B&K ST-140 power amplifier John Atkinson 1990
I got hold of the sample of the B&K ST-140 also reviewed by Guy Lemcoe last month (S/N 5506), which had been purchased by Stereophile. This amp has been highly recommended in the past and offers a benchmark performance at the NAD's approximate price level (though the latest version is somewhat different from the previous sample owned by the magazine).
The system I used with this amplifier is hardly representative of one in which it'll be used, but as I'm familiar with the sound of every component, I felt it would best reveal any differences. Source was the Kinergetics KCD-40 CD player that I review elsewhere in this issue; preamp was the Mark Levinson No.26, and speakers were Celestion SL700s single-wired with Monster M1. Interconnect between CD player and preamp was AudioQuest Lapis Hyperlitz, and Madrigal HPC between preamp and power amp. (I fitted each amp with Camac-to-phono adaptors so that I could plug and unplug the interconnects with impunity, the Camacs making the ground connection first.)
The two recordings mainly used for the comparative listening were Jennifer Warnes's duet with Rob Wasserman, "Ballad of the Runaway Horse," from the album Duets (MCA MCAD 42131), and the aria "Ah! Stigie Larve" from the Harmonia Mundi USA album Arias for Senesino (HMC 905183) (footnote 1).
For direct comparisons, levels were matched at 1kHz. Maximum continuous levels reached 10W or so (on an 8 ohm basis).
The B&K's sound was characterized by a rather lumpy upper bass with considerably less low-bass extension than I'm used to with the SL700s. Jennifer Warnes's voice had good body, however, with, as the Audio Anarchist is fond of saying, a "palpable" presence. This was offset to a considerable degree by an exaggerated lispiness to her voice. This recording does accentuate her sibilants, in my opinion, but via the B&K they came over as "sssh" rather than "sssss." This treble signature is, I assume, the "MOSFET mist" that Sam Tellig has always felt to be part of the ST-140's sound. (It is something that can be reduced in my experience with MOSFET amps, mainly the original Hafler DH200, by increasing the standing bias in the output stage, though with the very high rail voltages here, this could thermally stress the devices.)
On the Handel track, there seemed to be a slight accentuation of tape hiss, but the countertenor voice was nicely differentiated from the accompanying instruments. To sum up the B&K ST-140's sound, it has a quite musical, if untidy, presentation and represents good value at the price (within its measured output limitations into low-impedance loads (see sidebar).
I must admit to having a problem with Stereophile's continued enthusiastic recommendation of the ST-140, given the fact that the version now being produced by B&K differs in major ways from our original sample. The circuit appears to be the same, with its fully differential input circuit and a single pair of complementary Hitachi output MOSFETs per channel (though these are now mounted on the completely new printed circuit board rather than being connected to the driver transistors via twisted-pair wiring). An important difference, in that I have nearly always found it to make a subjective improvement, is that there now isn't a series output inductor to guard against potential instability with highly capacitive loads. (B&K states that the ST-140 is unconditionally stable without the inductor.)
The input still has to pass through a series electrolytic capacitor, but LEDs now appear to provide DC biasing rather than the strings of silicon diodes used in our earlier sample (SN 00755). The physical construction is totally different, as is the power supply, which now features more extensive star-grounding, different reservoir caps, and a toroidal transformer. The main change, however, is the raising of the voltage rails from ±49V to ±60V without any difference other than reducing the output-stage bias current from 160mA to 140mA. (B&K's John Beyer tells me that this change occurred in 1985.) This raises the specified clipping power into 8 ohms from 70W to 105W, but renders the amplifier much less suitable for driving speakers whose impedance drops much below 8 ohms (as do many speakers).
Further listening will be essential before I compile the next "Recommended Components," but I have to say that I think B&K were wrong to make this power-output change in what is such a highly regarded component—it renders the ST-140 much less compatible with real-life loudspeakers.—John Atkinson
Footnote 1: For those who insist that naturally recorded acoustic music presents components with a harder task and should therefore be more revealing of subjective differences—see J. Gordon Holt's letter in this issue arguing exactly this point—of these two recordings, the totally artificial Duets was the better at revealing the tonal differences between the amplifiers, while the naturally recorded Handel was the better at revealing soundstaging differences. Make of that what you will, gentle readers.—John Atkinson