Nagra VPA monoblock power amplifier Page 2
Now's the time to check the VPA's Load Meter for the best impedance match with your speaker. If you've chosen correctly (4, 8, or 16 ohms), and your speakers have a constant impedance with respect to frequency, the two pointers should be superimposed while playing music. If the red pointer leads the black, the load impedance is too high (an 8 ohm speaker connected to the 16 ohm tap, for example). The JMlab Utopia is a nominal 6 ohm speaker, and the meter clearly showed that the VPA's transfer function was best on the 4 ohm tap. During Immense Musical Moments the red pointers still led the black, but most often they were in sync. As with the PL-P, Nagra makes it easy to optimize signal throughput.
If you use other than Nagra-supplied tubes, use the potentiometers to adjust the 845s for the same cathode current. But I suspect that most customers will be getting factory-matched tubes from their dealers and won't be fiddling with bias. The VPAs operate without negative feedback on the output stage, which makes them very tolerant, Nagra claims, to variations in impedance due to frequency.
The main function selector on the front panel is a three-way rotary switch that feels "Nagraluxious" to manipulate, as Kathleen put it. Engineered for the field, the VPA is fitted with two mains fuses: one on the AC supply and another on the neutral return.
Circuit and transformers
The VPA's circuit is balanced from input to output. Nagra reminds us that a high-quality balanced input stage offers better isolation from noise than an unbalanced one. This is especially true, they point out, with long runs of interconnect "and when in the presence of a ground loop with associated current circulation." That's why Nagra chose a balanced input for the VPA rather than the single-ended topology found on their own PL-P. Does that makes sense? Manual: "Designing a high-quality symmetrical output is an extremely difficult task that increases the cost of the output stage, offers no audible advantages, and no technical advantages. For these reasons, the outputs of the PL-P (like the majority of preamplifiers) uses an asymmetrical output through high-quality gold-plated RCA connectors." Okay, that makes sense.
Tucked away neatly within the case, one ECC82 (12AU7) and one ECC83 (12AX7) per chassis serve as input and driver, respectively. The VPA sports a pure class-A push-pull output stage good for up to 50Wpc, unburdened by feedback on the tubes or transformers. "The advantage of such an output stage [is that it] allows the VPA to attack a wide range of loads (capacitive, inductive, or varying resistance) without the risk of oscillation or intermodulation. A short-circuit or accidentally disconnecting the speaker terminals for a short duration will not damage the VPA in any way." Good news for klutzy reviewers such as I.
The gapless-toroid output transformer was designed by Nagra and allows a low-frequency audio signal of 40Hz to be reproduced at full power, "when generally other vacuum tube amplifiers would only be able to manage about 50% of their rated full power at such low frequencies." Well, if you've got it, flaunt it, I always say. The transformers are mounted to one side panel with insulating washers. "High-frequency parasitic capacitance given off by the output transformer is very low despite the high voltage necessary to give a smooth frequency response up to 100kHz."
"Give me the luxuries of life and I will willingly do without the necessities."—Frank Lloyd Wright
The VPAs are unambiguous in their presentation: Their sonic qualities wrap around the listener in an exciting, virtual-reality kind of way.
The transparency was astounding, of such a quality as I've thus far enjoyed only from solid-state YBA components. But there was something uniquely appealing about the way tubes re-created tonal color, air, and space. Those big 845s, as implemented here, manifested a soundstage of such air and aliveness that on a few occasions it truly took my breath away.
The imaging was extremely holographic without being flashy in any particular way—the presentation was always totally, utterly palpable and complete. The separation of the performance from the background acoustic made it ultradimensional in every respect. Transparency is important in this regard; the totality of the original acoustic seemed transplanted to our listening room. Analog, as usual, was slightly more bloomy, and larger in overall aspect, than digital. But while listening to the dCS 972/Elgar combination, I didn't complain a "bit."