DAC Listening Sequel

INTRO: Thanks to everyone for their comments on feedback on the first round—both public and private—on the recently published NuForce uDAC-2 Listening Test. That challenge compared the line outputs of three different USB DACs and some have suggested comparing the headphone outputs might be even more interesting. The one thing keeping me from doing so was wanting to modify the Behringer UCA202 to have a better headphone output first. I’ve finally got that done, so here’s round two!

THE SEQUEL: This time around we have real DACs, playing real music, through real headphones—all captured as accurately as possible by a Benchmark ADC1. While sequels are often not as good as the original, hopefully it’s the opposite here. Just like before, I’ve done all I can to make sure this is as fair (and verifiable) of a test as possible.

YOU DON’T NEED HIGH-END GEAR: You can listen to the test files below on anything you like. Because the listening test is mainly about the differences between the recordings, not the absolute sound of any one recording, you don't need near-perfect gear to pick a favorite.

It's somewhat like looking at 3 different shirts at an online retailer. You can tell easily how they're different from each other by looking at the pictures of each shirt using your PC. You don't need a perfect computer display to tell that one might be a slightly darker shade of blue than another one. Most any computer display is good enough to show the differences. But you can't be sure of the exact shade of blue until you have the real shirt in front of you.

It's the same idea here. Your headphones and playback hardware don't have to be perfect to reveal the differences between the test files. But the more revealing your gear is, the more subtle differences might become more obvious.

OUT WITH JUST DANCE: Several mentioned they had a hard time hearing any differences with the previous Lady Gaga track. It was chosen because the high average level would hopefully reveal the clipping flaw I found in my review of the NuForce uDAC-2. But, in hindsight, it likely has too much distortion of its own.

ENTER ANNIE: I’ve replaced Ms. Gaga with a bit more refined (and obscure) female vocal track: Annie de Franco’s “Tis of Thee” from her Up Up Up Up Up Up release. It’s much less compressed but her voice still hits some fairly loud peaks and there’s some sibilance on the loud bits. Sibilance can be a revealing test as it tends to sound significantly more harsh if there’s much added distortion (sibilant tracks are often used to evaluate the upper midrange and high frequency performance of headphones and speakers). Tis of Thee is an otherwise fairly well recorded song with some interesting ambiance and low bass extension. The excerpt is about 19 seconds long (the legal copyright limit is 20 seconds).

BRICK HOUSE RETURNS: Lots of you seemed to like Sara K.’s Brick House so here it is again. This also allows comparing the headphone outputs to the Line Outputs of the previous trial (well, sort of, see the Tech Section).

KEEPING REAL SCORE THIS TIME! With the last round I was just looking for general comments. But it’s tough to summarize them in any objective way (although I’ll still try). So this time, I would like to be a bit more objective with the results. So if you want to comment, please include your favorite and a “runner up” (second most favorite) from one or both of the tracks. I’ll count the top choices as 2 points, and the runner up as 1 point when I add up the scores. So you can have up to four votes total (two for each song).

A “NEW” USB DAC (updated 3/16): This test includes the same three DACs including the $29 Behringer UCA202 from the previous listening test. But it also includes a modified version of the same UCA202 DAC. The modification significantly improves the headphone amplifier section for more output and a lower output impedance using just a few dollars worth of parts.

HEADPHONES USED: I could have picked one of my more “difficult” headphones, but I decided to use the very common Sennheiser CX300 16 ohm headphones for all but one file. They have a 16 ohm impedance and a relatively flat impedance curve (see the Tech Section below). For one of the DACs, I also used Ultimate Ears SuperFi 5 Pro headphones which have a much wider impedance range. I wanted to see if that track stood out in the blind tests.

CHEATING IS MORE DIFFICULT: Some enterprising folks “cheated” with the previous tracks by analyzing them in some clever ways that don’t involve listening. While you can still likely get some hints if you want to cheat, there are likely fewer clues here and some may not lead you to the right answers. I was more careful in how I prepared the files this time. Besides, the real fun is seeing how your ears do, right?

NO ORIGINAL TRACKS (yet): (revised 3/8/11) This test is mainly about how the DACs sound not about how accurate they are. Some manufactures, like NuForce, at least partly "design by ear" rather than by measurement and think their gear sounds better even if it measures worse. If the criteria is “the closest DAC to the reference is the winner”, companies like NuForce are more likely to lose. But if the criteria is “which one sounds the most pleasing” it should be a fair contest.  The Benchmark DAC1 is already included as a reference here. And after enough votes, I’ll release the original (as they came off the CD) reference tracks if anyone wants to go back and compare each DAC recording to the original or try making their own recording.

