Adcom GFA-565 monoblock power amplifier Measurements

Sidebar 3: Measurements

A look at fig.1 indicates that the Adcom exhibits a virtually flat frequency response curve, except at the very top end where it's 0.5dB down at 32kHz. Using the optional balanced inputs, the 0.5dB-down point was slightly lower, at 27kHz. (With a few noted exceptions, all of the measurements were made into the standard, unbalanced inputs.) The small-signal 1kHz squarewave (fig.2) shows a clean, square shape with a fast rise-time. The polarity was non-inverting. The Adcom's input impedance at 1kHz was 47k ohms, and its output impedance was uniformly low—ranging from 0.02 ohms from the bass to the upper midrange and increasing slightly to 0.03 ohms at 20kHz. The gain of the GFA-565 was 27.1dB into the unbalanced inputs. I found that the three gain settings on the balanced inputs—0dB, +4dB, and +6dB—actually gave quite different readings. The +4dB setting was actually 8dB down from the 0dB, the +6dB setting was down 14dB from the 0dB. Unweighted noise was 70dB below 1W into 8 ohms.

Fig.1 Adcom GFA 565, frequency response at 2.83V into 8 ohms (0.5dB/vertical div.)

Fig.2 Adcom GFA 565, small-signal 1kHz squarewave into 8 ohms.

THD+noise at 2.83V output (low power) (fig.3, upper trace) was virtually identical at all impedances. Two ohms is shown, and the slight rise at high frequencies was not evident at 4 and 8 ohms. Immediately after preconditioning, with the amp hot, the reading was lower (fig.3, lower trace). The readings appear to be largely noise. Fig.4 shows the same measurement into 8 ohms for the balanced input—the slightly higher value is unusual. The waveform of the distortion in fig.5 indicates a predominantly third-order characteristic overlaid with noise. This was taken at an output of 260W into 4 ohms at 0.003% distortion—any level lower than that and the waveform indicated nothing but noise.

Fig.3 Adcom GFA 565, THD+N (%) vs frequency at 4W into 2 ohms (measurement dominated by noise), top trace when amp warm, bottom when hot.

Fig.4 Adcom GFA 565, balanced input, THD+N (%) vs frequency at 1W into 8 ohms (measurement dominated by noise).

Fig.5 Adcom GFA 565, 1kHz waveform at 260W into 4 ohms (top), distortion and noise waveform with fundamental notched out (bottom, not to scale).

Fig.6 shows the spectrum from a 50Hz sinewave driven at 66W into 4 ohms. The second harmonic is negligible; the third harmonic is stronger (as expected from fig.5), but still down over 80dB. Fig.7 shows the high-frequency IM spectrum with an equal mix of 19 and 20kHz tones driven at 78W into 4 ohms. The sidebands visible at 18 and 21kHz are slightly accentuated by the Adcom, though they are still at an inconsequential level. Otherwise, no artifacts are present above the measuring system's noise floor, including the 1kHz difference component.

Fig.6 Adcom GFA 565, spectrum of 50Hz sinewave, DC–10kHz, at 66W into 4 ohms (linear frequency scale).

Fig.7 Adcom GFA 565, HF intermodulation spectrum, DC–22kHz, 19+20kHz at 50V peak into 8 ohms (linear frequency scale).

Fig.8 indicates the THD vs power for 8, 4, and 2 ohms at 1kHz, one channel driven (bottom, middle, and top curves respectively; line voltages of 117V, 115.5V, and 113.5V respectively). The maximum output power at the standard 1% THD level was 322.6W (25.1dBW) into 8 ohms, 564W (24.5dBW) into 4 ohms, and a rather breathtaking 904W (23.5dBW) into 2 ohms. In the case of the GFA-565, the "knees" of the distortion curves in fig.9 fall at 280W (8 ohms), 500W (4 ohms), and 850W (2 ohms)—figures which, in my judgment, define the maximum useful power of the amplifier.

Fig.8 Adcom GFA 565, distortion (%) vs 1kHz continuous output power into (from bottom to top): 8, 4, 2 ohms.

DC offset in the Adcom was under 8mV. It operated quite hot to the touch following its 1-hour, 1/3-power preconditioning, but suffered no ill effects from the experience.

The Adcom GFA-565's measured performance was impressive, to say the least, and certainly indicated why it sounded as solid and substantial as it did.—Thomas J. Norton