Over the last decade, there has been a nice proliferation of extremely high performing and inexpensive gear coming out of Asia available on places like eBay among other on-line merchants. Last year, I was scouring the 'Net looking for a decent headphone amp/DAC combination when I came across some good reviews of the AUNE X1.
The USB implementation is the bog standard BB PCM2707 (adaptive isochronous, only up to 16/48 sampling rate USB 1). The internal DAC is the BB PCM1793.
Adaptive USB: i7 PC --> shielded 3' USB cable --> AUNE X1 --> shielded 3' RCA --> E-MU 0404USB
TosLink & Coaxial: SB Touch (EDO plugin, WiFi) --> standard coaxial/TosLink --> AUNE X1 --> shielded 3' RCA --> E-MU 0404USB
July 2013 update:
As per recent testing procedure, here's the oscilloscope output from a 1kHz square wave at 0dBFS (24/44):
Impulse response of 16/44 signal:
Supposedly, these adaptive USB DAC's are prone to jitter (I'll consider more about this in a followup post), but let's have a look at the analogue output results for now:
Already this looks good for RCA output. Despite the asynchronous design, the 16/44 USB path measures a little better than going through the coaxial or TosLink SPDIF from the Squeezebox Touch. However, due to the limitation of the old USB1 interface, it is not capable of hi-res 24-bit data or higher sampling rates.
Nice to see that the 24/192 sample rate result looks very good, essentially not giving up anything measurable compared to 24/96 using the E-MU 0404USB.
Noise floor: (for 24/96)
THD: (for 24/96)
These are just the measurements of the analogue output from the AUNE X1 as a line-level DAC. It looks very good and can serve as an upgrade to the standard Touch analogue output with about an extra 4dB improvement in the noise floor and dynamic range with 24-bit music. It loses a little bit in the 16/44 performance for noise level but makes up for it in a pretty big way with improved stereo crosstalk (subjectively it sounds better through headphones as well)...
As I mentioned above, I bought this unit as a headphone amp and DAC which I've been listening to this past year using my MacBook Pro TosLink output while doing work at night. It sounds very good - tight bass, nice stereo separation, very detailed with my Sennheiser HD800 - and can easily drive the more demanding AKG Q701 headphones to headache-inducing levels. I've hooked it up to a bookshelf system to make sure the RCA out sounded good (it does as suggested by these measurements), but otherwise have only listened to them through the headphones. Subjectively I have not been able to tell a difference between the same 16/44 track played back using the adaptive USB vs. TosLink vs. coaxial.
These days I see there is a Mark II with upgraded USB input using the Tenor chipset (up to 24/96) which should be an improvement (I see what looks like a Mark III with Wolfson USB chip up to 24/192?). This is the level of performance you get for $200USD on-line. Commoditization of hi-fi is a very good thing.
In the next post, I'll have a look at jitter through this unit among other tests...