board which integrates with the large-screw terminal power-supply reservoirs.

This build quality is first-rate, tank like and a claim to longevity is supported by the 5-year warranty.

Sound
First impressions were of a thoroughly up-to-date, low-feedback sound, one of greater delicacy, air, and transparency than the sound traditionally associated with solid-state electronics. The KST-100 does have a good proportion of Krell character: a firm, confident approach, highly stable imaging, and a strong, highly controlled bass. I rate Krell's KSA-150B highly, and found it to have much in common with the KST-100. In this context, it was a touch more dynamic and a shade clearer than the KSA-80. Given that the '80 came top in its class only a couple of years or so ago, this is high praise indeed for the KST-100.

Taking the stereo performance in more detail, it was close to today's best in image depth and the transmission of hall and stage acoustic. It could help define the performance of top digital replay systems, and sounded genuinely transparent. Direct comparison with the '80 showed the latter to have a mildly cloudy effect with a shortfall in stage depth. Here the KST has moved rather closer to the ARC Classic 120. Specific focus was also close to the best, held both in terms of width and depth. Stage width was virtually to the limit of available program, and only fractionally less than the best monoblock amplifiers in percentage terms.

Possessing a fine tonal balance (to be discussed separately), the KST-100 was also capable of a strong presentation of perspective, with convincing layering of players in large orchestral recordings. I do not believe height is a property of electronics which are inherently neutral; however, certain combinations of room acoustic, phase, and perceived system frequency response, particularly in the 2–8kHz range, can give rise to sensations of image height which may be enhanced by the accurate and transparent reproduction of recorded ambience. In this latter respect, the KST was very capable.

The KST-100 conveyed a dynamic, lively quality well beyond its class, remaining interesting and alive over long listening sessions. Dynamics do not only relate to inner vigor but to an overall feeling of power, specifically dynamic range. Technically, this can be described as the range in dB between the inherent background noise and the maximum level possible with a given loudspeaker load. Subjectively, this also concerns an amplifier's ability to reproduce low- and moderate-level detail while being exercised by powerful ones, and its ability to sound unchanged and unstrained when crescendi arrive.

The power amplifier passed this test with traditional Krell imperturbability. The KST's bass was taut, agile, tuneful, and had good extension; I doubt that there is a class rival for bass precision. The broad midrange was highly neutral, in the KSA-150 mold, and this continued way into the treble register. This formed the foundation for its ability to portray perspectives. The mids sounded easy on the ear, very low in solid-state artifacts. Glare, hardness, etc. were all at very low levels or sub liminal.

This is a sensitive area. One's perception of this anomaly depends on many factors, including the treble quality and smoothness of one's chosen speakers, the "cleanness" of the sound of the system cables, and finally the preceding audio chain, both source and control unit. Cables with sweet treble and speakers without any lift in the high treble are advised for use with the KST-100.

Conclusion
In creating the KST-100, Krell has succeeded in trimming the fat from the audiophile KSA series. This recent introduction is definitely not a cheap version of the old KSA-80; rather, it is a cost-effective cousin of the new KSA-150. In absolute terms it equals the sonic performance of the KSA-80 and has a comparable power-output delivery, though it did not sound quite the same. Here you have to appreciate the class leading (exceeding) performance in terms of the bass speed of the KST and the lively, quick, detailed midrange of the KST and its high treble performance.

Given a wise choice in the matching system—for example, a clean-sounding analog cartridge or digital source, plus a speaker without any treble fizz—I would rate the KST highly, undeniably a class leader. Where this standard is required at a much higher power level, remember the facility for the use of a pair of KST-100s as 400W/8 ohm monoblocks. These would be leaders in their power class too.

With the option of balanced and unbalanced working, excellent finish, durable build quality, plus the five-year transferrable warranty, the KST-100 represents a worthy audio investment.

Measurements

Rated at a straight 100W, 20dBW into 8 ohms, and supplied with standard line voltage, the amplifier delivered a continuous 136W (21.35dBW) into 8 ohms, with a single channel driven. At both 4 and 8 ohms the power bandwidth was superb, with maximum falloff of only 0.25dB at the 20Hz and 20kHz frequency extremes. The flat-out maximum power (using a 10ms 1kHz toneburst) was 138Wpc into 8 ohms (21.4dBW) with 246W available into 4 ohms (20.9dBW) and 418W into 2 ohms (20.2dBW), comfortably meeting Krell's specification. This was backed by a peak-current capacity of 40A, sufficient for all but the most impossible speaker loadings. Krell claims an 800W short-term delivery into 1 ohm, and I can believe it!

Claimed at 0.125 ohms, my measured figure for output impedance was close at 0.15 ohms, rising to 0.2 ohms by 20kHz. This was where Krell's specified damping factor of 150 appears to be optimistic: 0.2 ohms for an 8 ohm load gives a damping factor of 8 divided by 0.2—40 in anyone's math. As a matter of interest, 0.125 ohms yields a figure of 64. The output impedance was more than sufficiently low regardless of the claims of yesteryear—I remember 4000 being claimed for an early Phase Linear design. Nevertheless, with very low impedance speakers and a moderate cable impedance, a very slight interaction is theoretically possible with a speaker impedance which was not uniform with frequency. Its moderate output-impedance value is a consequence of the low overall feedback factor adopted for the KST-100 in keeping with the philosophy of the KSA-150.

