Introduction

I received the Curious Cables USB a few months back and postponed this review until I was able to compare this venerable cable with at least a dozen other USB cables. We all know it’s a fantastic cable, but how does it fare against the giants of today?

So over the course of a few months, I was insane enough to interview over 20 audiophile USB cables, most of which I had on hand for direct comparisons. Each and every cable had their “flavor” and I believe most will be able to blind test with high accuracy, especially silver vs copper. Without exception, even the worse sounding audiophile USB cable sounded better than the generics I had on hand. Most generic USB cables are just peaky and disoriented. Once in a while, I’ll pop in a generic for a sanity check.

When you’re copying your bootleg movies from your PC to an external hard drive, it doesn’t matter if it takes 5 minutes or 5 hours, your bits are error checked and consistent. Barring any physical problems with your storage device, you’re guaranteed a consistent copy via retransmission of bad packets. For streaming media such as video from your webcam or audio to your DAC, an isochronous transfer protocol (USB has a few endpoint types) is used. What this means is that once the bits are sent from your PC, packets are never retransmitted regardless of CRC errors. If packets had to checked and resent, this will be clearly audible in your system.

Asynchronous & Isochronous Basics

Isochronous refers to the transfer type and asynchronous USB essentially refers to the location of the master clock (who controls the clock?). In the context of USB, isochronous and asynchronous are mutually exclusive notions.

USB initiates a transfer with a specific device with of one of four types: bulk, control, interrupt, and isochronous. Bulk transfers are not time-critical (USB hard drives) and frames will be retransmitted when an error is detected.

With isochronous transfers, a certain amount of bandwidth is allocated on the host to accommodate the frames/bits required per second by the audio stream. DACs could detect errors but frames will not be retransmitted. But what defines the notion of a “second”?

With synchronous USB, the clock resides on the host (your PC, server, etc). With asynchronous USB, the master clock typically resides on the DAC. The DAC tells the host when to send the packets which essentially defines what a “second” means. This way audio data is “packaged” and streamed properly and the DAC never misses a sample. This does not ensure the integrity of those incoming bits, however, it just guarantees a specific rate using the presumably more accurate/consistent master clock on the DAC.

USB Cable Design

To get a better idea of what goes into the design of a USB cable, let’s break it down.

We have the two data lines (D+ and D-), the 5V power leg, and the ground leg. USB works via differential signaling, much like balanced cables. What determines whether the signal is a logic “0” or “1” is the differential voltage differences. On the receiver end:

  • Differential Logic 1 – D+ is 200mV above the D- signal
  • Differential Logic 0 – D+ is 200mV below the D- signal

Put simply, on the transmitter end, when D+ is above 2.8V, it’s a “1” and if it’s below 0.3V it’s a “0.” D- is the inverse so when it’s below 0.3V, it’s logic “1.”

These signals are square waveforms and any slight slope, tilting, or mistimings are measurable and audible. With USB’s fast transfer rates (various voltages at 480 MHz+) and our noisy gigahertz speed PCs emitting electromagnetic radiation into the USB port, it doesn’t seem unlikely that ‘0’s are mistakenly represented as ‘1’s and vice versa. The digital signal is a bit more analog and fragile than you would think. Due to various environmental and internal factors, these voltages might be skewed one way or another. I’ve broken down some of these factors:

  • Crosstalk – The electrical interaction of adjacent conductors with one another. We want to isolate and prevent any “leakage” of energy to other conductors.
  • Capacitance – Electrons get temporarily “stuck” in the cable typically in the insulation/dielectric. When electrons are held and release, this nonlinear propagation attenuates the signal causing plenty of signal timing issues.
  • Inductance – As a signal is going across a wire, a magnetic field is created. This magnetic field will create eddy currents in the conductor. These eddy currents will generate an opposing magnetic field which will impede the propagation of the signal and could potentially convert some of that signal energy into heat. Typically, the larger the conductor, the stronger the opposing force.
  • Resistance – Impediment in voltage typically correlated to the length and gauge of a conductor. Generally speaking, the shorter the USB cable, the better. From my listening sessions, imaging and resolution seem to improve with shorter cables. Both of my Danacable TruStream USB cables are less than 2 ft.
  • Impedance Mismatch – USB cables are 90-ohm, a mismatch may cause reflections in the signal. Something like the UpTone ISO REGEN may help here.
  • Quality of Connectors & Termination 
  • Geometry – Unsurprisingly, conductor layout is very important in all cable design. Shielding approaches need to be in place to prevent noise from affecting the signal path.
  • Clocks – This isn’t cable related but an accurate clock on both the PC and DAC side are imperative to a clean, jitter-free, and pure USB audio signal.
  • EMI/RFI – External factors could introduce noise into the signal. Proper shielding will help mitigate this.
  • Conductor – Higher quality conductors won’t degrade the signal over reasonable lengths

