Adcom GFP-750 preamplifier Measurements part 2
Fig.3 Adcom GFP-750, Crosstalk (from top to bottom): L-R, R-L, balanced mode; L-R, R-L, unbalanced mode (10dB/vertical div.).
Distortion (fig.4) was very low in both active modes, if a little higher in unbalanced mode and at high frequencies. Despite (or because of?) its minimal circuitry, the GFP-750 is very linear. Fig.5 shows the spectrum of the balanced output driving 50Hz at 1V into a 600 ohm load. This is a punishingly low load for a preamplifier to drive; even so, the distortion harmonics are negligible in level, other than the third at almost -100dB, which is still extremely low. In unbalanced mode, AC-supply harmonics at 180Hz and 300Hz made their presence known, but were still well down (below -80dB) in absolute level.
Fig.4 Adcom GFP-750, THD+noise vs frequency at 1V into 100k ohms, unbalanced mode (top at 1kHz), balanced mode (bottom).
Fig.5 Adcom GFP-750, spectrum of 50Hz sinewave, DC-1kHz, at 1V into 600 ohms, balanced active mode (linear frequency scale).
Finally, fig.6 shows the GFP-750's percentage of distortion+noise in both balanced and unbalanced modes, plotted against output voltage into loads of 100k ohms and 600 ohms. At a 1% THD+N figure, the maximum output voltage in balanced mode was 26.5V (100k ohms) and 6.5V (600 ohms); in unbalanced mode, it was 13.3V (100k ohms) and 5.0V (600 ohms). All voltages are well above the level required to drive any known power amplifier well into clipping. Note that the shapes of these traces imply that the preamplifier offers its lowest level of distortion and noise at around 2V, the maximum voltage likely with a real-world power amplifier.
Fig.6 Adcom GFP-750, distortion (%) vs output voltage into (from right to left): balanced mode into 100k ohms, unbalanced mode into 100k ohms, balanced mode into 600 ohms, unbalanced mode into 600 ohms.
This fine set of measurements indicates good engineering.—John Atkinson