I'll say it right here: there are quite large differences in sound amongst CD transports. Yes, even when you only use the digital output, still you can hear differences in speed, attack, fluidity. It seems incredible, especially since many enigeers swear that this is impossible.
Below are some well known transports that have strong sound characteristics. I will describe the sound of each transport.
This is an older audiopile favorite. This machine is well-built, reliable and sounds excellent. It is still around on the second hand market and good value. It uses the simplest VRDS transport. One that is also used by some highend brands such as Wadia. That is probably why audiophiles like this machine; it is almost Wadia on the cheap.
As you can see here, this transport is actually less robust as it first appears. (picture taken from www.lampizator.eu) The sturdy looking bridge is actually just a plastic cover - for looks. Under that is a simple pressed steel plate and a standard brushed motor.
The green stabilizing platter is made from plastic, is very lightweight and wobbly. Actually this is the only part made by Teac. The rest is standard Sony. Only reversed here and there. The motor went up, and the disc clamp went down. The laser is also standard Sony.
You might be wondering why I showed these pictures, unveiling the true nature of this VRDS transport. Well, I stumbled across them on another website and was amazed by the descriptions found there. (http://www.lampizator.eu/lampizator/REFERENCES/TEAC-T1/VRDS-T1.html) After reading this report I just couldn't correlate it with my own findings. I've heard this transport and it still sounds much like a Wadia transport! Don't ask me how this is possible, seeing as everything else is Sony-made. Even the servo. But I swear: this thing does not sound like a Sony. I've had several Sony players and heard even more and you can always tell that it's a Sony. Normally you can easily distinguish a Sony transport by its airy, agile and slender sound s well as sounding slightly mechanical. All Sony transports and cd player that I've heard had these characteristics. The Teac T1 sounds completely different: it is full, warm, solid, slightly dark and natural sounding. In fact, it reminds me a lot of how the Wadia transports sound. I've had several Wadia's that used a similar transport and their sound has a lot in common with this Teac T1. It also has the VRDS tradeoff that the player has to run warm for a couple of cd's every time, even when you leave it switched on 24/7, before it sounds its best. So is this difference caused only by the fact that the motor is upside down and that the disc is covered by a green plastic platter? I don't know, but even though you might feel ripped off after seeing the pictures above, this is still an excellent player.
By the way, this is only the cheapest VRDS transport available. There are much better ones further down the line that do have a full cast-metal bridge.
PHILIPS CDM-1 AND CDM-4 IN VARIOUS PLAYERS
Meridian 602. This was their reference transport at the time and it sure looks very chic. It also operates very smoothly; the drawer is both quick and silent. It uses the Philips CDM-4 transport, which is a descendant from the CDM-1. The 4 is a more econimical version of the 1. It is still made from cast metal and it also sounds excellent but clever rethinking made it possible to reduce the amount of material used. A discription of the 602 sound follows below.
Well it certainly looks the part with its sturdy centrally placed VRDS transport and separated powersupply, digital section and output stages.
Marantz CD94MKII. It uses the CDM-1MKII transport. Since I had this player at the same time as the Meridian 602, I could easily compare the players. Of course I used the digital output in order to listen only to differences in transport.
This is the CDM-1 transport inside a Philips CD960 CD player. The same transport (and also a large portion of the electronics) are used in the Marantz CD94 and CD94MKII.
I've owned 2 Marantz CD94MKII's, a Philips CD960, 2 Philips CD880's and many other cd players that use a CDM1 as well as players that use the CDM-4 such as the Proceed PDT3, Krell KPS20i and the Marantz CD16. And I'm sure I left some players out. I guess you could say that I know a bit about these transports:-) As such, I am very familiar with their sound.
The old swing-arm mechanism is in fact an excellent mechanism. It was used by many highend brands until it was taken out of production because it was getting too expensive to make. After that many brands were forced to start using the el cheapo CDM12 mechanism in its many guises. Granted, the CDM12 was far from bad, but it's in a different league than the old swing-arm mechanisms.
