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So, my thoughts on this. The combo is pretty good to use as a DAC, but the DAC alone is not great because it's USB self-powered. That's severely limiting for a few reasons:
1) The power output of your attached computer system will introduce noise into the DAC signal chain.
2) USB power does not provide enough power to drive the pre-amp to levels necessary to pair this well.
3) The combo is not capable of solely amplifying hungry headphones, despite what the specs say. I've owned one, and I would say this is a good buy for a really good quality entry-level DAC/pre-amp to drive a separate amplifier for your planars or 600ohm cans. If you've got lower impedance cans, the ODAC+ O2 is really quite good.
So, yeah, grab this if you need a solid DAC. Don't waste your time with the standalone. The beauty of the combo is that it's externally powered, but still has USB inputs for audio signals (which means you can blank the power pins to achieve a better isolated signal).
Also on that note, for people using USB audio as the connection for their DAC. Don't buy expensive audiophile USB cables. Just get something that doesn't suck and then look up the pinout and use liquid electrical tape to cover the pins for power on both sides. That's basically all the non-powered audiophile cables do. Since USB is a purely digital signal, the majority of the noise introduced into the signal path from the computer is via the power connection. Eliminating this unnecessary connection when using an externally powered DAC unit will greatly improve sound quality by lowering the noise floor.
All that said, there are better DACs than the ODAC on the market, simply because the ODAC has a very weak pre-amp stage, even within the combo build. I consider the O2 to be a significantly subpar amplifier and would only consider this buy if I were pairing it with another amp unless I was driving low-impedance dynamic transducers. It's also lacking some potentially useful features as a DAC. It's a great starting point though, and I would encourage anybody just getting into audio to start here, as you almost can't go wrong.
EDIT: As Ming pointed out below, I had made an incorrect assumption based on my experiences. The ODAC+O2 combo still uses USB power to power the ODAC component, just as it would be in the standalone. Based on this, there is no functional difference in theory between the combo or the standalone ODAC. I have only ever had experience with the combo and made the assumption that it had external power split off the power input for the amp because I did NOT hear the tell-tale noise associated with USB power I've come to expect from demoing other USB powered DACs. This is a good thing, as it means that the optimizations they've done in the ODAC for their power circuit manage to eliminate the majority of the power noise (or at least what exists is below the DAC chip's noise floor).
Because of this, please be aware that you should NOT modify your USB cables when used with the ODAC. My advice does stand true for DACs which are not USB powered when connected for the purpose of USB data.
Thanks for writing this.
I'm curious to know what is your preferred DAC all things considered.
I was not aware of that. I improperly made the assumption that the power supply feeding the amp in the combo had it's rail split to feed the DAC via a DC-to-DC converter or similar technique. After fully reading through your link I actually have a lot more respect for this design. Having heard it only in the combo form I made that assumption largely because I didn't hear the tell-tale power noise I've come to expect when using inexpensive USB DACs (I've tried the gamut of them). His optimizations to deal with the shortcomings (which he notes in his post) for USB self-powered DACs are apparently very effective.
Given this information though, I would have some concerns about it's ability to create a true 2vRMS line output. It's very rare (note, I've yet to see it in any that I've had the chance to measure with test equipment) to see a USB self-powered DAC that can actually output a true 2vRMS line out signal. I note he shows that he's achieved this, which if true is extremely impressive. I have not myself measured the ODAC with any test equipment for voltage output, so cannot verify, but I did not notice anything in my demoing that would indicate it put out a low line-out signal, so I would not be surprised if it did manage it.
My preferred DACs are significantly more costly than the ODAC. Lately I've gained an affection for the Sabre ES9018 DAC chip in particular. I've had the opportunity to try a few DACs based on this chip and currently use a Yulong DA8 in my desktop stack. I considered the Yulong DA8 and the Benchmark DAC2 to be the best of the bunch that I had the opportunity to demo as far as being completely transparent and supporting the feature sets I needed.
One thing that I'll note is that I've always had the best experiences with DACs which were built based on hard science and strict measurement. While there are many very good products out there, the best of the best are built this way. Benchmark and Yulong both make use of extensive measurements in their design and build processes to ensure quality, as does the ODAC.
As I mentioned above, I've had positive experiences with using the ODAC (as part of the combo) when had the opportunity to demo it and I think it's a very solid DAC. I just don't consider the O2 to be a powerful enough amplifier for my needs as most of the headphones I own and use are higher impedance or are power-hungry (orthos).
I actually had the chance to demo the Benchmark a while ago and I really liked it with my AKG K1000 which are very picky on source.
But it's waaaay too expensive for a $500 headphones, unless you have a wide setup of worthy headphones and speakers.
Do you have any other suggestions for a DAC under 500 ?
I got my own setup with a DIY gamma2 DAC , but I'm sure others are in the market for a new DAC (after buying T1 or HE-500) so this timing is perfect.
Honestly, the ODAC is pretty darn good for it's price. The main things it's missing that I require in my setup is I need SPDIF inputs and balanced outputs. That's why I bought a better DAC. That and I was interested in experimenting with using DXD/DSD files and 32bit USB. I can't really hear the difference between a 32bit/384Khz DXD studio master and a 24/96 FLAC that originated from the DXD master. But it was worthwhile experimenting nonetheless. Note, the Benchmark doesn't support this, although it's DAC chip does. The Yulong does though with proprietary drivers under Windows.
If you have a functionally transparent DAC that provides enough power on the line out to reliably begin your signal chain, and it has a low enough noise floor to get the dynamic range you require, you're basically good to go at that point. There's nothing else a DAC is going to do for you, although sometimes the specifics of your setup may require certain inputs and outputs, and that's when you end up needing pay out the nose to get the former combined with the latter.