Both Kal Rubinson and Michael Lavorgna report in this issue on the positive effects UpTone Audio's USB Regen7 had on the sound quality of D/A processors fed USB data. Kal loaned me his review sample for a few days so I could see if it made any measurable differences to a D/A processor's analog output signal.
Unlike AudioQuest's passive JitterBug, which I reviewed in the September issue, the Regen is an active device that regenerates the USB datastream. The JitterBug is intended to reduce the level of radio-frequency (RF) noise on the balanced data connection, the 5V power bus, and the ground connection. In my measurements of the JitterBug for my review, I used four different test signals with three different D/A processors and one, two, or no JitterBugs. Only in two of the large number of test conditions did I find any measurable improvement in the reconstructed analog signal. Even then, the measured improvement was so small that it was hard to see that it could have had anything to do with the improvement in sound quality I heard the JitterBug make with the Mac mini I use as a music server.
One thing that emerged after the review was published was that Gordon Rankin, one of the JitterBug's designers, suggested using a very long USB cable to bring the measurable difference out from the DAC's noise floor. The long cable will marginalize the USB transmission to the point that the effect of the JitterBug will be more easily measurable. So to measure the effect of the Regen, I took a generic 3m USB cable and lengthened it with a 1.5m USB cable extender. This brought the total length of the cable close to the USB's maximum cable length of around 12'.
Using this long cable and my MacBook Pro running on battery power, I tested the Regen with a PS Audio PerfectWave DirectStream DAC and a Meridian Prime D/A headphone amplifier, using my Audio Precision SYS2722 analyzer and the same test signals I'd used for the JitterBug review. Sad to report, I found that the Regen made absolutely no difference in the D/A processors' analog output signals. With one exception: the levels of the power-supply–related spuriae in the Prime's output when powered by its wall wart were slightly higher in level with the Regen than without it.
However, I did find one idiosyncrasy that will affect A/B comparisons of the Regen's effect. I used Pure Music 2.0 to play my source files. When I disconnected the D/A processor from the MacBook Pro's USB port to insert the Regen then reconnected it, while Pure Music still sent the file's data to the USB port for playback, it no longer automatically changed the Mac's CoreAudio parameters to match the file. If I played a 16-bit/44.1kHz file without the Regen, then disconnected the DAC, inserted the Regen, reconnected the DAC, and played a 24/96 file, that file was truncated to 16 bits and downsampled to 44.1kHz. This is something that will be readily audible.
The correct procedure to perform A/B comparisons with the Regen using Pure Music was to deselect the DAC in use as the output device, relaunch Pure Music, disconnect the DAC, insert the Regen, reconnect the DAC, reselect the DAC with Pure Music, then relaunch Pure Music. Then, and only then, could a legitimate A/B test be performed.
Kal and Michael were careful to observe the correct protocol when performing their comparisons with the USB Regen. But I do wonder how many of the A/B tests published on the Internet were not done correctly.—John Atkinson
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COMMENTS
This is not a criticism, but it would have been informative to include a manufacturer's comment. Maybe the manufacturer of the Regen can offer an explanation, as to how the device can improve the sound, but that these improvements/differences could not be shown in the measurements that were conducted.
I have no trouble believing that this device can improve the sound without the changes being measurable; but an explication as to why would be interesting.
This is not a criticism, but it would have been informative to include a manufacturer's comment.
I have added the Manufacturer's Comment to this reprint.
On the question posed by another reader why a DAC would need to use the 5V USB supply, many designs power the USB receiver chip from the USB bus, to try to minimize contamination of the processor's power supply for the D/A and analog circuitry with RF noise.
John Atkinson
Editor, Stereophile
I have no trouble believing that this device can improve the sound without the changes being measurable; but an explication as to why would be interesting.
Thank you for your interest in learning more. At the beginning of his original review on AudioStream, Michael Lavorgna was kind enough to include our entire condensed "white paper" discussing some of the specific technical limitations of USB audio interfaces and how the REGEN addresses them.
You can find that at this link: http://www.audiostream.com/content/uptone-audio-usb-regen
Regards,
Alex Crespi
I guess these devices are becoming quite popular and new ones are announced like iFi purifier2. From the reviews it is not quite clear whether there is a sonic benefit from USB outputs of only computers or also of USB streamers like Aurender (the former is designed for any use and the later is supposed to be audio specific).
Or is the culprit the USB input receiver design of the DAC. So do different DACs (which use different input receivers) have similar benefits of these devices?
Most DACs (except some portable ones) are not powered by USB so why is 5V necessary (except maybe for initial hand shake).
You raise good points. I used the Regen with what I had but, absent engineering specs from Regen and from the USB devices to which one might connect it, one cannot generalize.
You ask who is the culprit here? I don't know but most finger-pointers suggest, as you do, that it is the influence on the DACs rather than out-of-spec USB sources.
As for the 5V, that is necessary to wake-up self-powered DACs.
It is hard for me to make the "bits are bits" argument when it comes to USB since it was developed as a general purpose interface that also carries power and has seen significant improvements since its inception twenty plus years ago, so it is not surprising that there would be further room for incremental, application-specific improvements. That being said, there seems to be far more bang-for-the-buck associated with the various regen products than the much maligned and far more expensive high-end USB cables.
Thanks to you both, John and Alex for posting the manufacturer's comment and white paper. Very interesting and informative!
I spent a lot of time wrestling with dirty USB power and I concluded the only thing to do is cut it out entirely. It's not reasonable to think that the noise pollution can be cleaned up by adding additional circuitry between the computer and DAC. Those circuits generate noise.
There are other options out there. I suggest folks go on to Google and search for "Dirty USB Power" and you'll find other solutions, better tailored to this problem than adding more circuits to the very important link between computer and DAC.
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