Seta Nano phono preamplifier Measurements
I measured the Seta Nano phono preamplifier with Stereophile's loan sample of the Audio Precision SYS2722 system (see the January 2008 "As We See It" and www.ap.com). I experimented with the grounding between the preamp and the analyzers to get the lowest level of noise.
Housed in an enclosure the size and proportions of an Apple Mac mini computer, the Nano crowds together pairs of balanced and unbalanced inputs and outputs on its rear panel, as well as rotary switches for adjusting the resistive loading of each channel, a ground terminal, and a trimmer to balance the channel gains. The balanced outputs provide only a flat response; the single-ended outputs can be switched between flat and RIAA-corrected responses with a pushbutton. Unlike the more expensive Seta Model L, the Nano lacks internal batteries, and is powered by a wall wart that supplies 15V DC.
All the Nano's inputs and outputs preserved absolute polarity (ie, were non-inverting). The voltage gain at 1kHz was close to specification, at 46.5dB for both balanced and unbalanced flat outputs and 58.4dB for the RIAA-equalized unbalanced jacks. Both figures are appropriate for typical low-output MC cartridges, where the typical output level of 500µV for a recorded velocity of 5cm/s at 1kHz will result in outputs of 105mV flat and 418mV RIAA-equalized.
Unlike the Model L, the balanced input impedance was consistently higher than that set by the loading switches on the rear panel. Set to "25 ohms," the input impedance was 59 ohms; "50 ohms" was 82 ohms, "100" was 142 ohms, "150" was 204 ohms, "200" was 273 ohms, and "500" was 974 ohms. The final position of the switch, which was how I measured the gain, gave an input impedance of 9340 ohms. Via the unbalanced jacks, the input impedance generally tracked the balanced figure, with the exception that I got anomalous results with the final switch position when I switched the Audio Precision source impedance to 600 ohms. (I measure input impedance by examining the drop in a preamplifier's or power amplifier's output voltage when I switch the generator's source impedance from 20 ohms, unbalanced, or 40 ohms, balanced, to 600 ohms.)
The output impedances were basically to specification, at 36 ohms balanced and 43 ohms unbalanced, though the latter rose to 72 ohms at 20kHz. This will have no audible consequences. The blue and red traces in fig.7 show the Nano's response from its RIAA-equalized outputs. The 10Hz infrasonic filter was in-circuit for this measurement; you can see that it rolls off the output below 20Hz, and is 3dB at exactly 10Hz. The error in the RIAA equalization is very low, meeting an excellent ±0.1dB limit, with a slight rise in the mid-treble. The odd step above 12kHz is small in absolute terms. The channel matching is also excellent, but this can be adjusted with the rear-panel trim control. The response from the flat outputs was the same for both the balanced and unbalanced jacks, and showed a rise of 1dB above 100kHz (fig.1, cyan and magenta traces). This suggests a peak in the response at some high ultrasonic frequency, which might correlate with the instability noted by Michael Fremer with the first sample of the Nano.
Fig.1 Seta Nano, balanced RIAA output response with 10Hz low-cut filter (left channel blue, right red), Flat output response (left cyan, right magenta, offset for clarity; 0.25dB/vertical div.).
Channel separation was excellent from both sets of flat outputs, at >90dB in both directions below 1kHz (not shown). It was about 5dB worse from the RIAA-equalized output, rising to 85dB in both directions at 20kHz, which is still excellent. The RIAA output's unweighted, wideband S/N ratio (ref. 500µV input at 1kHz) was moderate, at 47.4dB left and 44.6dB right. Restricting the measurement bandwidth to the audioband didn't improve these figures, though switching an A-weighting filter in-circuit resulted in respective ratios of 72.5 and 70.2dB. The noise is dominated by spuriae at the power-supplyrated frequency of 60Hz and its odd harmonics (fig.2). Via the flat outputs, the unweighted audioband S/N ratio was somewhat better, at 58dB in both channels ref. 0.5mV input, but the wideband measurement was worse, at 25.6dB left and 31.7dB right, presumably due to the circuit's sensitivity to RF noise.
Fig.2 Seta Nano, balanced Flat output, spectrum of noise floor, DC10kHz, ref. 1kHz at 500µV input (left channel blue, right red; linear frequency scale).
The noise affects the plot of the distortion present in the Nano's output against output voltage for both the balanced and unbalanced flat conditions (fig.3). Below 600mV output balanced and 1V unbalanced, the downward tilt to the traces reveals that any distortion is actually below the preamp's noise floor. The maximum output of the Nano's balanced output (at 1% THD) is 5.5V into 100k ohms, dropping only slightly to 4.7V into 600 ohms. Both figures are well above what is needed to drive the A/D converter of a high-quality soundcard into clipping, and the former is almost 36dB above a nominal 1kHz recorded level of 0.5mV at 5cm/s. The RIAA preemphasis of the signal on the LP will reduce this margin at 20kHz to around 16.5dB, which is fine. The unbalanced flat output clips at 2.8V, which will still be sufficient for a typical soundcard. The RIAA-equalized outputs offered a good overload margin of 17.3dB across the audioband.
Fig.3 Seta Nano, unbalanced (top) and balanced Flat outputs, distortion (%) vs 1kHz output level into 100k ohms.
In RIAA mode, the distortion at typical levels is lower than implied by the top trace in fig.3. Fig.4, for example, shows the spectrum of a 1kHz tone at 1V into 100k ohms from the unbalanced RIAA jacks. The power-supply components can again be seen, while the downward slope of the noise floor with increasing frequency is due to the RIAA deemphasis. The only distortion components that can be seen, however, are the subjectively innocuous second and third harmonics, at 82dB (0.008%) and 90dB (0.003%), respectively. At the same output level from the balanced or unbalanced flat outputs, equivalent to an input 20dB above the nominal 0.5mV level, the second and third harmonics lie at 70 and 67dB, respectively, with the second harmonic a little lower in the right channel; some fourth and fifth harmonics can also be seen (fig.5), though these are at a very low level. Dropping the signal level by 10dB eliminates these higher harmonics (fig.6).
Fig.4 Seta Nano, unbalanced RIAA output, spectrum of 1kHz sinewave, DC10kHz, at 1V into 100k ohms (left channel blue, right red; linear frequency scale).
Fig.5 Seta Nano, balanced Flat output, spectrum of 1kHz sinewave, DC10kHz, at 1V into 100k ohms (left channel blue, right red; linear frequency scale).
Fig.6 Seta Nano, balanced Flat output, spectrum of 1kHz sinewave, DC10kHz, at 300mV into 100k ohms (left channel blue, right red; linear frequency scale).
Finally (fig.7), at about the highest level the flat output will be asked to deliver with a typical soundcard, intermodulation distortion resulting from the demanding mix of 19 and 20kHz tones resulted in a difference tone at 60dB (0.1%), though the higher-order spuriae at 18 and 21kHz lie at 50dB (0.3%).
Fig.7 Seta Nano, balanced Flat output, HF intermodulation spectrum, DC24kHz, 19+20kHz at 1V peak into 100k ohms (linear frequency scale; left channel blue, right red).
It took me a while to get my head around the Seta Nano's measured performances, because I had to keep reminding myself of what were typical recorded levels on LPs before the cartridge's output had been equalized by the RIAA characteristic. I was less impressed by the Seta Nano than I had been with the Model L, mainly because I felt its performance was somewhat limited by its use of a wall-wart power supply.John Atkinson