Ultimate ES CD player of the last "classic" years, last evolution of the CDP-XA7ES with, give or take, the same mechanical build quality.
Three R-Core transformers (the same as in the CDP-MS1 / CDP-X5000 or SCD-1 / SCD-777ES) but not the big two-in-one Twin Core molded into a resin block as in the original XA7ES.
The other power-supplies are also somewhat reduced and definitely gone was the luxury of copper-plating the chassis as in the "real" X7 players of the 1980s, 1990s and said XA7ES.
The digital-to-analogue section is similar to that of the CDP-XA7ES : same Pulse and Current Pulse ICs.
Where the CDP-XA55ES really changes is in the added 24bit VC24 filtering and S-TACT signal treatment : the former for tailoring the low-pass filter's frequency response, the latter to refine the way digital signals are deciphered, handled and transmitted.
S-TACT :
The Synchronous Time Accuracy Controller pulse generator reads the pulse code data and forms the pulse waveform accordingly.
The pulse is formed using the clock generator, which is controlled by a quartz oscillator. For this reason, the pulse formed using S-TACT is extremely precise, and therefore generally referred to as quartz precision. Conventional systems incorporate digital calculating devices like a noise shaper on the clock generator side.
VC24 :
VC24 is a 24bit digital filter which, through sampling math, allows to overcome the abrupt low-pass characteristic of its curve above 20Khz... and therefore "play" with the higher part of its curve to "tailor" a bit the highs' frequency response. Four custom curves are preset, plus the normal/regular one.
The Feed Forward filter/function of the XA7ES is here cumulated/integrated to the VC24 chip.
S-TACT and VC24 are covered in the TECH sections of the SCD-1 TVK page - see there for the detailed details.
Other niceties inherited from the CDP-XA7ES are the cast-iron off-center feet (for less vibration transmission through layout symmetry), the Frame & Beam chassis, the all-DC analogue signal path and a reinforced diecast zinc base supporting the optical pickup.
Gone however were, for instance, the second anti-vibration top plate, the XLR and FET-charged outputs or SEPP headphones' amp : if the CDP-XA55ES shared the same KSS-273A (or 273B ?) laser as the original XA7ES, it "only" was an X5 and not an X7.
Pity Sony didn't carry a decade long evolution like those of the X7 series for the XA7 or XA5. But then, in 1999, SACD was finally there !
All about the chips inside the CDP-XA55ES, right here.