I didn't think you could bypass the iPod's DAC with any available cable. I thought you needed the Wadia i70 or similar dock. As far as I know, there are only a few that do this - and they all cost more than $100 new.
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I remember reading a review that stated that the USB port on the Marantz SACD/CD player SA8003(? I think) didn't use the better DAC that they used for SACD. I don't have the link off hand though.
So it may not be worth springing for a better cable than the standard one.
But if you really want a better quality cable, have a look at ALO Audio
Found the review
"The USB input is powered by an NEC uPD63901 microcontroller (designed for use in car audio head units), and it enables the SA8003 to play MP3, WAV, WMA and AAC files directly off a USB hard drive, pen drive, or iPod (it does not function as a USB DAC, as connecting a laptop to the Marantz with a USB cable does nothing). The uPD63901 does its own D/A conversion, so files read from the USB input completely bypass Marantz' CS4398-based digital section (and cannot be output via the Marantz digital outputs). The USB signal still benefits from going through the SA8003's discreet analog output stage, but overall the USB sound is not quite up to par with what the SA8003 does when playing a shiny disc. "
The player actually does put USB-loaded music files through its own DAC. It plays files from any kind of drive that can be connected w/USB (ipod, flash, etc.). The SQ difference between this and (e.g.) headphone-jack-to-RCA is quite noticeable. I'm still trying to decide if I can hear a difference between CDs, and lossless files from ipod/flash/etc.
I'm kind of surprised that (1) this feature of the player isn't more renowned, and (2) other players don't incorporate it. On the other hand, I suppose most folks listening to files on drives via USB have moved beyond physical discs (I'm not there yet), and just use DACs with this kind of input capability.
As I said, there are aftermarket ipod USB cables that are supposed to be an improvement over stock, and some are quite affordable. Audioquest has a bunch of cables of this sort. However, as Mateored suggests, the online consensus seems to be that you have to spend a fair amount of money to get a significant improvement over stock.
Based on responses here and elsewhere, it sounds like this may be a market that just isn't very active yet. I've only found one or two online discussions of these cables, that refer to actual experience by users. (There are, of course, "academic" discussions about whether they make any difference at all.) Frankly, I'm looking for someone to tell me that the less expensive cables are an improvement, but so far, as indicated, the word seems to be that money must be spent to get a significant improvement over stock.
For now, if anyone's interested, I'm looking at the Audioquest Cinnamon. A step above the Forest model, but not quite the Carbon, or the Coffee, or the $500+ Diamond. If/when I take the dive, I'll come back and share my experience.
I was actually a little confused at first, because I just don't know anything about the world of DAC inputs, and apparently, firewire and other sockets are common outs to DACs. (I know I sound like a caveman here - firewire-to-USB cables, eh? Your modern devices frighten and confuse me!)
Audioquest does make a bunch of those cables, and that's what I found when I began my search, but then I came across a bunch of AQ ipod cables that have what I'm looking for:
I was confused, because all of the other AQ cables come with the same model names/tiers (Forest, Cinnamon, etc.) Eventually, I was able to narrow my search and figure out what I was, and wasn't, looking for.
I know AQ isn't the only company that makes "high-end" aftermarket ipod cables. Just getting started in my quest for cable.
Well, I picked up an AudioQuest Carbon Ipod USB cable...they're supposed to have a burn-in period, but I think I immediately heard a noticeable improvement over stock as soon as I plugged it in - cleaner, tighter, slightly larger soundstage. Of course, I may have just heard $119. We'll see. More later.
I know I am a little late coming into this discussion but similar to Bthogan, I own a Marantz CD6004 CD player with the USB input that has been specified to be able to connect an iPod and bypass its internal DAC to make use of the Marantz's more superior DAC. I was so sceptical that I even contacted the Marantz's technical team to confirm that it actually does what it claims to do and the answer I got from them was a "Yes" apparently.
As pleased as I was with the answer, I am however convinced that it may not be the case and that the guys in Marantz was not honest with their answer due to my following encounters:
1) If the iPod's DAC is bypassed, then the link, i.e. the cable, between the iPod and the Marantz would have been a digital one. So, I ripped a CD in AIFF format into my iPod at the same resolution as the CD and played it back via the iPod and the sound reproduction was different compared to playing the CD. I would have expected the reproduction to be the same if the iPod's DAC was bypassed as it would have been an exact digital copy.
2) I then purchased an aftermarket iPod 30-pin-to-USB cable, the Oehlbach XXL and the sound reproduction was again different. In fact, the sound reproduction is different when comparing (a) the CD, (b) using the original Apple cable and (c) using the aftermarket cable.
Based on an ear test, the sound reproduction from the CD as the source was in my opinion the best sound reproduction.
In my conclusion, I believe that the Marantz CD6004 does not bypass the iPod's DAC which makes a lot of sense because as a "whole" package, it is priced rather inexpensive.
A disappointed enthusiast. Tom.