This is a review and detailed measurements of the Cyrus soundKey portable USB DAC and headphone amplifier. It is on kind loan from a member. It costs USD $98.99 from Amazon with prime shipping. It comes in a few colors.I like the flat form factor with detachable USB cord:I received two short USB cables, one of which you see in the picture above. I don't know if that is how Cyrus ships it or the owner sent it me as such.There are no volume controls so you would be relying on your software player for that.One of the differentiators for the soundKey is low power consumption according to the company. I will be measuring and reporting on that later in the testing. For now, there was no rise in the temperature that I could feel on the unit.As usual, we start with our dashboard view of 1 kHz tone:Typical of these small dongles, there output is too low to use the soundKey as a proper DAC at 1.7 volts (like to see 2 volts).Distortion numbers are disappointing with a SINAD (signal over noise and distortion) of only 76 dB. This is due to high second harmonic distortion. This puts the soundkey at the bottom tier of our rankings of all DACs regardless of type and price:For a high-end company, I would have liked to see this distortion much more under control.Turning down the input level a few notches did not improve the situation much.Another down note was the roll off in low frequencies:I don't expect to see this in DACs/headphone amps.Jitter spectrum was very clean but partially due to high noise floor:Linearity shows the soundKey to be fine for 16 bit audio (96 dB) but after that, there is a lot of deviation/lack of accuracy:Dynamic range likewise was "OK" for CD but not much more:On the right we see the dynamic range when outputting just 50 millivolts into 600 ohm load. This is how it ranks against a few recently measured devices:Most important parameters for a headphone amplifier are power ratings versus distortion. Here it is at 300 and 33 ohms respectively:The 300 ohm measurement shows a bit less power than Topping DX3 Pro in low gain. There is no clipping which is nice. Clipping comes our way though with 33 ohm load.Putting these in perspective, we get:Finally, here is the power consumption while idle and not:The soundKey is almost as efficient as Dragonfly Red.I started with my usual Sennheiser HD-650 headphones. Here, I was surprised that there was enough volume if you had recordings that were close enough to 0 dBFS. The sound overall was good and I could not detect much distortion even at max volume which is probably where the volume control will live.Switching to Hifiman HE-400i was less satisfying. Bass performance was not there. And at high volumes I thought the highs got a bit distorted.The Cyrus soundKey jumps into the crowded market of probable headphone dongles with good looks, form factor and functionality a volume control would have been nice though). On the down side, distortion metrics are poor. Better efficiency compensates for that if you use it on battery operated phones and such (you would need the Apple camera dongle for iOS support).Listening test results were good with Sennheiser and not so good with Hifiman. The later probably suffered from rolled off frequency response and clipping distortion.Overall, I am ambivalent about the soundKey. I see it a step up from $9 dongles but the $99 price is hard to swallow. I let you guys decide if it should, or should not be recommended.------------As always, questions, comments, recommendations, etc. are welcome.Going to the dentist tomorrow. Like to load up on a bunch of chewy candy just before I walk to give him a shock the moment I open my mouth. Please donate money so I can afford a good variety of candy by using: https://www.patreon.com/audiosciencereview ), or https://audiosciencereview.com/foru...eview-and-measurements.2164/page-3#post-59054 ).