BACKGROUND: (revised 3/7/11) After posting my review of the NuForce uDAC-2 the company has emphasized they compromised the measured performance of the uDAC-2 in the interest of better sound. For example, they have said they chose the volume control potentiometer, even when they’re aware it has significant channel balance issues, because it sounds better. They also have argued they intentionally allow the uDAC-2 to clip internally (creating higher distortion) because this makes the product sound better in real world use. So I wanted to provide a way for most anyone to compare the sound of the uDAC-2 against two other DACs under conditions that are as identical as possible.
THREE VERY DIFFERENT USB DACs:
- NuForce uDAC-2 ($129) - This is a popular second generation USB DAC from a company that specializes in mainly DACs. It sits above the $99 uDAC2-hp in their product line. It has line, headphone and digital outputs and uses a 24 bit/96 Khz chip. Here’s my detailed review.
- Behringer UCA202 ($29) – This inexpensive product is the lowest priced USB DAC from Behringer--a pro audio company that makes hundreds of products but only a few DACs. It has line, headphone, and digital outputs and uses a 16/48 chip. Here’s the full review.
- Benchmark DAC1 Pre ($1595) – This is an award winning, very well reviewed USB DAC/Preamp with excellent measurements. It has line and headphone outputs and uses 24/192 chips. The Benchmark is included mainly as a reference to help establish any “sound” the Benchmark ADC1 doing the recording might have. It represents a product well past the “point of diminishing returns”.
REFERENCE TRACKS: One track was chosen from two very different original CD’s:
- Audiophile Track – Sara K.’s “Brick house” from her Hobo CD on the Chesky audiophile label. This very well made recording has a lot of acoustic instruments and sounds, female vocals, and much more room ambience than a typical recording. It also has a wide dynamic range and only rarely reaches the full 0 dBFS digital signal level a few times. So the clipping problem with the NuForce uDAC-2 should be minimal with this track.
- Popular Track – Lady Gaga’s “Just Dance” from her The Fame Monster CD. This represents a typical current pop track that’s mastered to sound relatively loud by restricting the dynamic range. It was played back exactly as it was ripped from the CD with no digital level changes. To the possible benefit of the NuForce, the track itself has a lot of distortion which may well mask the clipping present in the uDAC-2.
WHY SHORT EXCERPTS: To avoid large file sizes, make downloading faster, and comply with copyright laws, a 15 second excerpt was chosen from each of the above tracks. The Brick House portion was chosen to represent as wide of variety as possible. The Just Dance excerpt was chosen from one the louder sections of the track to hopefully expose the 0 dBFS problem with the uDAC-2.
Short excerpts work especially well when doing ABX comparisons. Most people familiar with ABX testing end up comparing only a few seconds of music back and forth. The brain can remember a few seconds of a previous excerpt much easier than longer clips.
THE SOUND FILES: There are four FLAC sound files, and four MP3 files for each test track plus two more small files. If you would prefer to use just the Audiophile track, or just the Pop track, you can download just the four files for that track. There are a total of 18 files but you only needs as few as 4 or at most 10.
- 2 Reference tracks in MP3 & FLAC – These are the original tracks from the CD
- 2 NuForce uDAC-2 tracks in MP3 & FLAC – Recorded from the line output of the uDAC-2
- 2 Behringer UCA202 Tracks in MP3 & FLAC – As above for the UCA202
- 2 Benchmark DAC1 Tracks in MP3 & FLAC – As above for the DAC1 Pre
- 1 Copyright Notice – (keep the lawyers happy)
- 1 encrypted text file – More on this later
HOW THE SOUND FILES WERE MADE: The goal was to minimize as many differences as possible, and document the procedure sufficiently to let someone else reproduce it. That way someone can verify my results and/or compare other USB DACs in a similar way. The following apply to all 6 recorded files (see the tech section below for more details):
- The same two uncompressed native 16/44 reference tracks were played in Foobar 2000 for all three devices.
- The line outputs were used and carefully matched to same level.
- All three were connected to a Benchmark ADC1 professional A/D converter using the same cable and any channel balance differences were corrected.
- The files were recorded at CD quality to uncompressed files.
- The average volume of the files were slightly adjusted as needed to be as similar as possible.
- The files were tagged and compressed to both lossless FLAC and lossy MP3 formats.
MYSTERY FILES: At the suggestion of others, I’ve decided to make this a blind test and initially keep each file a mystery. There are some very good reasons for this. When a listener knows what they’re listening to, several kinds of bias influence their opinions. This has been proven many times in many ways. So to minimize bias, the files are named in a generic way. I’m also including a small encrypted file that contains descriptions of each file. When I reveal the source of each file, I will also reveal a password for the encrypted file. This allows anyone to verify I have not changed the assignments since the start of this trial. Here’s the basic plan (unless there’s consensus on improving it):
- Initially all the files will be “blind”. During this time I encourage anyone to download the FLAC or MP3 files, compare them, and hopefully comment on the differences either at the end of this article or in the appropriate forum thread—ideally pick a favorite or two out of the bunch. You can also download the Descriptions file if you want to keep me honest.
- When there’s enough feedback, I will reveal the two reference files. This will allow anyone to A/B compare each DAC file against the reference track. It will be interesting to see if this changes any preferences among the files.
- After a suitable time, I’ll reveal the remaining 6 files along with the password to decrypt the descriptions and publish a summary of the results.
MORE INFORMATION ON BLIND TESTS: For more information on why blind listening is better, you might want to check out one or more of these links:
- Dishonesty of Listening Tests (some good info by Sean Olive)
- Matrix HiFi Blind Test (a typical blind test and the surprising outcome)
- Blind Listening Tests (Hydrogen Audio – rather technical info)
HOW TO COMPARE THE SOUND FILES:
Download 4 or 8 MP3 Format Files – These are faster to download, and are very high quality, but use lossy compression which could mask very subtle differences. They can be played on most anything.
