Parasound HCA-3500 power amplifier Measurements
Unless otherwise noted, all measurements presented here are for the unbalanced configuration.
Following its one-hour, 1/3-power pretest, the Parasound HCA-3500's heatsinks were moderately but not excessively hot. Its DC offset measured a negligible 1.5mV in the left channel, 1mV in the right. The amp is noninverting, input to output—in the balanced mode, pin 2 is positive—and its voltage gain into 8 ohms was 28.5dB (27.9dB, balanced).
The HCA-3500's input impedance measured 47.4k ohms (67.6k ohms balanced). The maximum measured output impedance was 0.55 ohms (1kHz, 4 ohm load)—a slightly higher value than we normally see with solid-state amplifiers, but still low enough to have relatively little effect on the measurements with a simulated real load (see below). The signal/noise readings (ref. 2.83V, rounded to the nearest dB) were 98dB from 22Hz to 22kHz, 89dB from 10Hz to 500kHz, and 100dB A-weighted. (The corresponding balanced values were 96dB, 88dB, and 98dB.)
The Parasound's frequency response is shown in fig.1. Not much worth commenting on here, other than the slight mismatch in channel balance. The 10kHz squarewave (fig.2) is excellent, with a fast risetime, a bare minimum of leading-edge rounding, and no overshoot or ringing. (The 1kHz squarewave, not shown, produced a virtually textbook result.) The crosstalk in fig.3 is sufficiently low in level to be audibly irrelevant.
Fig.1 Parasound HCA-3500, frequency response at (from top to bottom at 6kHz): 2W into 4 ohms, 1W into 8 ohms, and 2.828V into simulated loudspeaker load, (0.5dB/vertical div., right channel dashed).
Fig.2 Parasound HCA-3500, small-signal 10kHz squarewave into 8 ohms.
Fig.3 Parasound HCA-3500, channel separation vs frequency, L–R top blow 3kHz, bottom above 3kHz (10dB/vertical div.).
The THD+noise percentage vs frequency curves in fig.4 indicate that the amplifier's distortion into 2 ohm loads is notably higher than into 4 or 8 ohms, but is still respectable even into this low impedance. The waveform of the distortion content into 2 ohms (fig.5) shows a heavily second-harmonic content. (The 4 and 8 ohm results were essentially the same, though with a higher noise content because of the lower distortion into these higher impedances.)
Fig.4 Parasound HCA-3500, THD+noise (%) vs frequency at (from top to bottom at 4kHz): 4W into 2 ohms, 2.83V into simulated loudspeaker load, 2W into 4 ohms, and 1W into 8 ohms (right channel dashed).
Fig.5 Parasound HCA-3500, 1kHz waveform at 4W into 2 ohms (top), distortion and noise waveform with fundamental notched out (bottom, not to scale).
Fig.6 shows the distortion spectrum of a 50Hz input signal at an output of two-thirds rated power into 4 ohms (335W). The distortion, with a second-harmonic content (at 50Hz) of –70dB or 0.04%, is low. However, a power supply component at 120Hz can be seen at just above the –80dB line.
Fig.6 Parasound HCA-3500, spectrum of 50Hz sinewave, DC–1kHz, at 335W into 4 ohms (linear frequency scale).
Fig.7 plots the IM distortion of a 19+20kHz waveform at 317W into 4 ohms. The 1kHz difference-frequency distortion is –66dB, the 18kHz artifact is at –64dB (approximately 0.045% and 0.06%, respectively). These are solid results. A similar spectrum at 185W into 8 ohms (not shown) produced comparable results. Note that, in both cases, the 19+20kHz signal visibly clipped the amplifier before the 2/3 power point could be reached—a limitation of many amplifiers. This is of no practical significance at the frequencies and power levels involved here.
Fig.7 Parasound HCA-3500, HF intermodulation spectrum, DC–22kHz, 19+20kHz at 317W into 4 ohms (linear frequency scale).
Fig.8 plots the HCA-3500's THD+noise percentage against output power at 1kHz. For these readings, an outboard load was pressed into service to supplement our normal test-bench load and thus reduce the strain on it. Though the amplifier is not rated into 2 ohms, it nevertheless puts out a solid 800W into this load. For the discrete clipping measurements (which put more of a strain on amplifier and load than the sweeps shown in fig.8), and because of the limits of our test-bench loads, I measured only the 1% THD+noise clipping into 8 ohms. With both channels driven, the HCA-3500 clipped at 351W (25.5dBW) in the left channel (power line 113V), and 353W (25.5dBW) in the right (power line 113.5V). With one channel driven, clipping occurred at 366W (25.6dBW, 115V line).
Fig.8 Parasound HCA-3500, distortion (%) vs continuous output power into (from bottom to top) 8 ohms, 4 ohms, and 2 ohms.
John Atkinson looked at the output power using a low-duty-cycle 1kHz toneburst that is more representative of a musical signal. With one channel driven, the Parasound amplifier proved a powerhouse under these conditions: 441W was available into 8 ohms (fig.9, black trace), 854W into 4 ohms (red), 1538W into 2 ohms (blue), and a whopping 2230W into 1 ohm (green), this last figure equivalent to a peak burst current of 47.25A! (The 1%, –40dB THD level is shown in this graph as the horizontal magenta line.) But the amplifier is clearly less happy working at such high levels into the very lowest impedances, revealed by the increase in distortion in the green curve above 1200W.
Fig.9 Parasound HCA-3500, distortion (%) vs 1kHz burst output power into 8 ohms (black trace), 4 ohms (red), 2 ohms (blue), and 1 ohm (green).
Altogether, this is a fine set of measurements for a powerhouse of an amplifier.—Thomas J. Norton