Counterpoint Pure Development NPS-400 energy amplifier

This review was first published in March 1995

Amplifier designers have often sought ways to combine the advantages of tubes with those of solid-state devices. Over the years hybrid designs of different strips have emerged, most of which use tube input stages with transistor output stages, eliminating the main weaknesses of tube amplifiers: e.g. B. They run hot, are unreliable because tubes have a finite life and are more expensive than a similarly powerful solid state design and have power hungry output tubes and output transformers.

Counterpoint Electronics has long been a leading proponent of this design approach. In 1992, they introduced their new Natural Progression mono power amplifiers, which build on the hybrid design topologies they had long used in their previous amplifier designs. Some of that technology has now been filtered into a pair of lower-cost natural progression stereo power amplifiers: the NPS-200A and the NPS-400A.

The imposing NPS-400A ($ 4,395) is big, heavy (65 pounds), and clearly well built. The front panel has a single two position power switch and a single LED indicator. This LED indicates either standby (red) or operation (green) and flashes during the transition from one to the other. In standby mode, the B + supply (high voltage supply) of the pipe heating is switched off. However, the pipe heaters remain on to reduce the heat cycle. The output stage remains switched on in standby mode.

On the back there are balanced and unbalanced inputs for both channels (each with a balanced / unbalanced selector switch) as well as an input muting switch (useful for swapping input cables without having to switch off the amplifier). There is also a switch that selects bridged operation and two pairs of five-way binding post outputs per channel (for double wiring, if desired). The high quality binding posts are finger tightening type. I strongly prefer hexagonal posts that can be used with a screwdriver (for audiophiles who can’t resist the urge to tighten beyond their wits). For each channel there are output protection fuses (together with internal fuses of the power supply rail) and a mains line fuse in the connection for the removable mains cable.

Inside there is a massive 2 kVA power transformer with eight large filter caps for the power supply at the front and in the middle. There are two vacuum tubes per channel (a 6DJ8 triode for amplification and a 6CA4 rectifier, which is used to power the 6DJ8’s high voltage) and eight solid-state output devices.

The NPS-400 Natural Progression design uses a bipolar transistor output stage with high current gain, high input impedance and not the IGBTs or insulated gate bipolar transistors of the Natural Progression Mono. However, the vacuum tube input stage uses the same configuration as the Monos ??, to quote Counterpoint, “a double triode configured as a common cathode differential amplifier with constant current bipolar cathode and anode loads”. If this doesn’t ring bells for you, don’t worry. The net goal is to reduce the input stage distortion, which should be so low that no loop feedback is required. In fact, the NPS-400 does not use any feedback at all in its gain stages [other than the output stage’s usual cathode-follower feedback—Ed.].

However, for the DC servo, a feedback loop is used to control the DC offset. This loop only contains the output stage where Counterpoint argues that the only significant DC offset errors are occurring.

sound
My thoughts about the NPS-400A remain ambiguous even after listening to it for a long time. It is only fair to say this in advance. While I usually try to use a maximum of two speakers (one primary and one alternative if necessary) for rating a power amplifier, with the counterpoint I used three. I also freely swapped the cables indicated in the “System” section.

The NPS-400A presented a large, expansive sound stage. The choir on Laudate (Proprius PRCD 9100) had a bloom with open textures and a convincing sense of space. The amp’s sound had life and detail, with a reasonably neutral, if only slightly forward, perspective. The counterpoint was also able to perform excellently on simply scratched material. In particular, the vocals via the Dunlavy SC-IVs and the Energy Veritas 2.8s had a spatial quality. The precision of the soundstage, especially the Dunlavys, was superb laterally, with a good, if not exceptional, sense of depth.

The counterpoint was not lacking in power either. While its bass quality varied from speaker to speaker, on the Energys, its size and weight were exceptional at best. It definitely powered a space of material with a strong bottom end. The Counterpoint’s bass effect was often uncomfortably good, but surprisingly less obvious with the Dunlavys. In fact, the operational word for the bass through the Dunlavys was “tight” – even a little lean (the Dunlavys sound tighter and slimmer in my large listening room than in Robert Deutsch’s smaller one). The same was true for the midbass, which was fuller and richer in the Energys, but less defined than in the Dunlavys.

However, during all of this, a slightly dry top end bothered me? It was almost grainy, though that term might make too much of the problem. Wasn’t this dryness always noticeable? Often I could just go with the flow. While the counterpoint was clearly doing more right than wrong, the drought bothered me often enough that I couldn’t ignore it; When I started comparing the NPS-400A to other amps, that dryness turned into an itch that I couldn’t quite scratch.

With the Mirage M-7sis, the hard percussion with the counterpoint sounded very slightly tangy – no real brightness (which is in the low to medium highs), but an aberration whose frequency seemed to be a bit higher. Highlights like those on Laudate turned a bit silvery. The top end of the Krell KSA-300S was noticeably cleaner and more fluid, with less stress on the tips and a more natural rendering of the ambience. The more expensive Krell has undoubtedly prevailed in this comparison.

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