Cyrus soundkey
One often sees Kickstarter appeals to create new products. But how often do you see the resulting product on retail shelves? One such is the Cyrus soundkey, a tiny little DAC and headphone amp, designed to give you decent sound from smartphones. Cyrus is no start-up, of course, rather an established UK hi-fi company of renown. So while its Kickstarter goal was £50,000, it ended up receiving pledges of nearly £80,000. And now we have the Cyrus soundkey in our hands.
Well, not so much in our hands as plugged into a shiny new Google Pixel 2 phone. The phone is using an app which is drawing FLAC music wirelessly from our network server and feeding it digitally to the DAC, which is decoding it and powering a set of headphones we are wearing.
But first, let’s understand what we have here. The Cyrus soundkey is a very small device, just 54mm long and slightly less than 8mm thick. And it weighs just 18 grams. It’s certainly easy to take with you.
At one end is a 3.5mm stereo audio socket for your headphones. You can also use it as a line output to an amplifier. At the other is a Micro-B USB socket. That’s for plugging into your phone’s power and data port. At this point we should note that even though the unit was aimed at smart-phones — which is why it’s so very small and light — it will also work with tablets and computers.
But it won’t necessarily work with all smart-phones. Just about all iPhones made in recent years will be fine, and very, very many Android phones. The latter do require what’s called ‘On-the-Go’ (OTG) capability. These days almost all Android phones do have that. After playing with a lot of Android phones lately, we’ve only come across two or three lacking that capability, and they’ve all been extremely cheap models.
The soundkey comes with a nicely styled Micro-B USB to Micro-B USB cable for use with many Android phones, along with a standard USB Type-A to Micro-B USB cable. You can use that one with a computer, or plug it into an Apple Lightning-to-USB Camera Adapter and plug the adapter into the Apple device. This works with the iPod touch and iPhone, as well as iPads.
Some super-compact DACs of this kind come with a standard USB Type-A plug on the end so that they plug straight into a computer. We think we prefer the Cyrus approach with short adaptor cables. We always fear that a solid DAC poking out the side of notebook computer, or from the bottom of a smartphone, is more likely to be damaged, or cause damage to the socket to which it’s connected, if accidentally struck.
For broad compatibility, the unit uses the USB Audio Class 1.0 protocols. The good thing about that is that you can plug it into any Windows computer going back years. USB Audio Class 2.0 requires a driver for all versions of Windows (at least before the Windows 10 Creators Update from earlier this year), although it works fine with Macs and Android and iOS.
The not-so-good thing about USB Audio Class 1.0 is that it’s restricted to a maximum of 24-bit/ 96kHz sampling. Still, most of what you’ll be listening to is 16-bit/44.1kHz sampling, and only a minority of high resolution goes beyond 96kHz.
One other practical matter. Cyrus Audio says that the soundkey draws only 50mA of power, or half of the typical draw. Since it’s powered by your phone, that reduces the impact it has on the phone’s battery life.
Cyrus soundkey
We had a good listen using various headphones and wired earphones, and a variety of platforms. We used the Pixel 2 mentioned — we had to use a generic USB Type-C to Micro-B USB cable for this — and a Samsung Galaxy S7, and a Samsung Galaxy Tab A tablet. All worked perfectly well, recognising the DAC and routing their audio to it.
Likewise our iPad Mini 4, our Windows 10 computer, and our Mac Mini. Having checked that everything worked, we did most of our listening and testing with Android phones and the Windows 10 computer.
Clearly Cyrus’ high fidelity heritage has been brought to bear. As we’ll see shortly, just about all the tech stuff ticks the relevant boxes, but we were impressed just as much with the sound. There was excellent control of all our headphones. We mostly used three sets of head-gear. First, there was our old Sennheiser HD535 headphones — they have an open design and are quite insensitive and have a fairly high 150-ohm impedance, which can make things difficult for portable gear. Then there was our Oppo PM-3 headphones — which are quite sensitive, closed-back and which offer only 26 ohms impedance. That impedance is even across the full range of audio frequencies. Finally, there were our Sennheiser Momentum earbuds, which are rather high quality as these things go, and even lower impedance at 18 ohms.
All our music sounded excellent. Had we not known what device was being employed, we’d have been hard put to say that it wasn’t a high-end DAC and headphone amplifier. Even with fairly complex music playing at quite a high level, the Cyrus soundkey remained smooth, yet powerful. 
Just as importantly, it tended to impart as slight sense of distance to the music, so there was less of the ‘in our head’ experience, and more of an ‘out there’ experience.
Noise? It was completely inaudible to us, regardless of platform, and that includes the Windows computer plugged into power. And even with the old high-impedance low-sensitivity Sennheiser headphones, there was ample volume available. Which is impressive, given that the system is relying on the 5V USB power supply. (We note that Cyrus says that the device is suitable for headphones only between 16 and 64 ohms, lest others not be loud enough. That proved not to be a problem for our listening.)
There was really only one problem: the higher sensitivity devices had to operate with the volume control right down near the bottom of the range when we used it with Android devices with our preferred USB Audio Player Pro app. That made it hard to adjust the level precisely. But perhaps it was the player as much as anything. This one is optimised for use with external DACs, but for some reason the ‘hardware volume control’ slider within the app wouldn’t work. With Spotify there was no such problem. A good listening level was in the middle of the phone’s volume range.
We ran some measurements, confirming noise to be impressively low. With 24-bit/96kHz signals it delivered -103.5dB (A-weighted) noise levels. Repeating the test using a computer running on power and running from battery, the power produced a little breakthrough at 50Hz and its harmonics, but that remained inaudible. Most of the noise floor was below -130dB and the 50Hz peak was at -110dB. The noise levels with 16-bit audio were at -96.8dB, A-weighted.
With 96kHz sampling, the high-end response was extended, down by 1dB at 36kHz and 1.5dB at 44kHz. Bass rolled off too, which was unusual, down by 1dB at just under 30Hz and 1.5dB at 20Hz. With 44.1kHz sampling the top end was maintained all the way to 20kHz, down perhaps 0.1dB, above which the output hit a brick wall.
We really enjoyed the Cyrus soundkey — practical, convenient to carry, and delivering excellent sound into just about any sensible set of headphones. 
Cyrus soundkey
Cyrus soundkey portable headphone DAC & amp
Price: $169
+ High quality sound
+ Extremely portable
+ Easy to use
- Limited to 96kHz sampling
Inputs: Micro-B USB
Outputs: 1 x 3.5mm headphone/line
Dimensions (whd): 54 x 23.5 x 7.9mm
Weight: 18g
Warranty: One year