Nakamichi IA-1z
A five-channel integrated amplifier with Dolby Digital on board, a volume control accurate to the resolution of the test bench, and a power rating so conservative it borders on false modesty (Audio, December 1997, p.60).
Overview
Nakamichi built its name on tape transports, but the IA-1z shows the company could put the same obsessive streak into a home-theater amplifier (Audio, December 1997, p.60). It is a 5.1-channel integrated amp with Dolby Digital (AC-3) and Dolby Pro Logic decoding built in, plus dual-zone and dual-source multiroom capability. Nakamichi pushed two circuit ideas hard in its literature: Harmonic Time Alignment in the power amp stage and a Precision Resistor Array for volume control. Motorola 24-bit DSP chips do the decoding, followed by 20-bit DACs. Analog sources pass through 18-bit A/D converters before surround decoding.
Nakamichi understated the power number. The company rates the IA-1z at 100 watts per channel into 8 ohms in stereo and 80 watts per channel in its five-channel surround modes, and gives no 4-ohm rating at all. Into 4 ohms the same testing measured clipping at 200 watts continuous and 225 watts on tone burst. Past that point the protection relay stepped in, and the amplifier showed no sign of strain. Distortion at rated output measured roughly half of Nakamichi's own 0.1% specification.
The Precision Resistor Array is the kind of detail that separates a Nakamichi from a commodity receiver. Across the most useful 55 dB of the control's range, no error could be detected at all. Below that, stepping in 2-dB increments, accuracy held within ±0.03 dB down to -61 dB and ±0.1 dB for the next three steps. The mute circuit was factory-set for total muting but adjustable anywhere from 10 to 46 dB of attenuation.
Analog frequency response in stereo mode came in flat and extended - within +0, -0.22 dB from 20 Hz to 20 kHz, with the -3 dB points below 10 Hz and out at 91 kHz. The left and right channels tracked each other in both response and level with virtually no imbalance below 10 kHz. The subwoofer crossover sat at about 80 Hz, 3 dB down, with nominal 12 dB-per-octave slopes on both sections. The reviewer would have preferred a sharper low-pass slope but allowed that Nakamichi's choice was not unusual. Damping factor measured 115 at 50 Hz despite the relatively modest global feedback, and the input circuit was essentially overload-proof.
Surround performance held up to the same standard. In Pro Logic mode, THD + N in the main channels stayed under 0.05% from 30 Hz to 13 kHz at rated output. The center channel was slightly lower still, and the surround channels, in the reviewer's surprised phrasing, were almost as quiet as the mains. Channel separation at 1 kHz mostly landed between 65 and 70 dB. The weakest path, surround to left front, was still at 44.7 dB, and center-to-surround separation reached a superb 81.7 dB, which is where it matters, since nobody wants dialog bleeding out of the rear speakers. The surround channel's 7.1-kHz rolloff was judged almost exactly on target for Pro Logic. In Dolby Digital mode, front and surround channel levels balanced within ±0.15 dB, and THD + N at 1 kHz and 0 dBFS ranged from 0.0109% in the left front to 0.0276% in the right surround.
The DAC section is where the review found the only real caveats. THD + N at 0 dBFS peaked at 0.198% at both the preamp and speaker outputs, dominated by the DAC's sampling-rate crossmodulation at 18 kHz rather than by the power amp. Signal-to-noise came in at 84.9 dB A-weighted and 75.7 dB CCIR-weighted relative to 0 dBFS, with dynamic range of 82 dB unweighted and 85.2 dB A-weighted. The review's verdict on the converters was measured. They showed more detail than a high-end Sony CD player on some recordings, and were among the better sets seen in A/V receivers and amplifiers of the day.
Two infrared remotes shipped with the unit - the full-featured, programmable RM-1m and the simpler RM-1s for source selection, volume, power, and muting. The RM-1m carries separate control banks for tape/VCR transports, CD and laserdisc players, and tuner or TV, each with its own power pad. It also handles surround mode selection, the Dolby Digital dynamic-range and LFE attenuators, system setup and calibration, two-zone operation, on-screen display toggling, and balance. A laserdisc player connected to the "LD" input set frees the second Dolby Digital input for another source. The amp accepts remote amplifiers, and power comes in through a removable heavy-duty IEC grounded line cord.
Collectors should treat the IA-1z as a niche piece - a Nakamichi for people who care about Nakamichi engineering rather than Nakamichi tape decks. Its digital section dates it to the first wave of Dolby Digital hardware. The amplifier half is overbuilt by any reasonable measure. The converter half is competent for its moment but not exceptional. Anyone buying one today is buying the Precision Resistor Array, the Harmonic Time Alignment topology, and a power supply that shrugs off 4-ohm loads, wrapped around decoding that a modern processor will better.
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