An audio signal needs to be heard in real-time. Anything less would introduce some level of artificialness of the music. The USB standards themselves don’t even take into consideration the inductance or capacitance of a cable, which really only affects an audio or video signal. Many of these electrical parameters could introduce jitter into the signal (timing errors), which all human ears are unequivocally sensitive to. It’s apparent the quality of the material and geometric design used for a USB cable is correlated to the level of signal deformation. Coming up with the right dielectric that matches well with the choice conductor material and figuring out how to arrange all of it is a big part in USB cable design.

This isn’t Rob Woodlands’s first rodeo in audio. He also manufactures the highly esteemed Bullet Plugs, and had a few creations of his own: the Bayonet plug and Cable Pods.

So what was Curious Cable’s Rob Woodlands approach to USB cable design?

Curious USB construction:

  • Physically isolated 5V, fully shielded mini coax power leg. Most of the noise on a cable will come from this interaction. There’s no better way to get this noisy leg away from the data lines than moving it outside the cable. This is the only cable I’ve tested with a fully isolated power leg.
  • Shielded ground leg to prevent the transmission of noise to the data lines. The relationship between the data and ground lines are of utmost importance.
  • Unshielded data lines
  • Pure silver conductor. In my opinion, this accounts for the “dimensional” sound you get with the Curious USB.
  • Out of all the cables I’ve tested, the Curious USB seems the most laborious to construct. Kudos for not cutting corners here.

Setup

  • DACs
    • Chord Mojo
    • Chord DAVE
    • Chord Hugo
  • Source
    • MacBook Pro Late-2013
    • SOtM Trifecta + Roon
    • SOtM tX-USBultra
    • UpTone ISO REGEN

Music

  • Man Plaba – Harry Belafonte
  • 1812 Overture – Pyotr Tchaikovsky
  • Heart of Gold – Neil Young
  • L-O-V-E – Jenna Mammina
  • Gypsy Eyes – Jimi Hendrix
  • Contact – Daft Punk
  • Over the Rainbow – Jane Monheit
  • Under the Table – Banks
  • Hurt – Youn Sun Nah
  • You Go It! – The Cannonball Adderley
  • Homecoming – Dave Holland
  • Back to Black – Amy Winehouse
  • Hurt – Johnny Cash
  • Moby Dick – Led Zeppelin

Comparisons

This cable took about 100 hours to break-in. Although silver provides an enormous soundstage, air, and resolution, copper just sounds more musically shaped (especially for voices) and tonally accurate to my ears. I find silver to be initially impressive but not as involving over time. Of course, this depends on the purity of the silver and Neotech silver and gold/silver stuff is probably the best I’ve heard.

Kimber KS-2436-AG ($1,195 @ 1.0m)

When I first A/B’ed the Kimber with the Curious, I thought they sounded very similar as far as tone and resolution. Over time the Kimber AG is the better sounding cable by providing a smoother and more coherent listening experience. The Curious is slightly leaner with more “exciting” overtones. Whether Kimber AG is worth over three times the price will be something you’ll have to decide for yourself. To my ears, I’m not even sure the Kimber AG is double the sonic performance of the Curious.

Danacable TruStream USB ($895 @ 2.0m)

This is my reference USB cable. It’s not the most detailed cable but, in my opinion, probably the most musically accurate. In comparison, the Curious USB provides a more spacious soundstage with incredible depth and resolution. It’s more transparent and quiet. The Danacable is smoother, has more accurate timbre, and provides more body and weight to the music. Although I would prefer the more holographic sound of the Curious, I felt more at home with the Danacable.

Phasure Lush USB ($269 @ 1.5m)

This cable is aptly named. It’s a very tonally rich cable. Probably the polar opposite of the Curious USB as far as tone. If you’re a resolution chaser, you won’t like this cable as much as the Curious USB. In ways similar to the Danacable, it has a denser and sweeter sound. Romantic if you will. Swapping between the Curious and the Lush and I get a presentation for different moods. The great thing about the Lush cable is that it has the ability to layer the sound like cables at least twice its price. It’s a bit like the warmer counterpart of the Curious USB. The price/performance ratio of both of these cables cannot be understated.

