Here is an example. I have been comparing the O2/ODAC to the Apogee Duet 2. The Apogee is obviously the more expensive DAC that is also used by mobile professionals. The Apogee representative told me the sound would be close between the headphone out and the line out, the line out being the better SQ. So I played the same music through the headphone out of both DACs. I first discovered that the soundstage of the Duet has much more depth. There was more of an analog coloration to the sound. Some higher frequency sounds were clearer like that of a cymbal. The ODAC/O2 on the other hand seemed to be brighter. This does not make sense when I looked at the FR graphs of both DACs. This examination showed the Duet to have the high frequencies boosted, not the ODAC/O2.
Also, there were sounds that would show up more clearly with this ODAC/O2 compared to the Duet. In one case, there was a sax playing the melody and a guitar playing counterpoint. On the O2/ODAC, the counterpoint showed up clearly. On the Duet, this guitar was much more subdued and at times was almost lost in the other background instruments being played. Also a specific example of the differences is with the song "Old Time Rock and Roll" by Bob Seger. There is allot of reverb used at the start of the song. With the O2, you can hear the definite echo, but on the Duet, the reverb is very subdued. Now this is comparing the headphone out on both DAC/amps. The line outs may tell a different story which should be the case with the Duet.
So what is going on here? Is the Duet still the more accurate of the DACs? If so, what is the O2/ODAC doing to the sound?
Bob
Also, there were sounds that would show up more clearly with this ODAC/O2 compared to the Duet. In one case, there was a sax playing the melody and a guitar playing counterpoint. On the O2/ODAC, the counterpoint showed up clearly. On the Duet, this guitar was much more subdued and at times was almost lost in the other background instruments being played. Also a specific example of the differences is with the song "Old Time Rock and Roll" by Bob Seger. There is allot of reverb used at the start of the song. With the O2, you can hear the definite echo, but on the Duet, the reverb is very subdued. Now this is comparing the headphone out on both DAC/amps. The line outs may tell a different story which should be the case with the Duet.
So what is going on here? Is the Duet still the more accurate of the DACs? If so, what is the O2/ODAC doing to the sound?
Bob
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Got the Duet schematic or taken a look at the board? The O2 has a about a 1.1 ohm output impedance. So it's quite likely the Duet's is substantially higher. A lower damping factor is less accurate but can be subjectively more pleasing in cases where it masks other limitations.
It does look as though the Apogee's the more transparent, yes. The ODAC's likely adding noise - its endemic to S-D based converters and having on-board opamps adds even more noise. The noise is modulated with the signal hence confuses the interpretation of acoustic space by our perceptual processing.
There's a design fault in the ODAC which doesn't help matters - the wrong output cap has been specified for the Micrel LDO regulator feeding the ES9023. Correcting this will likely make some small improvement to the transparency. Adding power supply filtering and more local decoupling to the ES9023 will help a lot more.
I have the 250-ohm DT880s. So the damping factor with the Duet is 8. Is this adequate?
Bob
I think you are on to something! Maybe 250-ohm headphones for a 30-ohm Duet is not a high enough damping factor? I do find the base boomy and the treble lacking compared with the O2.
Bob
For playing music (classical, jazz, rock) over the headphones without any distortion.
Define an acceptable limit and do the math for your case. Be cautious when reading up, however; typical answers for a desirable minimum damping factor are in the range of 20 to 50 but they're usually based on relatively idealized models of driver behaviour.
For example, the Focusrite VRM Box + Vsonic GR07 combination I use has a damping factor of about 4.5. But no boomy/flabby/resonant bass quality I can detect. Perhaps because the cellulose membranes in the GR07s show no sign of suspension resonance in impedance measurements. That should make them less reliant on electrical damping than most cans.
For example, the Focusrite VRM Box + Vsonic GR07 combination I use has a damping factor of about 4.5. But no boomy/flabby/resonant bass quality I can detect. Perhaps because the cellulose membranes in the GR07s show no sign of suspension resonance in impedance measurements. That should make them less reliant on electrical damping than most cans.
It appears be working much better with the 600-ohm Beyers. This does surprise me since everyone has been telling me that a damping factor of 8 is adequate. I guess it depends on the specific combination of DAC/amp and headphones.
Bob
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