Adcom GFA-555 power amplifier 1989 Measurements
The Adcom GFA-555 was a powerhouse. Output at clipping was 231.7W into 8 ohms (23.6dBW), increasing to 331.9W into 4 ohms (22.2dBW). Like the PS Audio 200C, however, the GFA-555 blew output fuses when asked to drive a 2 ohm load to high levels. Again, the power supply seems much more robust than the output stages. Incidentally, I have owned a GFA-555 for about three years and never had a fuse blow during normal (music-listening) use.
Frequency response was very flat over the band (fig.1), with a slight (0.2dB) rolloff at 20Hz and 20kHz. There was about a 0.1dB level difference between channels. Output impedance was extremely low at 0.05 ohm over most of the band, rising to 0.07 ohm at 20kHz. THD was constant across the audible band at about 0.04% with 5W output into 8 ohms.—Robert Harley
Fig.1 Adcom GFA-555, frequency response at 1W into 8 ohms (0.2dB/vertical div/.).