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The Femto’s digital volume control attenuates in 0.5dB increments to drive a power amp directly. Since I didn’t have a power amp on hand—my main amp is an integrated—I did not test this feature. The selectable digital filter offers a choice of three different low pass settings - 50, 60 or 70kHz. I stuck with the default 50kHz setting as it sounded more balanced. The other two tilted up the top end too far for my liking. The power switch is placed discretely on the left cheek, suggesting that the Femto should remain powered up for optimum performance.

The DAC chip is ESS Technology’s ES9018 Sabre as used already in the 24/192 though the Femto uses two, one per channel which should offer greater channel separation and dynamic range. As with the 24/192, the USB input uses the now ubiquitous XMOS chip set, which operates asynchronously i.e. the DAC controls the data flow instead of the computer. This supposedly offers lower jitter levels than the computer-controlled synchronous mode.

In case you’re wondering what Femto means, it is a direct reference to the claimed miniscule jitter levels of this clocking circuit, i.e. 500 femto seconds. One femto second is one quadrillionth (.000 000 000 000 001) of a second. Digital gear of recent vintage generally possesses jitter levels in the higher picosecond range but some newer products such as the Femto are pushing it lower still. Jitter is generally described as minute timing errors that contribute to the less than pleasant aspects of digital playback. To my ears, music played back with excessive jitter comes across as grainy, two-dimensional and lacking focus while the harmonic envelope of instruments and voices is smeared or plain clipped off. More importantly, jitter seems to affect the timing and flow of music. I am finding as I am sure many others have, that digital has improved dramatically in the last couple of years. It is finally offering some of the ease, flow and naturalness usually associated with decent vinyl playback. It’s these traits that so impressed me with the 24/192 DAC and to an even greater degree with the Femto.

Peering under the hood, the substantial linear power supply with separate transformers for the digital and analog sections sits in its own shielded enclosure away from the main circuit board. The 24/192 I reviewed used either the 5V supply from my laptop’s USB bus or a switch-mode wall wart. However, the 24/192 is now available with a more robust outboard linear power supply (CLPS). I had the opportunity to try one with the 24/192 after my review and it certainly pushed that DAC’s performance upward.

Those expecting to see an interior stuffed with discrete components might be disappointed with the Femto’s smallish board and use of surface-mount devices (SMD) although once hearing the Femto, any apprehension will surely evaporate. Dominating the board are three large milled-aluminum enclosures which contain Calyx’s impressively spec’d clocking circuits.

My primary source was a HP laptop running either Windows 7 or Windows 8 beta with J. River Media Center 17 and JPlay. All my music files are stored as WAV on an external FireWire-connected 1TB hard drive. I keep FLAC backups on a pair of portable USB hard drives. For the odd silver disc yet to be ripped I use my CEC TL-51X transport. All file-based music was played back natively without upsampling. I played recordings from 16-bit/44.1 up to 24-bit/192kHz and my comments below are not specific to any one bit rate/sampling frequency although the well-recorded high resolution stuff sounded absolutely terrific.

I tried a number of power cables and as expected each had an effect on the overall sound. There wasn’t a huge difference no doubt a tribute to the robustness of the Femto’s power supply but my overall favorite was Sablon Audio’s Gran Corona. The same was true with the Femto’s input circuitry. I found little difference between Transparent Audio and Cardas USB cables or with RCA leads from DH Labs, MIT and Sablon Audio. The Femto needed a few days of continuous play to fully come on song but the lovely silky smoothness of this machine was evident from the start.