Clarity Cable Natural USB ($1,250 @ 1.0m)

Tonally similar to the Curious USB (It’s the silver silly). The Clarity Cable has a slightly better spatial reproduction and is even more detailed. Timbre is also more accurate and overall smoother. That said, the Curious USB gets 85% of the way there. For what it’s worth, I prefer this cable over the aforementioned Kimber AG for its natural sound. As you could probably infer by now, the Curious Cables USB is looking like more than just an incredible value in comparison to the giants.

Kimber KS-2416-CU ($495 @ 1.0m)

Although I have a proclivity towards copper, I preferred the Curious USB over the Kimber Cu in this comparison. You get so much more sonic benefits at a cheaper price. The Kimber Cu has more accurate timbre but sounds quite flat in comparison. It completely lacks any delineation across performers and really doesn’t have enough air and resolution to sound convincing.

Chord Silver Plus USB ($100 @ 1.0m)

The Curious USB immediately replaced my Chord Silver Plus. It wasn’t even close. Every cable in this lineup sounded better than the Silver Plus. The Chord sounds overly smooth and dulls out all the texture in the music.

Purist Audio 30th Anniversary USB ($1,500 @ 1.0m)

The priciest cable of the bunch, I believe it beats the Curious USB on all accounts aside from the Purist having a more tamed top and low end. Some may prefer the more present and weighty low and top ends of the Curious. Both have dimension and detail but I feel the Purist does it in a more coherent and realistic way. The Curious USB is the more energetic and sparkly cable.

Audience AU 24 SE USB ($595 @ 1.0m)

This cable is a very neutral sounding cable. In comparison to the Curious Cables USB, it’s a bit bass light and relaxed. I found the Curious to be more engaging and fun to listen to. Acoustic decay, maco/micro-dynamics, and resolution were better on the Curious USB cable. The Audience has a calmer, more mellow, and easygoing presentation. In a way, it’s smoother and doesn’t “try too hard.” The Curious USB is definitely more vivid with a healthier dose of clarity.

Vovox textura IC USB ($390)

Like the Lush cable, this cable may complement the Curious USB cable. It doesn’t have close to the amount of layering or dimensionality (probably its largest shortcoming), but it’s superbly musical and provides more meat on the bones. It has better timbre and tonal balance but isn’t as resolving or airy as the Curious USB cable. I find myself going between these cables depending on genre and mood.

Comparisons with other USB cables: 

A few commenters were curious about comparisons to other USB cables. I’m planning a full comparison chart soon but some of these cables are difficult to source. A few guys from the CA and Head-fi communities were kind enough to send their cables in. Please contact me if you’d like to do the same and I’ll add update my impressions.

Conclusion

After swimming with USB cables for the past few months, I’ve concluded these cables should be treated as components (no wonder why some are priced as components). They are an integral part of your digital playback system and should be selected with care. Unlike most interconnects I’ve heard, USB cables have more potential of sounding unacceptable in most systems.

If it isn’t obvious from the comparisons, I found the Curious Cables USB to be more than just a great value. At its price point, it might be the only USB cable you really need. Most would be hard-pressed to spend three times the amount for an appreciable difference in sound quality. Those who are using the Curious USB for their USB streamers and conditioners (ultraRendu, SOtM trifecta, UpTone ISO REGEN, etc) are getting a hefty bang for their buck. What’s even better is their 280mm Hugo Link for $160. This is currently the cable I’m using on the go with my Mojo and Hugo. There is no better portable audiophile-grade USB cable out there. Period.

In summary:

  • Performance gets really close to those of other top-shelf silver USB cables costing many times more. Of course, a small group of us are willing to pay the premium for just a bit more coherence, smoothness, and tonal/timbre accuracy. Just know that diminishing returns applies heavily.
  • Very clean, pitch black background. Copper USB cables can’t touch this.
  • Incredible soundstage and depth. Typically for this level of musical dimensionality, you would have to pay at least double. This is what makes the Curious USB so compelling, especially for their Hugo Link cables.
  • Slight grainier upper-mids with a slight emphasis in the upper octaves. Other than this, musical integrity is intact.
  • A great balanced sound that’s never irritating or fatiguing.
  • I personally prefer a richer and denser mids and lows but this might require using copper for the signal.

I really enjoy this cable and believe it to be one of the gems in audiophile land. After listening, I’d suspect many will look no further than the Curious Cables USB. As always, I speak with my wallet and have purchased a few Curious USB cables for our systems.

Curious Cables offers free shipping and a 30-day money back guarantee. They’ll even pay for the shipping back to Austrailia. If you’re one of the 1% who hasn’t satiated their curiosity yet, the Curious Cables USB is worth interviewing.