Parasound Halo JC 2 line preamplifier Measurements
Operated with either balanced input and output or unbalanced input and output, the Parasound Halo JC 2 offered a maximum gain of 14dB, as specified. Surprisingly, it offered 20dB gain with an unbalanced input and measured at the balanced output jacks. Both unbalanced and balanced outputs preserved absolute polarity (ie, were non-inverting), with the blue front-panel Polarity LED off. (The XLRs are wired with pin 2 "hot.") Both unbalanced and balanced input impedances were close to specification, at 27k ohms at low and middle frequencies. The input impedances dropped a little at 20kHz, to 22k ohms, though this difference will be inconsequential. The output impedance was higher than specified, at 257 ohms unbalanced and 295 ohms balanced. These are still usefully low values, however, and remained constant across the audioband.
With its volume control set to the maximum, the JC 2's frequency response at 1V into 200k ohms was flat within the audioband and down by just 0.6dB at the top of the audioband, with superb channel matching (fig.1, blue and red traces). This wide bandwidth was maintained into low impedances. However, there was some interdependency of the bandwidth and the volume-control setting. The magenta and green traces in fig.1 show the preamp's response at the same output level, but with the volume control set to unity gain (1:00). The response is now –0.35dB at the top of the audioband, and a very slight level mismatch (0.1dB) between the channels is evident. Neither will have any subjective consequences, however.
Fig.1 Parasound Halo JC 2, balanced frequency response at 1V into 200k ohms with volume control set to maximum (left channel blue, right channel red) and to unity gain (left channel green, right channel magenta). (0.25dB/vertical div.)
The JC 2's channel separation was superb, at >120dB below10kHz in both directions, a tribute to the circuit board and component layout. The preamplifier also offered very low levels of noise. With the volume control at its maximum but the input short-circuited—very much the worst case—I measured an unweighted, wideband signal/noise ratio (ref. 1V output) of 96.7dB. A-weighting improved this figure to 108dB, which is superb.
Harmonic distortion was also vanishingly low, revealed by plotting the JC 2's THD+noise percentage against its balanced output voltage (fig.2; the top trace at 5V output was taken into a very low 600 ohm load; the lower trace was taken into 100k ohms). Both traces slope down with increasing voltage below 2V, due to the measurement's being dominated by noise rather than distortion; a constant noise level becomes a smaller proportion of the signal as the level increases. The true distortion begins to rise out of the noise above 2.5V, which is eminently sensible engineering: this is close to where the partnering power amplifier will start to be driven into clipping. The JC 2 itself starts to clip in a "soft" manner, the beginnings of the flat top of the waveform having rounded edges. At our standard definition of clipping as being when the signal is suffering 1% THD, the Parasound preamp in balanced mode clips at 10.5V into 100k ohms and 7.2V into 600 ohms. The unbalanced figures were 10V and 6.2V, respectively.
Fig.2 Parasound Halo JC 2, balanced distortion (%) vs 1kHz output level into (from bottom to top at 5V): 100k, 600 ohms.
I plotted how the THD+N percentage changes with frequency at a level of 3.5V, in order to be sure I was measuring distortion rather than noise. The results are shown in fig.3. Midrange THD+N into 100k ohms is between 0.0008% and 0.0009% (bottom pair of traces), with only a slight increase evident into 600 ohms (top traces). The THD does rise slightly in the top two audio octaves, this presumably due to the decreasing amount of corrective feedback available in this frequency region, but the absolute level of the distortion is very low.
Fig.3 Parasound Halo JC 2, balanced distortion (%) vs frequency at 3.5V into: 200k ohms (bottom), 600 ohms (top). (Left channel blue, right channel red.)
The spectrum of the distortion into high impedances consists almost entirely of the second and third harmonics (fig.4), which would be subjectively benign at much higher levels than are offered by the Parasound preamp. The level of the second harmonic increases only slightly into 600 ohms (fig.5), but the third is higher in level, and some higher-order harmonics make an appearance, though the absolute levels are still negligible. There were some higher-order sidebands evident with the demanding HF intermodulation test (fig.6), but the difference component at 1kHz was very low in level.
Fig.4 Parasound Halo JC 2, balanced spectrum of 1kHz sinewave, DC–10kHz, at 3.5V into 200k ohms (linear frequency scale; left channel blue, right channel red).
Fig.5 Parasound Halo JC 2, balanced spectrum of 1kHz sinewave, DC–10kHz, at 3.5V into 600 ohms (linear frequency scale; left channel blue, right channel red).
Fig.6 Parasound Halo JC 2, HF intermodulation spectrum, DC–24kHz, 19+20kHz at 3V peak into 200k ohms (linear frequency scale; left channel blue, right channel red).
Overall, the Parasound Halo JC 2 offers superb measured performance, as I have come to expect from John Curl designs.—John Atkinson