You're right, I've been playing around with TDA1541A based players over the years, including my own DAC designs using a slave clocked transport and the 855 is the most musical and relaxing player I've ever found - I'm no Audiophool by any stretch, but it's a perfect example of taking a pretty much standard design and, with an optimal PCB layout, taking it to the next level, something that Philips themselves couldn't seem to manage. An example of how sensible supply layout and signal routing can make all the difference in a mixed signal design.
Again, desperately avoiding the audiophool label, the mods I've found most effective...
- A separate +5V regulator for the TDA1541A (just a basic 7805 with 220nF output cap feeding the lifted end of the fusible resistor). The main +5V feeds pretty much everything, including the decoder, micro, front panel, and rather noisy SAA7220 digital filter. It has quite a lot of hash on it, even with local decoupling.
- Swapping the fusible resistor on the -15V to the TDA1541A for a 33uH inductor. Inproved decoupling on a sensitive rail.
- Inserting series resistors in the I2S lines from the SAA7220 to the TDA1541A, 120R on BCK, 390R on WS and Data. The 7220 is a MOS ic with pretty much 5V output swing whereas the 1541a uses ECL logic and only needs a minimal voltage swing. The resistors help to reduce noise injection into the 1541a substrate (it is possible to do much better). As I mentioned, the SAA7220 is a very noisy digital filter IC, it pulls nearly 200mA @ 5V.
Basically, it's all about keeping noise out of the DAC, nothing mystic.
It's possible to go further - the 11.2896MHz master clock is a simple Xtal on-chip inverter based oscillator on the SAA7220. The level of noise on the chip causes a lot of jitter. This then gets piped back to the 7210 decoder without any particular care or terminations (which shouldn't actually matter as the 7220 re-clocks the I2S at it's inputs) and is also divided by 2 and used as the BCK to the DAC. External clocking seems desirable. The 855 is a good platform with a good solid chassis and plenty of free space just where you'd want it.
As with all consumer products, models are designed to fit their allotted space in the price range. Cheaper products are supposed to sound worse than more expensive ones. Rotel rather messed that up, the multibit 855 / 955 sounded better than the more expensive sigma-delta 865 / 965 (and achieved slightly better specs for THD+N and dynamic range too). An example of 'doing it right' can be seen in the Arcam Alpha 5 / 5 plus / Delta 70. If you look in the service manual you can see the differences between variants - minor supply decoupling improvements, 7220 on chip clock vs external, adding I2S data reclocking before the DAC etc.
Sorry, I'm blathering at this point.
P.S. I picked up my 855 for less than £30 incl. last year, listed as intermittently skipping. No, not the laser, the top of the CD clamp had worn a divot in the pressure plate and was causing enough lateral vibration to affect tracking. An easy fix!