Apogee Stage & Mini-Grand loudspeakers Mini-Grand part 5
The subwoofer amplifier was amenable to change: In place of the KSA-300S, I substituted the Hafler Trans-Nova 9500, rated at 375Wpc into 4 ohms (vs the Krell's 600Wpc into the same impedance). I had to reset the DAX for unbalanced inputs and outputs (the Hafler only has single-ended inputs). The subwoofer level on the DAX was reset to account for the Hafler's different input sensitivity.
The Mini-Grands' overload margin greatly improved with the change in amplifiers. Some material was still out of bounds—"Rhinefalls" from Staccato 2 and the drum on Tropic Affair, each of which is problematic for many loudspeakers—but some was significantly improved. Däfos played considerably louder though still required some caution. The other recordings mentioned above sailed through without a hiccup. The pumping continued on the same recordings as before; though it may have been slightly less noticeable, I was beginning to feel that some sort of infrasonic filtering (perhaps in the DAX) might be appropriate, and might well cure the last vestiges of subwoofer overload. But with the Hafler, the bass finally had a more than acceptable dynamic range.
Interestingly, the previous overload had been a combination of what seemed to be woofer bottoming and port noise (a "chuffing" sound). Although the latter is not truly overload, it can be confused with it. With the Hafler in place, the bottoming was reduced significantly and the "chuffing" virtually disappeared. These two observations are both consistent with reduced workload on the woofers.
With respect to bass quality, however, there were some minor sacrifices with the Hafler. The bass was a bit less punchy, tight, and defined than with the Krell. The effect was not pronounced, however, and could easily be lived with—especially when you consider that the money you'd save with the Hafler would almost cover the price difference between the Mini-Grands and the larger Studio Grands. Of course, with the Studio Grands you might find the Krell's somewhat better bass performance more desirable. Who said life was easy? But for most of us who can't afford such options, the Hafler, in addition to putting less stress on the subwoofers, does a perfectly fine job driving them.
What about using less-expensive amplification all around? I had another Hafler amp on hand—a Trans-Nova 9300. Apart from the fact that the 9300 and 9500 have different input sensitivities which require a bit more jiggering around with the DAX's level controls, hooking up the Mini-Grand with all-Hafler amplification was no big production. Here I was also able to use Cardas Hexlink all around, as four short, unbalanced lengths of it were available to run between the DAX and the amps.
The result was unsurprisingly effective, considering what we already know about the Haflers' performance. The main changes from the Krells: the sound was a little more forward, somewhat less divorced from the loudspeakers' locations; the sense of depth was less alarmingly real, but still effective; the top end was a bit drier and less liquid; and the bottom end, to repeat what I noted above, wasn't quite as tight and punchy. Individually, none of these differences is worthy of more than a passing comment. Collectively they add up to a somewhat less compelling performance.
That said, I have to note that the savings—about $15k—will buy you a packed-to-the-gills, completely tricked-out economy car, or, again, a complete Apogee Studio Grand. The Krell—particularly on the top end—is worth the difference for those able to pay for the added performance. But I wouldn't hock the Mercedes or auction off the mink for it. I loved the performance of the Mini-Grands (bass power handling, but not bass quality, excepted) with the Krells; I could live happily with the Haflers. And there are other cost-effective options as well. I haven't heard the complete Mini-Grand system driven with Aragon 4004 Mk.IIs, but I suspect, based on my experience with that amplifier driving the Stages, that it would be a good combination. And while we haven't yet tried the new Classé amps with the Stages, we found that earlier Classé models worked well with them. In this case, however, our usual caution about careful amplifier selection goes double, to ensure that the chosen subwoofer amp, in particular, won't overdrive the Mini-Grand's subwoofer.
I believe that good old-fashioned infrasonic filtration—a single, small change—would greatly increase the Mini-Grand's practicality. Such filtration is merely good design practice with ported systems using boosted low-frequency response, yet my measurements of the DAX show no evidence of rolloff down to the 10Hz lower limit of our Audio Precision test set. A sharp rolloff below, say, 15-20Hz would most certainly increase the Mini-Grand's output capability and would probably eliminate the last vestiges of overloading at anything like a reasonable playback level.
Footnote 4: Reflex loading leaves the drivers unloaded below the tuning frequency.—Thomas J. Norton