Adcom GFA-535 power amplifier Measurements

Sidebar 3: Measurements

Following its one-hour preconditioning at 1/3 power, the Adcom was hot to the touch. Its input impedance measured a comfortably high 93k ohms, and its voltage gain inot 8 ohms measured 27.2dB. The output impedance was under 0.04 ohm from 20Hz to 1kHz, and under 0.06 ohm at 20kHz. DC offset measured 0mV in the left channel, 3mV in the right. Wide-band signal/noise (ref. 1W into 8 ohms) measured 88.2dB, unweighted. The GFA-535 II was noninverting from input to output.

Fig.1 shows the GFA-535 II's frequency response—essentially flat across most of the spectrum, with rolloffs at the extremes. The response in the bass was down 0.6dB at 20Hz. The 10kHz squarewave response is shown in fig.2. The gradual high-frequency rolloff is reflected in its slightly rounded leading edges. The 1kHz squarewave (not shown) is distinguished only by a slight downtilt on the top of the waveform (and a mirror-image uptilt at the bottom), caused by the extreme low-end rolloff. The crosstalk (fig.3) is nearly perfectly symmetrical (the same from either channel to the other) above 1kHz, with the left-right crosstalk slightly higher below this frequency. As is typical in most two-channel components, there is a gradual increase in crosstalk as frequency increases—usually the result of inter-channel capacitive coupling.

Fig.1 Adcom GFA 535 II, frequency response at 2.83V into 8 ohms (right channel dashed, 0.5dB/vertical div.)

Fig.2 Adcom GFA 535 II, small-signal 10kHz squarewave into 8 ohms.

Fig.3 Adcom GFA 535 II, channel separation at 100Hz): L–R, R–L (5dB/vertical div.).

The manner in which the Adcom's small-signal THD+noise varies with frequency into various loads is plotted in fig.4. The rise in distortion as frequency increases is common to most amplifiers; the absolute level of distortion here remains low, even in the worst case. The waveform of the residual THD+noise with a 1kHz signal, taken at an 11W output into 4 ohms, is shown in fig.5. The distortion here is primarily second harmonic with noise—as is also the case into both 2 ohm and 8 ohm loads (not shown).

Fig.4 Adcom GFA 535 II, THD+N (%) vs frequency at (from top to bottom): 4W into 2 ohms, 2W into 4 ohms, 1W into 8 ohms (right channel dashed).

Fig.5 Adcom GFA 535 II, 1kHz waveform at 11W into 4 ohms (top), distortion and noise waveform with fundamental notched out (bottom, not to scale).

The distortion spectrum of the GFA-535 II with a 50Hz input and an output of 67W into 4 ohms (2/3 rated power into that load) is shown in fig.6. There is some power-supply hum visible (120Hz at lower than –85dB). The distortion components are well down in level, the second harmonic at 100Hz being the largest by far at –78dB, or about 0.012%. Fig.7 shows the spectrum resulting from a combined 19+20kHz input—the intermodulation of these two frequencies—at 67W output into a 4 ohm load. The largest artifact here is at 1kHz, –76.8dB or about 0.015%, followed closely by a slightly lower –77.7dB (0.14%) at 17kHz. The spectrum for 40W into 8 ohms (not shown) has slightly lower distortion components, but is not dramatically different.

Fig.6 Adcom GFA 535 II, spectrum of 50Hz sinewave, DC–10kHz, at 67W into 4 ohms (linear frequency scale).

Fig.7 Adcom GFA 535 II, HF intermodulation spectrum, DC–22kHz, 19+20kHz at 67W peak into 4 ohms (linear frequency scale).

The 1kHz THD+noise vs level curves for the Adcom are shown in fig.8. The power output does increase with decreasing impedance, but—as one might expect with a modestly priced amplifier—does not double with each halving of impedance. The results are good otherwise, with low distortion up to the break-point, or knee, of the curve. The discrete clipping levels (1% THD+noise) were measured at 80W (19dBW) left and right, 113V line, both channels driven, and 80.8W (19.1dBW), 113V line, with a single channel driven into 8 ohms; 114W (17.6dBW) left and 113.2W (17.5dBW) right, 114V line, both channels driven; and 113.6W (17.5dBW), 114V line with a single channel driven into 4 ohms. The separate power supplies for each channel are evident in the nearly identical readings for one or both channels driven. Blowing fuses defeated attempts to measure the power output at clipping into 2 ohms, but the THD+noise vs level curve of fig.8 indicates that this should be just over 150W (15.75dBW) with one channel driven.

Fig.8 Adcom GFA 535 II, distortion (%) vs 1kHz continuous output power into (from bottom to top): 8, 4, 2 ohms.

Altogether, the measured performance of the Adcom GFA-535 II gives no cause for complaint. It is clearly not too comfortable with sustained delivery into very low impedances, blowing fuses when forced to drive 2 ohms for more than a few seconds. But it will drive significant power into these impedances for brief periods. And sustained performance into very low impedances is not, in any event, normally expected in this size and price class. Overall, the GFA-535 II lives up to its reputation, and is a solid performer on the test bench.—Thomas J. Norton

Company Info
Adcom, LLC
PO Box 2668
Sedona, AZ 86339
(602) 773-1909
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