Or, Download 4 or 8 FLAC Files – These take longer to download and require a player that supports FLAC but use lossless compression for ideal sound quality. For a Windows PC, I recommend Foobar 2000 as it’s free and accurate. But many other players work including VLC, MediaMonkey, etc. If you have a portable player, several will play FLAC files including the newer Sansa players, Cowon, HiFiMan, and most anything running the Rockbox firmware. Some home music players/devices also work such as SlimDevices (Logitech), Sonos, etc.
Listen To The Files – Using a FLAC compatible player (see above), or most anything for MP3, listen to the excerpts. I would suggest using the most revealing setup you have. Most people find even moderately good headphones to be more revealing than speakers, but it all depends on what you have and your preferences.
ABX Compare The Files (optional) – For the most revealing comparison, if you have a Windows PC and some decent headphones or speakers, you can use Foobar 2000 to do your own solo blind test. Here’s how:
- Download and install the small, simple, and free Foobar 2000 player
- Follow these directions for installing the ABX comparator (hint: start from File > Preferences > Components in Foobar 2000).
- Once the ABX component is installed and shown in the list of Components, you can perform an ABX comparison by opening the two tracks at the same time (File > Open then hold down the Ctrl key to select both tracks and click Open), stop the player if it automatically starts playing, and then right click on either of the two tracks in the playlist and select Utilities > ABX Compare. From there clicking the “A” button always plays track A, and the “B” button always plays track B.
- If you’re not familiar with ABX and/or Foobar, this YouTube video (not by me) is helpful: ABX Audio Testing with Foobar 2000 (by homebrewedmusic)
Share Your Observations: Please note what differences you might hear and comment at the end of this article, or in a forum thread if you want. But, for now, if you’re fairly certain you know the origin of one or more of the excerpts, please keep it to yourself to keep it “blind” for the others. Or send me a private message.Ideally, pick your top favorite or two to help me “score” the results.
THE FILES: With the listening test over, the files have been removed to save bandwidth.
HEAPHONE VERSION: (updated 3/7) Some of you asked for a similar test with the headphone outputs, and I’ve done that with a few other twists here: Headphone DAC Listening Test Sequel
RESULTS (updated 3/16): The results have been published. This has been an interesting experiment, and if there’s enough interest, I may do it again as I’ve learned a lot from these two tests.
SARA K IN AUDIGY:
LADY GAGA IN AUDIGY: Believe it or not, this is just as it comes off the CD completely unprocessed (by me, obviously the recording engineer at the music label had other ideas):
- The reference tracks were ripped at their native format of 16 bits/44.1 Khz without any level changes (i.e. no normalization or volume leveling). Steinberg’s Wavelab version 6.1.1 was then used to extract the 15 second segments from both tracks.
- The Just Dance track already had peaks of 0 dBFS and was left exactly as it is on the CD (as you see above!). Like many pop CD’s, this one was clipped during the mastering process. The Brick House excerpt was normalized to 0 dBFS so the same gain settings could be used during recording for both.
- Both tracks were played in Foobar 2000 version 1.1.4 in WAVE format. Foobar was set for 0.0 dB gain with no equalization, DSP, plug-ins, or other processing for each of the recorded files.
- The operating system was Windows XP SP3 with all current Microsoft updates. The master volume slider was set to full and all other inputs were muted. XP and Foobar were configured to deliver a bit accurate stream to all three DACs.
- The Behringer UCA202 has fixed level line outputs which measure approximately 1.2 volts RMS into 100K with a 0 dBFS input signal. So this determined the output level for the other two DACs. The volume controls on the uDAC-2 and DAC1 were adjusted to match the UCA202’s line output within 0.05 dB (matched to approximately +/- 0.01 volts RMS). The levels were set playing a digitally (Wavelab) generated ideal 0 dBFS 1 Khz stereo sine wave file in Foobar 2000.
- The same professional Neutrik cables were used to connect each DAC to the Benchmark ADC1 for recording. The levels on the ADC1 were set to –1.0 dBFS using the same 1Khz 0 dBFS file used to set the DAC output level. The clip hold indicators were enabled on the ADC1 and never illuminated during any of the recordings. The full measured specs of the ADC1 are available here ADC1 Specs. The –1.0 dBFS recording level was chosen to avoid clipping from frequency response variations due to the DACs.
- Any channel imbalance in the DACs (not the excerpts) was removed by adjusting the ADC1’s level controls as necessary to within +/- 0.05 dB.
- Wavelab was used to record each track also at the native format of 16/44. This format was chosen as the majority of people evaluating the files likely do not have the ability to play back 24/96 files without sample rate conversion. This way the test files can remain bit accurate from the output of the ADC1 to anyone who plays them.
- Each recording was trimmed at the start to match the reference track (within about +/- 0.5 mS). The ends of the files were trimmed to match the overall length of the reference track.
- Each track was analyzed for perceived volume using Wavelab’s Average RMS Volume Analysis. The loudest track in each set was then used as the reference and the other 3 tracks were normalized to the same average volume in the left channel to within +/- 0.01 dB. Notes were kept on each track’s data.
- The files were then saved as WAV files, in no particular order, adding unique names to the end of each.
- The files were then tagged in MediaMonkey version 3.2.4 and saved as FLAC with a compression level of 5.
- Another set of matching files was made and saved in 192K VBR MP3 format using LAME with the well established “–alt –preset –standard” settings.
- The Description file was made and encrypted using the free 7Zip file compression utility. The password will be revealed later.