nigel

48Kbps AAC is not very good. You can easily hear the difference, using iTunes Radio streams. BBC Radio 3 is available in both 48Kbps AAC ("Low") and 320 Kbps AAC ("High"). Switching between the two easily shows the difference.

In my experience, a USB to SPDIF converter doesn't make any difference with my NAD M2. The sound is equally superb whatever the source. I bought a Musical Fidelity V-Link after reading the good reviews of it here, and I no longer use it. It sounds exactly the same as the Toslink output from a Mac Mini, or (with 16/44.1 sources) from an Airport Express. So, in my experience, the M2 really isn't bothered by any jitter in the source. If you decide to try it for yourself, however, I'd be interested to know if you hear the same thing, particularly as we have almost identical systems.

Not only is the jitter vanishingly low (see John Atkinson's measurements in the Stereophile review of the M2), it is also inaudible. My M2 sounds equally free of jitter when fed via Toslink from either jittery sources like an Airport Express, or from well-reviewed converters like the Musical Fidelity V-Link. In fact, I don't bother to use the V-Link any more, as the NAD M2 sounds equally fantastic without it.

I just heard a radio documentary about Pink Floyd on BBC iPlayer which I think would be of interest to others: http://www.bbc.co.uk/iplayer/episode/b014w3ld/The_Record_Producers_The_Record_Producers_Pink_Floyd_Special/ Richard Allinson and Steve Levine look at the work of Pink Floyd in the latest edition of Record Producers. The programme concentrates on the early days of the band and the development of their sound from the late 60s through to the mid 70s. Amongst the contributors to the programme are Joe Boyd, Floyd's producer for their first single Arnold Layne; John Leckie, who engineered the band's album Meddle; and Clare Torry, whose vocal performance on The Great Gig In The Sky is one of the highlights of Dark Side Of The Moon. Plus personal recollections and studio secrets from their engineer Alan Parsons. There are new interviews with guitarist David Gilmour and percussionist Nick Mason, and Richard and Steve have access to the original multitrack recordings of some of Pink Floyd's best work from thie period. These highlights include the four track tapes of Arnold Layne; their masterpiece Echoes (which began as an eight track recording - but later expanded to 16); the 16 track recordings of Money and Great Gig In The Sky from the seminal Dark Side. album, and 24 track recordings from their 1975 release Wish You Were Here, including the epic Shine On You Crazy Diamond. With a few extra surprises along the way, this edition of The Record Producers is not only an unmissable programme for Pink Floyd fans, but also a "must listen" for any lover of quality popular music

As a side note, I fail to understand why only (?) 2L is selling multichannel FLACs!? Chandos also has some recordings available at their download store The Classical Shop: http://www.theclassicalshop.net/formats.aspx

Again, I was exposed to identical files that had been altered to have different levels of jitter. So I learned what jitter sounded like. After that I could identify it at very low levels, at least low enough to differentiate between a file with 100ns jitter added, and another identical file with only 30 ns added. I couldn't identify files with lower levels of jitter than 30ns. How do you add jitter to a file? After the jitter was added, the files would no longer be identical, would they? Or did you mean that the jitter was added to the audio stream as the file was playing?

@rgbyhkr From the answers you received, it seems the the M51 is basically an M2 with a traditional line-level output. The M2 eliminates the need for separate analogue power amplification by doing the analogue to digital conversion at speaker power levels. This has huge benefits for the overall sound, as they can eliminate noise and distortion from all sources (power supply, EMI, RFI) by their DDFA (Direct Digital Feedback Amplification) method described here: http://www.computeraudiophile.com/content/NAD-M2#comment-57564 Now it looks like they have released a version of the technology for people who are not ready to replace their traditional power amps, by replacing the M2's speaker-level output with the M51's line-level output. As a DAC, I believe this will be one of the best sounding DACs out there. But an M2 will always beat an M51 plus traditional analogue power amplification, because the traditional analogue amplifiers will still suffer from all the noise sources that they always do (see all the threads here about different sounding USB cables, music players, operating systems etc. etc) without the M2's ability to cancel it out via DDFA.

It even works with the compressed $1 iTunes version of the song. Not only that, it even works with the free 30 second iTunes preview of the track, played through the built-in speakers of my MacBook Air sitting on my lap!

On OS X 10.7, with iTunes 10.4.1, I followed mwheelerk's instructions, and got exactly the same result here. The final PNG file was not the same size as the original (it was bigger) which suggests that the JPEG really was converted back to PNG at some point.

Some record companies have it right. My favourite classical music label, Chandos Records, have their whole catalogue available for download in lossless 16/44.1, and an increasing number of recordings also available in 24/96 "studio quality". http://www.theclassicalshop.net/

Interesting. Looks like the new M51 DAC incorporates some of the technology from the M2 Direct Digital Amplifier, which also has the 35bit lossless digital volume control.

My limited understanding of png is that it is an uncompressed format (similar in comparison to WAV or AIFF audio files) and that jpeg is a compressed format (similar in comparison to Apple Lossless or FLAC). http://www.computeraudiophile.com/content/Questions-use-differnt-file-formats-artwork-iTunes-png-and-jpg#comment-95848

Standard external drives are designed to run continuously. Switching them on and off can actually cause more wear and tear than leaving them on 24/7.

Unfortunately Apple doesn't seem to have any public bugzilla or similar to report and track bugs. If you are a registered developer, you can use Bug Reporter, see http://developer.apple.com/bugreporter/

I'll try exiting iTunes BEFORE ejecting the external next time and see if that maintains the location to the external. Good idea, but also, and probably more important, is to make sure the external drive is powered on, connected, and mounted (visible to OS X) before you start iTunes. Or you could do what I do and leave the Mini and external drive running all the time. You may find your missing rips in the default iTunes location on your Mini's internal hard drive Music->iTunes->iTunes Media Have you looked there?