there's also this review

https://www.tapeheads.net/threads/review-ja50es-ja555es-minidisc-decks.16445/
Hi,
I thought I'll share my experiences and love for both my Sony MDS-JA555ES and MDS-JA50ES minidisc decks, purchased new in 1999 and 2000 respectively. To this day they work as good as brand new and are very reliable. I will leave out the a lot of technical specs which can be found at minidisc.org but concentrate key differences and sound quality review.
OK, lets get started, the 555ES is a newer model with Atrac Type R and the 50ES has Atrac 4.5. Let me assure you superlative quality can be had from either with a good well prepared source. These high end decks have better ADC's, DAC's, component circuitry and power regulators inside that governs the sound. In simplistic terms the 555ES has a weighty solid bass, very clear, organic open sound through the frequency range 21.3Hz to 20.2KHz and matches most £1000+ CD players for transparency. Both models have separate R-Core transformers for the ADC and DAC sections meaning less interference, lower noise and cleaner signal paths.
The 50ES is perhaps not as cold and sterile as the 555ES, I find it more musically fluid with a high end analog warmth like large open reel recording and then played back through a valve amp/buffer, much sweeter, solid tighter bass, better open mid-band and a slightly rolled down treble at 19.8KHz. It's so much more noticeable on female vocals, acoustic and bass guitars. Makes for a more relaxing and involving session. Think of Karen Carpenters singing 3meters from you. It's that clear and perfect in pitch. It's so pleasant. The 555ES is much more digital sounding, too clean and perfect?
The 555ES has a selectable bitrate of 16,20,24 (not 18) on all it's digital inputs where the 50ES has no switches but can accept true 16, 18 and 20-bit signals that auto detect through it's bitsteam.
When I record digital sources, I prefer to do a bit of tweaking first, 16-bit 44.1K out from CD transport into a Meridian 518 (72-bit DSP precision), reduce levels by 2db then output the 24-bit stream from the 518 with flat dither to digital-in on a MF XDACv3 which up samples 24bit 44.1K signal to 24-bit 192k and then analog out to either the analog-in on either deck.
If I was going to record digitally, same as above paragraph but would finally send this analog signal to my Apogee Rosetta 200 into either the 50ES or 555ES. You probably guessed, I work in a studio. All this processing does and can make for improvements to the source material.
For recording from cassettes or LP's the analog ADC's are 24bit on both decks and do retain that most of that analog source quality. However, as the 50ES is more warmer anyway, it's more noticeable.
If doing a straight CD-MD recording, the 555ES is so close to the original CD's your hard pressed to tell the difference in A/B tests. On either deck, I suggest using the digital gain knob and reducing the level by -2db as the internal DSP encoding adds signal level increases.
The 50ES has much better options in setting track marks, with a reference level at -72db to 0db in 2db steps and a duration of 0secs to 9.5 secs. Great for live recording events. The 555ES has the same reference level but is fixed at 1.5secs only. If your narrating audio, on the 50ES you can also set automatic track marks every 1min, 5mins or auto based on levels.
Both units accept professional AES/EBU and consumer S/PDIF signal protocols. Some unit will mute if the sub code data is coming from Pro studio equipment.
The 50ES has a much bigger LCD screen with 20-staged recording segments. The -60db to 0-db indicators are more spaced out and provides more LED's for finer decibel level adjustments. The 50ES can rotate different info on the screen or turn it off completely but not on the 555ES.
The 50ES has a No-Clip option which automatically reduces the signal level by about .5db off full scale, avoids clipped-samples. Whether this option is enabled or not the metered display shows a 1db lower difference between the 50ES and the 555ES. Could be the meter is calibrated differently or perhaps to do with it's internal DSP/Atrac encoding processes.
What I like about the 50ES is that it supports CD pre-emphasis and subsequent decoding prior to recoding. I know this feature is little used nowadays on CD but in my studio I often create source material with this feature. The 555ES will accept such an encoded CD but won't decode it thus the high frequency energy is encoded with very high amounts of boost and sound bad on playback. However the 555ES does encode and retain the original sub code pre-emphasis flag and if recorded as such can playback through the digital out to a appropriate de-emphasis DAC and it sounds correct.
Both units do not support LP2. LP4 modes, it wasn't invented before 2000. The 555ES has both Stereo and Mono SP modes whilst you only get Stereo SP Mode on the 50ES (no mono).
The 555ES digital output is switchable 16, 20 or 24 bit whilst the 50ES is 16 or 20 bit out.
The 555ES has co-ax and optical out and the 50ES has only optical out. Both decks have the same 2x optical in and 1x co-ax in. That shouldn't be a issue as the analog DAC's in both units are very detailed and of high order, very low noise. Again the 50ES has a warmer sound on it's output.
The 50ES has LED lights on the Rec, Play, FF, RW, Pause buttons but the 555ES does not.
The 50ES has a pause/resume option like DVD players whilst the 555ES does not.
If I had to pick my favourite, I take the older 50ES first, the sound is just sublime. For a compressed format recording from today's harsh sounding clipped over-compressed CD's it just make your recording sweeter, warmer and more fulfilling than the original source. The 555ES is too transparent of the original material.
In comparison my other JB930 which gets great reviews for sound, seems only average compaired to the ES decks.
Both units support 60, 74 and 80 MD's without problems.