Solid State Power Amp Reviews

For this modular audio component set—a review of the matching NSC 222 streamer preamp with a matching NPX 300 power supply is forthcoming—we first cover the standalone NAP 250 100Wpc stereo power amplifier, a new version of Naim's 200-series amplifier, now so extensively revised that it must be regarded as a new model.

Naim has long embraced two upgrade paths: the usual substitution route—replace an inferior component with a newer or better one—and progressive power-unit augmentation for components already installed. Setting aside for now the former route: Among many other changes, these Naim "New Classic"–series components may be more easily combined not only with other components from Naim but with items from other audio makers. This enhanced compatibility includes Naim legacy systems: All can now be mixed and matched more easily.

Naim Audio's enduring, compact 200-series stereo power amplifier has been relaunched as the NAP 250, a key component of their new 200 Series. It has been updated massively in its technology and the compatibility of its connections, yet the primary circuit concept, which dates to the mid-1970s, is essentially unchanged. Recent development work has focused on helping the amplifier match up well with the new NSC 222 streamer-preamp, the NAP 250's intended companion. The new 200-series power amplifier is now called, simply, NAP 250, dropping the "DR" of the previous model. But while the suffix is now omitted, the ultralow-noise DR technology has been retained in the active, fully regulated power supplies; indeed, it has been improved.

For comparison, I had to hand a very recent sample of the now-superseded NAP 250 DR.

If it's not broke, fix it anyway
Over the years, I have tested several iterations of the 250. The outgoing "DR" version of the NAP 250 performed well in my recent evaluation in HiFiCritic (Vol.16 No.1). However, to better match the design and connectivity concepts envisaged for the new 200-series system, numerous operational and system changes were applied. That engineering process was allowed to run and run, resulting in a major design overhaul, sufficient to merit a fresh appraisal.

Externally, the traditional, green-illuminated Naim logo is now instead illuminated white, which may prove disconcerting for some diehard Naim fans. The brightness may be adjusted to suit ambient light intensity on all the units via a system control app or a back-panel button, but green cannot be restored. The old, shallower, all-alloy profile has been replaced by a new one with flanking sections of thicker alloy with a polished, recessed center section in black acrylic, to better harmonize with the rest of the 200 series.

Traditional aluminum-alloy construction is maintained to avoid the possibility of ferromagnetic interaction of the active electronics with the casing. The old, nonstandard connection to the single unbalanced dual-channel XLR input has been supplanted by a pair of standard-format XLRs for the L and R channels. These are now fully balanced, with higher input impedance (47k ohms balanced, up from the original 18k ohms) so they draw less current from the signal source. With the appropriate cable termination, the NAP 250's balanced inputs may be driven from unbalanced sources thanks to the universal, self-balancing input-circuit topology. This connection, which Naim refers to as "legacy," has a somewhat lower input impedance, specified at 34k ohms, but that's still usefully higher than 18k ohms, the value on the previous "DR" version. Though output impedance isn't specified, if the specified damping factor (36 with an 8 ohm load) is accurate, it is about 0.2 ohms (footnote 1).

The loudspeaker output connections are unchanged from earlier models: Naim-standard, spaced 4mm sockets, for which Naim also makes an inexpensive, proprietary, reliable bipolar plug designed to ensure unambiguous polarity. Non-paired 4mm plugs may also be used. The NAP 250 incorporates eddy-current suppression at the loudspeaker output connections, in common with the reference-level Naim "Statement" power amplifier. In most amplifiers, these connections are made through a conductive-alloy back panel, which gives rise to a circulating current in the chassis-plate region between the + and – connectors. In Naim amplifiers, this minor issue is nulled by introducing a small horizontal "insulating" slot between the standard Naim connectors at the standard 19mm spacing.

Performance upgrades include increased 8 ohm–rated power, now a full 100Wpc into 8 ohms with both channels driven, rated very conservatively at 0.1% THD+N. Rather more power is available for lower impedance loads. The amplifier is now rated at 190Wpc (at 1% THD+N) into 4 ohms with both channels driven. A 2 ohm "flat-out" toneburst rating of 300Wpc (up from 250Wpc) is consistent with the increase in peak current to a specified 28A. This increase in headroom promises better overall linearity.

