... or is there a weak link somewhere?

Laptop (w uncompressed music) >> WiFi >> Airport Express >> optic digital signal >> Optic/Coax Converter >> digital signal >>
>> BL5 w internal DAC >> digital signal >> BL5 w internal DAC

As I understand things the entire chain from computer to BL5 must ensure perfect bit-to-bit transfer (since it would otherwise be impossible to use WiFi etc for transfer of data files, programs etc). Therefore the digital signal that reaches the BL5s must be an exact copy of the
uncompressed music file stored on the laptop. This means, if I am correct, that a high end CD-player will not improve anything, and since
we are using the internal DACs of the BL5s there is no need for any other high end DACs.

Is there any fault in this reasoning - because if not - any loss of quality will occur in the BL5s, for example poor quality of the built-in DACs. However, I suppose these are of very high quality - and therefore - this system (very inexpensive up to the BL5s) will be "absolutely high end" :-)

 /Munin
 

Working backwards, you want as few steps as possible between the speakers and the source.

I would recommend having a Mac mini, with a soundcard that has a coax s/pdif OUT attached via Firewire to the mini. You then connect the soundcard directly to the BL5s using a coax cable, skipping the wireless portion.
You can control playback using either Screen Sharing on your laptop or an iPod Touch configured as a remote with Remote Buddy installed on the Mac mini. (You can also attach the Mac mini to a screen or television).

To extend storage space, you can have the Mac mini connected to a stand-alone server offering expandable memory. Such as the ReadyNAS+.

You can, of course, also have a soundcard attached to the laptop, but then it's not very mobile. The advantage of using a soundcard, and going directly to the BL5s from that, is that you're not limited by the 16-bit/44.1kHz limitation of the Airport Express, and can play back higher resolution files. The interior DACs on the BL5s are limited to 24-bit/96kHz, but that's as much as you'll need.

There are lots of good soundcards on the market, your Apple dealer will be able to recommend some. Or else try chatting with a Pro Audio store that caters to musicians, sound engineers - and these cards are not expensive.

Here's an inexpensive one: http://www.djmmusic.com/itemdesc.asp?ic=EC-AUDIOFIRE2&eq=&Tp= 

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As I'm using my BL5s for a variety of purposes, I've chosen to go via a non-B&O receiver that has a number of different optical and coax digital in and out connections, as well as a surrond processor. Check out the Cambridge Azur 540R version 3. 

Do you really think a good soundcard would be necessary? After all, jitter isn't really an issue any more, in fact it hasn't been an issue with computer soundcards for a number of years.

The tricky bit is that the BL5s are coax s/pdif and 24-bit/96kHz -- while the Airport Express is 16-bit/44.1kHz and optical toslink s/pdif.

The AE clips the resolution, and the optical/coax requires a converter.

By using a soundcard that has a coax s/pdif OUT you eliminate the need for a converter, and make it possible to feed up to 24-bit/96kHz files to your BL5s without any resampling of resolution.
Of course, nothing wrong with going the conversion route originally outlined - except that if it's absolute high-end that's required, then I'd give it a rethink. (Particularly now that www.musicgiants.com, Linn records and others are offering high-resolution downloads.)

I see what you're saying, the only shortcoming at the moment is that the vast majority of source material at the moment is still 16 bit 44.1 KHz (i.e. CD based) and is still likely to be for the forseeable future (especially since the mass market is currently heading the other way toward mobile, compressed music).

Optical/electrical S/PDIF converters are not very expensive (say less than £30) and shouldn't affect the music in any way.

stax 

stax:

Optical/electrical S/PDIF converters are not very expensive (say less than £30) and shouldn't affect the music in any way.

stax 

I agree, Stax. Though you'll find audioholics who are willing to spend thousands of pounds to fight the holy fight against jitter, etc.

The sound card above costs $139 - a smart choice. Works as a straight converter, if you want that, but can also be attached to your computer and provide the link described above. If the original poster had specified another pair of speakers, I would have recommended the setup he suggested - but since he specified the BL5s and wanted absolute high-end, then I would allow for 24-bit/96kHz. About 20% of my music is in that format.

Here's some info on the Apple Express' capabilities, from someone who really checked it out.

From this discussion, and very recent: http://discussions.apple.com/message.jspa?messageID=7075423#7075423 

I found yesterday an interesting article, maybe somebody is interested on that information too: http://www.stereophile.com/accessoryreviews/505apple/

First, it is important to note what the AE doesn't do. Whether you own a PC or a Mac, the AE works only with iTunes v4.6 or later and is limited to music files that iTunes can read; ie, 16-bit data only.(*) These data, though, can be in any file format that iTunes recognizes, from lossy MP3s at the low-quality end of the spectrum to Apple Lossless and lossless AIF or WAV files at the high end. It is also important to note that the AE functions only at a 44.1kHz sample rate. When you play 32kHz or 48kHz data, iTunes sample-rate-converts the data in real time before sending it to the AE. I tried using the Nicecast utility for OSX ($40) to feed iTunes with 24-bit data and 88.2kHz data from the inexpensive but excellent Amadeus audio-editing program for OSX. High-speed files were played back at half speed and hi-rez data were truncated to 16 bits, according to RME's useful DigiCheck utility for the PC.

Some audiophiles have dissed the AirPort Express on the grounds that its digital output is not bit-accurate. However, I found that this was not the case, that the data appearing on the AE's digital output were identical in the original file. To check this, I compared a WAV file with a duplicate that I had captured on my PC from the AirPort Express's S/PDIF output. I used iTunes on my PowerBook playing a version of the file encoded with Apple Lossless Compression to feed data to the AE. The files were bit-for-bit identical, proving that the AirPort Express is transparent to the music data (as is ALC, for that matter).

One operational glitch that didn't affect my system to any significant extent but did do so for some readers was the fact that, as the AE doesn't have a local clock circuit, when the incoming data is interrupted, as it is when you change songs in iTunes, there is no longer a digital output to feed the DAC, which loses lock as a result. Both the Levinson and the Benchmark DACs that I use deal gracefully with the lost lock and are quick to unmute when the new song is sent to the AE. I therefore missed the fact that with some other DACs, the first seconds of a song might be cut off.

(*) I have used Airfoil to send music from other programs, such as browsers, VLC, etc.