Updated version of the earlier and already very successful TA-D88B, the TA-D900 mainly added a "bass-boost" switchable circuit, a supplementary plug-in unit to handle very low frequencies and a glass top plate.
It did keep the steep 24dB/octave filter slope and the overall circuit design - in fact it is the same.
Differing from the TA-D88B are the plug-in "units" - or the scripting on them : blue for the original, red for those tagged with ESPRIT. The inside of the units is however identical.
Available on special order was a Unit 0 for very low frequencies (50 / 80 / 100Hz), just as was (later) added for the TA-D88B the Unit S1 offering 62,5 / 100 / 125Hz crossovers and Unit S3 for 3,5 / 5,6 / 7Khz, plus a "blank" Unit S with only one center crossover frequency set at 650Hz ; these "S" units are all scripted in white.
The filtering is based on the Bessel function for better group delay ; the circuit are all DC and dual-mono, the enclosure and materials (aluminium) all non-magnetic.
Opening the circuit is a 100% negative feedback applied to the first-stage differential amplifier for high input impedance and low output impedance, with a current-mirror loaded 2-stage differential amplifier to reduce distortion to a very low level.
A dual-Fet differential amp (first stage), dual transistors (second stage's differential amp) plus current mirror also suppress DC drift.
The low channel (output 1) has a DC amplifier in the input buffer, low-pass (Bessel) filter and output buffer stage.
Outputs 2 to 4 have their buffer amps work as source-follower amplifiers with constant current diodes and FETs designed for high frequency circuits ; the cascode-connected high frequency FET is used at the source follower with again a 100% negative feedback is applied for high input impedance and low output impedance.
When all frequency switches are set to "flat", a full-bandwidth signal is output at the "OUT 1" terminals, allowing for direct comparisons between a multi-amplified system and a reference single-amp system.
Sealed relays activate muting circuits for each outputs when changing a frequency setting and when opening the top lid.
Precision parts are used throughout : 99,99% pure copper is used for the PCBs and wiring, film caps, low-deviation & low-distortion resistors etc.
The ELNA For Audio power-supply caps were selected after "detailed study of everything from the electrical foil, electrolytes, lead material and plating to the capacitor's internal impedance".
The bass-boost function allows to add a maximum of 6dB by 1.5dB steps around 50Hz with a T-type filter added in the NFB loop of the output buffer amp.
Like all the ESPRIT components, the build-quality is... quite something.
One has to see any of them "in the flesh" to understand this : Sony, as usual, didn't care to have its own high-end audio production photographed well, let alone in detail.
As for all Sony high-end components starting in the late 1960s, an individal inspection card was delivered with each ESPRIT component where one could see that the published specifications were rather quite very conservative - something many a hi-fi reviewer noticed, whether pre-Esprit, ESPRIT, ES or R1.
The inspection card of my own (ex-)D900 showed S/N ratios bettered by up to 9dB (!) and distortion dwindling from 0,05% to as low as 0,002% (!!). Yes, Sony is a discrete brand.
The one thing, however, which isn't too cool, is the black anodizing of the aluminium parts : it is extremely thin and the ESPRIT components all get scratched very easily.
That anodizing also has a tendency to uncover a slight magenta (or yellow-ish) undertone throughout the years due to UV exposure.
Mine was a NOS in its never opened box so it had remained purely black.
But I still think "pure black" didn't fit well the notion of ESPRIT and that something closer to the "B" color of the 86s and 88s pre-Esprit would've been much better...
The one thing, however, which is cool is the beautiful TAC-91 cabinet made of wood and artificial marble which Sony produced... but never advertised anywhere.
Those TAC cabinets are therefore nearly impossible to find and were the only reason why, unlike the pre-Esprit, the ESPRIT components had detachable rack handles instead of the fullt-tilt one-piece extruded front plate of the pre-Esprit.
Sony will remain Sony.
The ESPRIT D900 was designed by Masaru Nagami, general manager of Sony's ESPRIT division.
Alongwith the TA-N900 monoblock, the TA-D900 probably is the most common of the series : as used in many a japanese high-end rig, distributing music to many a high-efficiency JBL or Altec stack. Even Accuphase bowed down to the TA-D88B for the 70s and TA-D900 for the 80s.
And the D900 was good enough to remain available as late as 1987 and still be tagged "Exciting Product" by Stereo Sound the same year !
Sony, sometimes, managed to remain Sony :)