Continuously working beyond the rated power will cause a low-noise cooling fan to turn on, after that a temporary thermal shutdown. (Naim calls the amplifier "party-proof.") Reset is automatic. The amplifier runs cool at modest power, barely warm to the touch and better for the environment.

Naim designer Steve Sells explained to me that in light of the prevalence of high-definition digital audio, the 250's bandwidth specification has been extended; the frequency response is now specified at 1.4Hz–100kHz, –3dB. The previous NAP 250 DR was rated at a much narrower 20Hz–20kHz but also with narrower limits: –0.5dB. The specified voltage gain is now 29dB, or a factor of about 30 times. The input sensitivity remains quite high at 1V for full-rated output, comparable with earlier Naim product ranges. This moderately high sensitivity—higher than most amplifiers from American designers—makes the NAP 250 a better match with direct-connected, volume-controlled DACs that output a nominal 2V full scale.

The new NAP 250 also enjoys a power supply redesign. The chunky, 1kVA-rated toroidal core transformer is continued, but regulation is improved, with lower copper losses (thicker wire!). Revised thermal annealing after assembly is said to lower internal stresses. Reservoir capacitors with a higher working voltage are fitted; this is well known to lift performance by helping to reduce artefacts that can occur in these internally complex, wound foil, passive components; pure capacitance they are not!

One thing that hasn't changed is the long-established, quasi-complementary, all-NPN transistor output topology. Circuit enhancements include liberal use of discrete, multiple-transistor constant-current stages with improved supply-rail noise rejection. The entire circuit board has been relaid for more logical signal paths. Superior, low-capacitance, thermally conductive aluminum-nitride heatsink insulators, previously used on the output transistors, are now also fitted to the regulators. Their lower capacitance should reduce stray coupling to the chassis. The time constant chosen for output-stage biasing has been lowered to the infrasonic, aiding low-frequency phase response. Full regulation of the supply rails has been retained.

Because the amplifier is now fully balanced, the input stage comprises two Naim input gain stages per channel. This format is claimed to deliver "an unexpected improvement in sound quality," which Naim says is supplemented by the accompanying redesign of the low-level, DR-stabilized supply rails. ("DR" stands for "discrete regulator"; regulation is not carried out by an IC chip.) Those low-noise "DR" regulators are also deployed for the subsidiary power supply rails and serve to reduce the noisefloor.

A retuning of the overall feedback/stability margin is claimed to contribute to better sound quality, augmented by greater output-current headroom, useful for driving lower-impedance loads down to a newly specified 2 ohms minimum impedance. Also worth noting is the retention of the traditional 60dB channel separation specification, constant over the full audio bandwidth. More separation is technically possible—but would it make the sound worse? Naim argues that it would (footnote 2).

When installed in an all-Naim system—let us say with its partnering NSC 222 streaming preamplifier—the amplifier powers up silently in four stages. This process may be synchronized optically with the other matching audio units, avoiding unwanted noise from earth leakage currents. By this and other means, stray electrical noise is rendered inaudible.

The NAP 250 sounds pretty good quite soon after powering up. A soft-start power-breaker module derived from the Statement design helps reduce stress on the mains input fuse. An auxiliary power supply initiates startup then shuts down once the amp is fully powered. Under normal use, the heatsink fins flanking the case will silently cool the amplifier, but if it is driven unduly hard or at high ambient temperatures, a low-noise, variable-speed cooling fan starts up. To meet global efficiency regulations, standby is invoked if the NAP 250 is silent for more than 20 minutes.

Fully regulated power supplies tend to give the subjective impression of significantly more power, for example, with the bass sounding as if the power transformer and reservoir are considerably larger than the ones fitted. Frequently there are performance gains in other areas such as transparency and micro detail, but when judged subjectively, supply regulation is still not a completely convincing substitute for more power, such as one with a massive 3–4kVA transformer having typically 300,000µF of reservoir capacitance. The difference becomes evident when driving larger speakers capable of extended low frequency at high power, frequently with a taxing, low-impedance loading. With their ultra-heavy loading, such speakers can often reveal the amplifier power supply's character in the resulting sound quality, even though most amplifiers are designed to avoid this unwelcome association by using negative feedback to maintain a low output impedance and thus to better meet the reactive current demand.