Adcom GFP-565 preamplifier Measurements
The Adcom GFP-565 exhibited a flat frequency response measured from the CD input to Norm outputs, as shown in fig.1. RIAA accuracy was also very flat, deviating less than 0.1dB across the audible band (fig.2). Phono overload was a high 110mV at 1kHz. THD and noise, again measured from CD input to Norm out, was less than 0.006% across most of the band, rising slightly above 12kHz (fig.3).Robert Harley
Fig.1 Adcom GFP-565, line input frequency response, 250mV output level (0.2dB/vertical div.).
Fig.2 Adcom GFP-565, RIAA error (0.5dB/vertical div.).
Fig.3 Adcom GFP-565, THD+N (%) vs frequency (Hz) at 1V output level.
To investigate the difference I heard in the low-frequency performance between the Adcom GFP565's Bypass and Main outputs, I looked at the shape of a raised-cosine waveform (from the Japanese Audio Society Test CD, track 88) as reproduced by both outputs. It can be seen from fig.4 that the Bypass outputs reproduce the waveform extremely well, perhaps even better than when reproduced by the line stage of the expensive, DC-coupled Mark Levinson No.26 preamplifier (not shown). Via the Adcom's Main outputs, however, there is considerable overshoot on the negative side of the time axis (fig.5), which is presumably due to the coupling capacitor and which might well correlate with my subjective impression of less-well-defined lows.
Fig.4 Adcom GFP-565, Bypass output, response to raised-cosine pulse (100ms time window).
Fig.5 Adcom GFP-565, Main output, response to raised-cosine pulse (100ms time window).
Note also that the Adcom's line stage is polarity-correct. I discovered, however, that switching in the Adcom tone-control stage inverted signal polarity, something to be aware of in A/B comparisons. I was also surprised to find that switching in its "High" filter gave a slight gain reduction (0.85dB) at 1kHz. Obviously, purists should use the Adcom's Bypass outputs to get both the best bass and the most consistent performance.John Atkinson