Impressions: Purist Audio Design's 30th Anniversary USB Cable

This is pretty crazy for me. It's a little jarring that, to date, the most expensive audio-related item I've held with my own two hands is a 3 meter USB 3.0 cable. At $1900, it is about as close to the opposite of cheap as I can reasonably imagine.

Let's start with the build. For $1900, it better be stellar, and it is. The cable is substantial, thick and beautiful. The heat shrink is well-aligned, it features the logo along with some other information printed in gold lettering at both terminal ends. In the middle, near as I can tell, is a ferrite choke that I assume serves to prevent interference in use on devices that carry power and data over the same cable.

The connectors themselves are nothing special, fairly run-of-the-mill USB A male to USB B male. In use, I've had no issues with intermittent connectivity or bad connections, thankfully.

Over the next two months, I will continue to test and use this cable with every single device that I can. I'll be having a friend help me do blind tests between this and a variety of other USB cables. I have a firm belief that cables do not change sound, especially error-detecting digital, like USB, but I will put that belief to the test as rigorously as I can. After I've exhausted every potential test that I can, I'll write up a full review of it including whatever conclusion I have reached. 

About USB and Error Detection

Inherent within current USB standards is a CRC (cyclic redundancy check) on every transmitted packet. Without going too far into the technical side of this, this essentially means that on every packet of data transmitted, there is a check performed that confirms that the data sent by the user was received successfully. In isochronous transmission, which is used for USB audio transmission, when a packet is confirmed via a CRC to have an error, it is dropped from the stream. This means that when there is an error in data transmission over USB to your Amp/DAC/etc. the packet that had the error is dropped and not re-sent. This manifests as, well, a lost bit of data, usually a single sample.

The main factor that drives these errors is electrical noise in the cabling used. Electrical noise in cabling is most frequently introduced as a result of a ground loop. A ground loop most frequently occurs when power and data are sent through the same cable at the same time, as the power communicates through the cable to the device and then back to the source over the grounded shielding of the cable itself. Because of the nature of computer power, there are inherently very small current spikes that occur relatively randomly during power and data transmission. These cause incredibly small voltages between the USB audio device and true ground. Although these are usually in the range of a few thousandths of a volt, microphones and other audio devices only generate a few thousandths of a volt anyway, and they can thus cause the hum, buzz, and general chaos that you hear when this happens.

These issues can be solved using a very simple ferrite bead (also called a ferrite choke). That’s what the little bulbous, cylindrical clip-on at the USB-A male end of a cable is. It collects this high frequency interference, removing it from the chain, and negating the buzz. There are a million and a half $5 cables with this feature. This $1900 cable also has one. Tangentially, any higher-end DAC/amp will be separately powered via a dedicated power supply, fundamentally removing this issue from even being a possibility.

More Reading

Below are a bunch of links that I found helpful when writing the above blurb:

http://www.beyondlogic.org/usbnutshell/usb3.shtml

http://allen-heath.helpserve.com/Knowledgebase/Article/View/1002/201/qu-windows-driver-understanding-and-troubleshooting

http://forum.cockos.com/showthread.php?t=113721

http://www.techhive.com/article/3063590/home-audio/how-to-get-rid-of-hum-and-eliminate-other-noises-from-your-audio-and-video-systems.html

https://techsupport.cambridgeaudio.com/hc/en-us/article_attachments/201787081/Audiophile_s_Guide_To_Bit_Perfect_USB_Audio.pdf

Disclaimers

This cable was provided to me for review by Purist Audio Design. I am not being paid by anyone to write this review, to endorse the product reviewed, or for the content that I put in the review.

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