Adcom GFP-750 preamplifier Sidebar 3: Measurements
I agree with Wes Phillips about the sheer quality of the Adcom GFP-750's construction. One thing that had me confused was that the passive option is selected when the red LED above the switch is illuminated. For some reason, I assumed that the light being on meant that the active stage was engaged. Serves me right for not reading the manual!
Once I got that out of the way, I started to measure the GFP-750. The mute control gave a full mute to silence; the unity gain setting of the volume control in active mode was at the 2:00 position; and the preamp was noninverting from the unbalanced outputs and from the balanced XLR jacks (pin 2 is wired as hot). (Throughout this section, "unbalanced" means an unbalanced source into the GFP-750's Aux 1 jacks and an unbalanced output from its RCA jacks; "balanced" means a balanced source into the balanced CD input XLRs and a balanced output from the XLR jacks.)
The maximum gain in active mode was 8.33dB unbalanced and 14.33dB balanced. In passive mode with the volume control set at its maximum, the output dropped from unity gain by a negligible 0.145dB. The input impedance was 21.1k ohms in active unbalanced mode, this dropping to a low 1945 ohms in unbalanced passive mode. Both figures are lower than specified, but this should not be a problem unless the source used has a high source impedance, such as some tube CD players.
The output impedance in active mode was a little higher than specified, at 970 ohms unbalanced, 1920 ohms balanced. This should present no problems. In passive mode, the output impedance will depend on the setting of the volume control. I checked at three positions: with the control wide open, the calculated impedance was 14 ohms, presumably due to the Audio Precision's 25-ohm source impedance being shunted by the potentiometer resistance; with the control at 10:00, the output impedance rose to 168.75 ohms; and at the 2:00 position it was 512 ohms. None of these figures will give rise to problems unless the Adcom is used with very long lengths of highly capacitive cable.
The frequency response in passive mode was, as expected, ruler-flat. In active mode (fig.1), a small but negligible rolloff can be seen above and below the audio band. A note on the excellent channel balance seen in this graph: It appears that any imbalance in the volume-control tracking and the gain of the active stages has been optimized for the balanced CD inputs. Repeating the measurement for the unbalanced Aux 1 input in active mode gave the response shown in fig.2: there is now a 0.4dB channel difference at the 2:00 volume-control setting used for this measurement. This is trivial, and was easily corrected by setting the balance control to 11:45, but it did puzzle me at first.
Fig.1 Adcom GFP-750, frequency response into 100k ohms, balanced active mode, with volume control set to 2:00 (0.5dB/vertical div.).
Fig.2 Adcom GFP-750, frequency response into 100k ohms, unbalanced passive mode, with volume control set to 2:00 (0.5dB/vertical div.).