Balanced Audio Technology VK-5 preamplifier & VK-60/75 power amplifiers Page 5
I compared balanced and single-ended connections between the VK-5 and the VK-60, adjusting the volume control to match levels to within ±0.1dB. (The balanced connection was retained between the SFD-2 Mk.II and the VK-5.) The main test piece was "Winter Wonderland," track 13 on the Big Band Basie CD (Reference RR-63CD), and parts of the Carmen suite from the PopeMusic sampler. Tonal balance, harmonic accuracy, depth, and detail were very similar, but some of the dynamic punch was attenuated with the single-ended connection. There was no noticeable difference in noise level. (That is, once the ground was lifted. You didn't miss that bit under "System & Setup," did you?)
The performance with the single-ended connection was more than acceptable, but, given a choice, I can't imagine why anyone would want to listen to these components that way. As a check on the VK-5's interface with single-ended amps (as opposed to a balanced amp driven in the single-ended mode), I hooked up the VK-5 to the YBA-1 Alpha and the Acurus A150 (watch for reviews in Stereophile and the Stereophile Guide to Home Theater, respectively), and found no undue noise or other compatibility problems.
Although most of my listening was to a single VK-60 configured as a stereo amplifier, I also spent some time listening to a pair of them, each amp bridged to produce 120W rather than the 60Wpc of the stereo configuration. For anyone considering BAT amplification, this is an important comparison: the extra 3dB of power will cost you an additional $4495. Also, audiophile lore has it that while power is increased by bridging, sound quality tends to be degraded, due, perhaps to the doubling of output impedance. Putting aside the matter of cost—and, of course, most people can't—does a pair of VK-60s perform any better than a single one?
In a word, yes. The most clearcut difference is in noise level. A single VK-60 has very little noise by tube amplifier standards: in a quiet environment, without music playing, I could just hear a bit of hum/buzz from the speakers when sitting in my listening chair. The noise was low enough that it was masked even by music played very softly, so I wasn't really concerned, but it was there. However, the bridged VK-60s were so quiet that I had to put my ear within inches of the tweeter and the midrange to hear any noise at all. (Of course, this means that the VK-5 must also be very quiet. In fact, muting the VK-5—which shorts its outputs—had no audible effect on the noise level.) Apparently, this is one of the benefits of the type of bridging used by BAT: much of the noise "cancels out."
A power increase is expected to bring with it a greater potential for high-level dynamics, and it did. For me, a single VK-60 has more than enough power to drive the 91dB-sensitive Dunlavy SC-IVs to higher-than-comfortable levels—most of the time. At let's-see-how-loud-she'll-play level, a single VK-60 does start to run out of steam, and the extra 3dB of headroom makes a difference. Even at levels below clipping, the monoblocked VK-60s had a little more, well, authority. The low bass seemed just a bit firmer and more extended. There was no increase in grain or any other untoward sonic effect.
Using a pair of VK-60s does have a potentially problematic side effect: heat. Each amplifier dissipates a lot of heat; spend time with a pair of them in a small room without air conditioning during the summer (which describes my situation), and pretty soon you may be yearning for a class-AB solid-state amp. Other than that—and the cost—there's no downside.
Up to now, my description of the VK-5 and VK-60's sonic characteristics involved implicit comparisons with live music (or, to be precise, my memory thereof) in the context of a general knowledge of what's available in high-end audio. Now it's time to make some explicit comparisons with other equipment (footnote 6).
To compare with the VK-5, I had on hand two preamps considered to be among the best: the Sonic Frontiers SFL-2 and the Convergent Audio Technology SL-1 Signature. The SFL-2, given a rave review by Russ Novak (Vol.17 No.11), is a fully balanced line-stage tube preamp that Dick Olsher, Bob Harley, and I have used as a reference. I've used it while reviewing several amps and have found it to be an excellent preamp, deserving of the praise it has received.
Sonic Frontiers: As good as the SFL-2 was, I felt that, in matched-level comparisons as well as in long-term listening, it was bettered by the VK-5. Listened to on its own, the SFL-2 sounded very good indeed: smooth, wide-ranging, good sense of space, no obvious grain or harshness. However, changing to the VK-5 resulted in the music opening up in a way that was quite exciting and, I think, closer to what was on the record.
Fig.6 Boulder 2050, HF intermodulation spectrum, DC-24kHz, 19+20kHz at 599W into 4 ohms (linear frequency scale).