What all Philips swing-arm mechanisms have in common are very subtle and fluid highs. The treble is so very smooth and velvety and the sound in general so natural and free flowing that this transport is often solely responsible for any given player to sound "analog". It doesn't matter which brand you listen to; if tthe CDM1 is inside, it can never be an aggresive sounding player. Just why that is is unknown to me. I can imagine that the magnetically driven lens assembly is very free in its movement, unlike modern plastic worm-wheel mechanisms and the lens was made from glass instead of plastic.
The strange thing is that the 602 sounded mediocre at best. Sure it was subtle and never aggressive, but it sounded slow, rounded off and undynamic. I've heard this before in an older Marantz CD94. I think it is because of age. Transports do age and may start to produce more jitter. Whatever the cause, it is important to note that once you're comparing 10 year old transports, that age becomes a factor. I hope that this was also the case for the Meridian 602 because it was simply completely bettered by the Marantz CD94MKII on al accounts.
The beautiful CEC TL1x Belt Driven CD transport. Not only the rotation of the disc is done by belt, but also the movement of the lens assembly.
The use of rubber belts, heavy platters and contraweights in a digital player is controversial to say the least. For one, cd's rotate at varying speed. Fast at the beginning and slow at the end as opposed to constant in a record player. This is why a contraweight is perceived as unneccesary and even unwanted in any disc reader. Rubber belts on their turn prevent the player to react quickly to changes in speed thus making the transport slower in response to starting up and track skipping. But according to CEC, these downsides are outweighed by the advantages which include less vibration transfered to/from the disc. But the proof of the pudding is in the eating.
The CEC sounds incredibly fluid and smooth, yet is very expressive, fast and dynamic. It bests the Marantz CD94MKII (CDM-1) in detail, focus, soundstage width and overall smoothness but the CD94MKII is actually a bit more fluid in the highs while being more dry overall. The treble is drier but also more fluid. I know that sounds contradictory but this is how I hear it. The CEC is more continuous but hi hats can be somewhat more coarse than on the Marantz. The CEC is my reference cd transport.
SO WHY DO THESE TRANSPORTS SOUND DIFFERENT?
Bits are bits, aren't they? Well yes, and no. There are pages and pages you could write on this subject but it is largely abracadabra for me too. I do know enough about it to be able to put shine a light on this matter. First you should know that there are many stages between the signal coming from the lens and the signal leaving the player in spdif form. These stages can all have their influence on the sound because in digital, timing is everything, and everytime you change or modulate the signal you risk changing the timebase. Then there's the precision of the clock is very important for the same reason. Also important is the precision of the rotation of the disk. The more it fluctuates, the more faults will be read and the more the error correction has to jump in. Which brings us to the matter of error correction. This is done in several stages, not just one overall correction. The output in spdif form has to be fabricated, it doesn't come off cd that way. The IC that combines clock and data has an influence on the sound, as well as (the presence or absence of) a capacitor or coupling transformer in the spdif output. Simply put, you're not only listening to the pure data, but also to many circuits that are in place after the signal has been read off disc. Actually, you can measure the presence of a capacitor and coupling transformer in the spdif output, so at least that part is no black science:-)
TRANSPORT PLACEMENT AND USE OF HIGH END FEET
This is also controversial and I can understand why. If the quality of any digital signal depends only on timing, why should different feet give a different sound? Well, maybe it influences the way that vibrations are conducted to the inner parts of the player hence causing the laser to mistrack. Or maybe there's more to it than that. But I do know that feet make a large difference to the perceived sound. Soft rubbery feet make the sound soft and slow and hard cones make the sound more delineated and improve transient response.
POWERCORDS AND DIGITAL INTERLINKS
This will not come as a surprise to you by now. After all, if you were a cynic you probably would've stopped reading by now:-) Indeed, power cables make a difference, even when connected to a transport. Why? I honestly don't know. But they do. You can hear the same differences as when they are used for amplifiers and other equipment. Digital interlinks also have an influence on sound. Again, why I don't know, but I know that they do. You can make the sound faster and more dynamic or gentler and more fluid.
Transports do have their own sound signature. This is easy to hear. Look for differences in speed, dynamics and fluidity. The differences are perhaps not night and day but absolutely clearly audible. The differences do excist. Whether they matter to you is a different matter:-)