Likewise, while full-power distortion figures were wholly satisfactory, they set no records. The specification indicated a 0.1%, –60dB figure "under all conditions," whatever that means. With a rated-power measurement of –63dB at low frequencies, worsening to –54dB by 20kHz, there was broad agreement with the spec. At 1W, a moderate volume cruise level, the harmonic distortion content had fallen to below –83dB over the audible range. A spectrum analysis showed that the distortion harmonics fell rapidly and progressively with order; eg, third at –65dB, fourth at –75dB, and fifth at –80dB with negligible high-order products.

The amplifier was checked for high-frequency intermodulation with an 8 ohm load, a drive of equal-amplitude 19 and 20kHz tones reaching peak level. Here the amplifier proved to be highly linear, delivering a 1kHz difference tone at –88dB, or 0.004%. This good behavior can be seen in the resultant spectrogram (fig.1). Even at 1W, an excellent –70dB spuriae figure was recorded on this test. Another modulation test concerns driving the amplifier to 2/3 rated level into a 4 ohm load, with a low-frequency signal of 37.5Hz, this harmonically related to the 50Hz UK line frequency. This test stresses the power supply and seeks to explore supply modulation appearing at the audio output. With fig.2 covering a dynamic range of 96dB (including some spectrum-analyzer noise-floor "grass") and a linear frequency range of 200Hz, no 50Hz-related negligible supply-modulation effects breakthrough can be seen, indicating excellent power-supply rejection and Channel balance held to a 0.02dB tolerance and DC offset at the output registered as 0mV; ie, less than the 1mV instrument sensitivity, a fine result. On frequency response, there was considerable low-frequency extension, to 1.2Hz, –0.5dB with a trace of treble rolloff, reaching –0.5dB at 18.5kHz; the –3dB points were at 1Hz and 52kHz. Using the DC mode of the Hewlett-Packard 3561A spectrum analyzer, the low-frequency response was plotted (fig.3), revealing an interesting but innocuous 2dB of lift at 2.2Hz before the final rolloff.

Easy to drive, the input impedance was virtually as claimed, measuring 48k ohms in parallel with 95pF of capacitance. A 1.6V input was required for program clip or maximum output, nearly 10dB above typical maximum levels from normal CD sources of 0.7V. This explains the 10dB of gain provided in the matching KSL and means that CD sources of normal 2V peak output level will not produce a loud enough level for direct connection. An input of 145mV was required for 1W output. Signal/noise ratios were fine, the "A"-weighted result of 84dB corresponding to a subjectively biased assessment while the 101dB unweighted reading confirmed the absence of electrical hum. (Both results are referenced to the KST-100 full rated output.)

Channel separation was checked, and in single-ended mode was satisfactory at 71dB (1kHz) and 55dB (20kHz). In balanced mode, just 45dB separation was measured at 20Hz, compared with 60dB unbalanced, this "averaged" result being partly to do with the command power-supply arrangements and the input balance circuitry which doubles as a phase splitter for monaural operation. In monoblock mode, with two KSTs used for stereo, each channel would now deliver a basic 400Wpc into 8 ohms, 800W into 4 (both 26dBW), with the recommendation that speaker loads known to rate lower than 5 ohms be avoided as the peak current limit of this powerful amplifier is approached. In this mode I can see the KST as a worthy choice for studio monitoring, where its more compact package and superior value make sense compared with the bigger Krells. The sound quality of the mono KST amplifier will leave most "professional" amplifiers for dead. A trial short circuit at half-power resulted in a harmless spark and successful protection operation. This is a fine set of results from a well-designed amplifier

Specifications

Description: stereo power amplifier. Output power: 100Wpc/8 ohms (20dBW), 200Wpc/4 ohms (20dBW), 400Wpc/2 ohms (20dBW), 800Wpc/1 ohm (20dBW). Output impedance: 0.125 ohms. Damping factor: 150. THD & IMD: less than 0.1% under all conditions. S/N ratio: 96dB A-weighted. Slew rate: 50V/µs. Input sensitivity: 1.5V for full output. Input impedance: 47k ohms.
Dimensions: 6.4" H by 18" W by 14.8" D.
Manufacturer: Krell Industries, Milford, CT 06460 (1991). Krell Industries, LLC, 45 Connair Road, Orange, CT 06477-3650 (2017). Tel: (203) 799-9954. Fax: (203) 799-9796. Web: www.krellonline.com.

The manual and original power cord are also included. The unit is boxed and ready for shipment. It weighs in at over 72 pounds fully packed. Shipping is a flat $125.00, buyer pays 3% Paypal fee (@$60). Offers considered.