Pioneer A-91D Amplifier
From where we stand, the flagship model in Pioneer's redesigned Elite Series is this integrated amp. The A-91D distinguishes itself on two counts: in the circuit-component and circuit-construction refinements employed in the interests of signal purity and in the comprehensiveness of its built-in digital switching and conversion circuitry.
Several of the measures taken are typical of those being employed these days to banish every conceivable source of signal contamination. For example, the A-91D's 65-pound weight is partly a result of its cast-iron transformer cases filled with damping fluid (to minimize vibration and maximize heat dissipation). Also typical of this trend are the concern for separation of functions in the internal layout and the use of intersection shielding, shortest-possible-path signal routing, and nonresonant mechanical construction. There is a honeycomb pattern stamped into the chassis panels to enhance mechanical rigidity, and the massive central finned heat sinks also adopt a honeycomb design to suppress fin vibration.
The internal layout assigns separate shielded bays to various functions, rather like industrial or military electronics. Most unusual is the digital bay, which contains separate digital-to-analog converters (DACs) for each channel with four-times oversampled digital filters. Its input is a digital "bit stream."
The switching for this section is exceptionally well thought out. There are direct-digital connections for as many as five digital-audio components, two of them digital recorders (either DAT units or digital-connection-equipped PCM adapters). Digital 1 is a fiber-optic input connection; digital 2 and 3 are "coaxial" (pin-jack) electrical input connections. DAT 1 offers either optical or coaxial options for both input and output; if a plug is inserted into the electrical option, it overrides the optical feed. DAT 2 provides a coaxial input and output only. A major advantage of the optical connections is their freedom from possible spurious radio-frequency radiation, which could leak into nearby analog circuitry.
Both electrical and optical direct-digital input links deliver the digital bit stream to the built-in DACs, which convert only the signal that has been selected for monitoring or recording through the amplifier's analog section. All the digital outputs deliver the unaltered bit stream from a direct-digital input. For this reason, you can't record the digital output from a CD directly onto a DAT deck. The sampling rates aren't even the same, and (unlike the built-in DACs) DAT electronics won't switch to the CD sampling rate. The myriad of switch settings necessary to accommodate all the permissible combinations of deck-to-deck dubbing (among two analog and two digital machines) is covered in a chart taking up half a page in the manual.
In addition to the five direct-digital sources, the A-91D has provisions for six analog sources: two tape decks, two aux, one tuner, and one CD player. The back-panel analog Tape 2 connections are intended primarily for insertion of a signal processor. The front-panel selector/monitor switch is set apart from the main switch array, together with a switch that selects the DAT 2 processor as the source for the built-in DAC. Individual stepper buttons serve as recording selectors for the analog and digital sections. The entire switching scheme is rather complicated, but evidently its ramifications have been carefully considered.
The remaining front-panel controls are quite straightforward. The main power switch is on the left; below it is a pilot light to show when the DAC is processing a bit stream (at any of the standard playback sampling rates: 32, 44.1, or 48 kHz). Below that is a headphone jack. To its right are the speaker selector (A, B, A + B, or off), the bass and treble controls, and the balance adjustment.
At the right end of the front panel are the volume control, a true muting switch (total output cutoff-not just 20-dB attenuation), the phono mode selector (MM/MC sensitivity, each with or without an infrasonic filter), and some mode selector buttons. The latter include one that steps from stereo through left-channel mono and right-channel mono (with the specified input fed to both outputs) and back. The others choose loudness compensation and "direct" operation.
The direct mode cuts out the tone controls, the loudness and mono/stereo controls, and the processor (Tape 2) loop. This is what we consider the standard mode for such an ultrapurist model, and Diversified Science Laboratories made most of the A-9 ID's very impressive measurements in the direct mode.
We found only one performance anomaly with the A-9 ID: With the tone controls activated but at their detents, the amplifier has a noticeable rise in the bass response (particularly below 100 Hz), shelving at about + 1 dB below 50 Hz.
The tone controls themselves are satisfactory in operation. Effective bandwidth handled by the bass control is altered somewhat by its degree of rotation. The maxima start digging in just below 1 kHz and have most effect (about 12 dB of boost, 9 dB of cut) below 50 Hz. Similarly, the treble alters response slightly for at least an octave below 1 kHz. Moderate rotation of the treble control shelves response by 5 kHz or so, while maximum rotations deliver about 10 dB of boost or 9 dB of cut at 20 kHz. The loudness compensation has only a slight shelving effect on the treble (+3 dB, relative to 1 kHz, from about 6 kHz up in DSL's lowest-level test) but boosts the extreme bass by as much as 10 dB.
Phono response is extremely flat above 100 Hz despite very slight (roughly 0.1-dB) rises centered at about 500 and 10.000 Hz. At the bottom end, the fixed-coil (MM) trace shows a tiny rolloff without the infrasonic filter option engaged, while that for the moving coil is down a moderate amount (1-1/2 dB at 20 Hz). Because the infrasonic filter is available only through the phono input and can't be measured in our usual way (through an aux input), the slope of the filtering is difficult to quantify. Suffice it to say that it is fairly steep, since it brings fixed-coil response down almost 3 dB at 20 Hz and 31 dB at 5 Hz (the middle of the warp frequencies). Moving-coil response with the filter on is down 3 dB at about 22 Hz, 4 dB at 20 Hz, and 39-1/2 dB at 5 Hz.
In the distortion tests, no measurement exceeded our 0.01-percent reporting threshold. The amplifier section delivers 120 watts (20.8 dBW) into 8 ohms. Lowered speaker impedance increases the A-91D's current (and therefore power) moderately at each step with no sign of struggle. Pioneer appears to have opted for a well-regulated power supply, rather than spectacular dynamic-head-room figures.
All told, the A-91D is a worthy member of Pioneer's Elite family. Above all, we are impressed by the care that has gone into its design: the details of chassis layout and construction; the evidently high component quality; the comprehensive, thoughtful approach to analog and digital switching; the handsome, well-ordered appearance; and the solid, hefty operating feel. That it sounds great almost goes without saying.