THE COMPARISON:

• NuForce uDAC-2 with Sennheiser CX300 headphones
• Behringer UCA202 (unmodified) with Sennheiser CX300 headphones
• Behringer UCA202 (modified) with Sennheiser CX300 headphones
• Benchmark DAC1 Pre with Sennheiser CX300 headphones
• “Mystery DAC” with Ultimate Ears SuperFi headphones (only for “Tis of Thee”)

INSTRUCTIONS: See the previous NuForce uDAC-2 Listening Test for more detailed instructions and a link to an ABX video (not by me), but the basics are:

• Download either or both sets of tracks in MP3 (faster & more compatible) or FLAC (lossless) format
• Compare them carefully--ideally with Foobar and ABX to make sure any differences are real and not imagined or based on the comments and votes from others (see the Previous Review for more on blind listening)
• Pick the DAC that has the sound you most would like to own and listen to, then pick your “runner up” (second most favorite) sounding track for each song, and either post a comment here, to an applicable forum thread, or message me privately.
• If you think you have a good idea about which DACs were used to record some of the tracks, please keep it to yourself or message me privately until others have had a chance to do their blind listening and pick the favorites.

THE FILES (updated 3/16): The listening test has been completed, and to save bandwidth, the files have been removed.

RESULTS (updated 3/16): The results have been posted! If there’s enough interest, I may conduct future listening tests as I’ve learned a lot from doing this one.

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TECH SECTION (optional reading):

NEW AUDIO TRACK: Here’s what Tis of Thee looks like in Audacity:

HEADPHONES USED: The impedance of the Sennheiser CX300 headphones mostly stays between 16 and 17 ohms so they’re a fairly easy load. The trace in gold is the impedance, while white is the phase angle in degrees:

The Ultimate Ears SuperFi 5 Pro’s have a much wider impedance swing from about 8 ohms to over 80 ohms (they’re rated at 21 ohms):

LEVELS: A “comfortable” listening level was chosen for each of the two headphones. This was rounded to 400 mV RMS with the Sennheiser’s, and 200 mV RMS with the SuperFi’s, playing a 0 dBFS 1 Khz tone with the headphones attached as the load. The same level was set for each DAC tested using the volume control on each of the DACs

HEADPHONE ISOLATION: The headphone earpieces were carefully isolated from the outside world and each other, and the room kept as quiet as possible during recording to eliminate stray sounds picked up by the CX300’s (or crosstalk between them).

CABLING: A high quality 3.5mm “Y” cable was used to split the output of each DAC to feed both the headphones and the ADC1. The same cabling was used for all the DACs.

GROUND LOOP NOISE: (updated 3/8/11) A ground loop was formed because there are two USB connected devices, and one of them (the ADC1) is also grounded via its power cord. When the ADC1 is used with balanced connections, this is not an issue, but there’s (almost) no such thing as balanced headphone outputs so I had to use unbalanced connections. The input gain of the ADC1 has to be set 9 – 15 dB higher in this test than it was in the previous line output test because the headphone outputs are much lower than the line outputs. This raises the noise by 9 – 15 dB making previously inaudible ground noise audible—both the NuForce and the Behringer had audible “USB noise”. Ideally you want everything in a system grounded at a single point. Once you have 2 or more grounds, the potential for loops exist. This is why HRT, for example, isolates the ground and USB connection for their Streamer II DAC.

To eliminate the ground loop, and all the associated noise, I used a very high quality Jensen audio isolation transformer designed for the purpose to isolate the DACs from the ADC1. The transformer may have some slight “sound” of its own but it measures and sounds very good by itself and should be relatively transparent. But, technically, the set up is not identical if you’re comparing these headphone output files to the line output files of the previous Listening Test. This would have been true anyway in terms of the input gain on the ADC1 being 9+ dB higher changing the noise floor even without the transformer. But having the transformer in the signal chain may very slightly alter the sound in other ways.

ENCRYPTED FILE: The encrypted was revised to add the new names. It has the same password as the old one which will be revealed later. Those who downloaded the older one can save it if you want to verify my honesty as to what’s what.

EVERYTHING ELSE: The rest should be the same as the last Listening Test.