Parasound Halo JC 1 monoblock power amplifier Measurements
Before I performed any measurements, I ran the big Parasound Halo at one-third power into 8 ohms for an hour, after which the heatsinks were way too hot to touch. I had one mishap: Before I measure anything, I experiment with the grounding between the amplifier under test, the AC supply, and the Audio Precision System One test setup, in order to reduce power-supply spuriae to their lowest level. With balanced drive, there were no problems. However, after I'd performed a full set of balanced tests, I hooked up an unbalanced connection and again started to experiment with grounding. With one configuration—understandably, I didn't note which it was—the amplifier went into high-level ultrasonic oscillation. In the short span of time it took me to yank loose the AC cord after seeing the oscilloscope screen go crazy, the room had filled with the smell of toasted components.
Starting up the amplifier again with the grounds properly made gave a tiny, heavily distorted signal. The Halo JC 1's rear-panel AC fuse was intact, but when I lifted the top panel, I found two of the four internal 12A fuses blown. Replacing them—not an easy job for someone with short, stubby fingers like me—seemed to restore the amplifier to full health. The moral: Always begin your setup with the Parasound properly grounded.
The voltage gains into 8 ohms were 28.2dB unbalanced and 28.1dB balanced, and neither input inverted absolute polarity. (The XLR jack appears to be wired with pin 2 hot.) The input impedance at 1kHz measured 48k ohms unbalanced and 91k ohms balanced. Each of the Halo's 18 flat-pack output devices stands on a 0.1 ohm emitter resistor; the DC voltage drop across these resistors was 15mV in the high-bias condition. As the output is taken from the common point between each pair of complementary transistors, this voltage is equivalent to a standing output-stage bias current of 1.35A, which in turn implies that, into 8 ohms, the JC 1 operates in class-A up to 29W. This is slightly higher than the specified 25W, but the difference will be inconsequential.
The Halo JC 1's output impedance was a very low 0.045 ohm over most of the audioband, this rising very slightly to 0.06 ohm at 20kHz. As a result, any modification of the amplifier's response due to the interaction between this impedance and the loudspeaker's impedance will be negligible. The JC 1's frequency response (fig.1) was identical through the unbalanced and balanced inputs, with just 0.1dB of rolloff evident at the top of the audioband. The -3dB point was 95kHz, which means that the Parasound's reproduction of a 10kHz squarewave was excellent (fig.2), with only a slight rounding of the waveform's leading edges apparent.
Fig.1 Parasound Halo JC 1, balanced, frequency response at (from top to bottom at 2kHz): 2.83V into dummy loudspeaker load, 1W into 8 ohms, 2W into 4 ohms, 4W into 2 ohms (0.5dB/vertical div.).
Fig.2 Parasound Halo JC 1, small-signal 10kHz squarewave into 8 ohms.
The JC 1's unweighted, wideband signal/noise ratio (ref. 1W into 8 ohms) was good at 73.3dB, this improving to 87.8dB when the figure was A-weighted. Any distortion was buried below the noise at the usual small-signal levels, so I measured how the THD+noise percentage varied with frequency at 12.65V RMS, equivalent to 20W into 8 ohms. Fig.3 shows the results: a very slight rise in treble THD is magnified into lower impedances, but not to any level that might be considered significant, especially as the THD content at moderate levels is almost pure—and sonically benign—second harmonic (fig.4). There was no significant difference between the amplifier's distortion performances in its two bias states.
Fig.3 Parasound Halo JC 1, balanced, high bias, THD+N (%) vs frequency (from bottom to top at 4kHz): 12.5V into 8 ohms, 4 ohms, 2 ohms, and at 2.83V into simulated loudspeaker load.
Fig.4 Parasound Halo JC 1, balanced, high bias, 1kHz waveform at 40W into 4 ohms (top), 0.0153% THD+N; distortion and noise waveform with fundamental notched out (bottom, not to scale).
As the output power increased, the second harmonic rose in level and the third, fourth, and fifth harmonics made appearances (fig.5)—but these fell in level with increasing order, something that tends to correlate with good sound quality. Intermodulation distortion was also acceptably low (fig.6), with the 1kHz difference component lying at -94dB (0.0015%). Note that these last two graphs were taken at 635W and 600W into 4 ohms!
Fig.5 Parasound Halo JC 1, balanced, high bias, spectrum of 50Hz sinewave, DC-1kHz, at 635W into 4 ohms (linear frequency scale).
Fig.6 Parasound Halo JC 1, unbalanced, high bias, HF intermodulation spectrum, DC-24kHz, 19+20kHz at 600W into 4 ohms (linear frequency scale).
With continuous drive, the Parasound clipped at 545W into 8 ohms (27.4dBW)—way above the specified 400W. ("Clipping" is defined, as usual, as the power level where the measured THD figure reaches 1%, and is shown in fig.7 as the horizontal magenta line.) With a low-duty-cycle 1kHz toneburst more representative of music, the Halo was a powerhouse. Its clipping power increased by 0.3dB into 8 ohms, reaching 586.5W at 1% THD (27.7dBW, fig.7, black trace), with 1154W available into 4 ohms (27.6dBW, blue), 2255W into 2 ohms (27.5W, green), and no less than 4.2kW into 1 ohm (27.2dBW, magenta). The latter is equivalent to an output current of 64.7A!
Fig.7 Parasound Halo JC 1, distortion (%) vs 1kHz burst output power into: 16 ohms (red trace), 8 ohms (black), 4 ohms (blue), 2 ohms (green), 1 ohm (magenta).
This is excellent measured performance. The Halo JC 1 is not only the best amplifier to come from Parasound, it ranks up there with the best high-end heavyweights.—John Atkinson