Media Server Reviews

I measured the Esoteric N-01 with my Audio Precision SYS2722 system (see the January 2008 "As We See It"), using both the Audio Precision's AES/EBU digital output and USB data sourced from my MacBook Pro running on battery power with Pure Music 3.0 playing WAV and AIFF test-tone files. Apple's USB Prober utility identified the Esoteric N-01 as both the "N-01" and "ESOTERIC USB AUDIO DEVICE" from "TEAC Corporation," its serial number as "0 (none)," and confirmed that its USB port operated in the optimal isochronous asynchronous mode. Apple's AudioMIDI utility revealed that, via USB, the Esoteric N-01 accepted 16-, 24-, and 32-bit integer data sampled at all rates up to an extraordinary 1.4112MHz. Its optical, coaxial, and AES/EBU inputs accepted PCM datastreams with sample rates up to 192kHz.

The Esoteric N-01's maximum output level at 1kHz was 5.07V from its balanced outputs and 2.535V from its unbalanced outputs, the latter 2dB higher than the CD standard's 2V. The outputs preserved absolute polarity with the XLR jacks wired with pin 2 hot. The unbalanced output impedance was a very low 26 ohms across the audioband. (The specified impedance is 20 ohms.) The balanced output impedance was 53 ohms at 20Hz, and 31 ohms at 1 and 20kHz, inconsequentially higher than the specified 20 ohms. Unusually, the balanced and unbalanced outputs can't both be active at the same time.

The Esoteric's impulse response in ORG (original) mode was an almost perfect pulse (fig.1; ignore the tiny amounts of pre- and post-ringing, which are due to the SYS2722's anti-aliasing filter operating at a sample rate of 200kHz). In the upconversion modes—2Fs, 4Fs, 8Fs, 16Fs, and DSD—the impulse response had a single cycle of pre- and post-ringing visible (fig.2). There doesn't appear to be a digital reconstruction filter in ORG mode, and with white noise sampled at 44.1kHz (fig.3, magenta and red traces, footnote 1) there was a slow rolloff above the audioband, disturbed by nulls at 44.1 and 88.2kHz. Consequently, the aliased image of a full-scale 19.1kHz tone (cyan and blue traces) was hardly suppressed at all. This rolloff steepened as the upsampling ratio increased (fig.4), the 16Fs upsampling mode showing the greatest amount of stop-band attenuation (fig.5).

As expected, in ORG and the low upsampling modes the frequency response rolls off at the top of the audioband, reaching –3dB at 20kHz with 44.1kHz data (fig.5, green and gray traces): The response for the top audio octave extends a little higher at higher sample rates (fig.6), but the ultrasonic output still rolls off relatively early. The channels were well matched from the balanced and unbalanced outputs. The Esoteric N-01's channel separation was superb, at >125dB from 20Hz to 1kHz, and still 115dB at 20kHz. The low-frequency noise floor was virtually free from any power-supply–related spuriae (fig.7), but low-level sidebands flank the 1kHz tone, spaced at 120Hz intervals. Experimenting with connecting or disconnecting the ground connection between the Esoteric and the Audio Precision didn't affect this behavior. While performing this test, I noted that the 1kHz waveform on my oscilloscope was stepped rather than smooth (fig.8).

Increasing the bit depth from 16 to 24 with a dithered 1kHz tone at –90dBFS (fig.9) dropped the noise floor by almost 24dB, which suggests resolution of around 20 bits. With undithered data representing a tone at exactly –90.31dBFS (fig.10), the three DC voltage levels described by the data were well resolved in all modes. With undithered 24-bit data, the result was a clean sinewave (fig.11).

The Esoteric N-01 produced very low levels of harmonic distortion. Even with a full-scale 50Hz tone into a punishing 600 ohms (fig.12), the third harmonic lay at just –82dB (0.008%), and the second harmonic was even lower, at –100dB (0.001%). With the Esoteric driving a high 100k ohms, both the second and third harmonics lay at –100dB. The poor ultrasonic rejection visible in fig.3 resulted in poor rejection of the aliased images with a full-scale mix of 19 and 20kHz tones (fig.13), though there was very little aliased energy in the audioband, and actual intermodulation distortion was vanishingly low in level, even into 600 ohms. This graph was taken in ORG mode; the picture was identical with the 4Fs, 8Fs, and DSD modes. However, as expected from fig.5, these images were completely rejected in the 16Fs mode (fig.14).

When I tested the Esoteric N-01 for its rejection of word-clock jitter, using undithered 16-bit J-Test data fed to its AES/EBU input, the odd-order harmonics of the low-frequency, LSB-level squarewave were higher than they should have been (fig.15). (The correct levels are indicated by the sloping green line.) Perhaps of greater significance were the higher-level sidebands spaced at 120Hz, which can be seen more clearly with 24-bit J-Test data (fig.16). As with the 120Hz sidebands in fig.7, this behavior was not affected by experimenting with the ground connection between the N-01 and the analyzer, and was identical with every input. One possible cause for these sidebands would be the presence of 120Hz hum on the DAC chip's voltage reference pin.

Other than that, the Esoteric N-01 offers respectable measured performance.—John Atkinson