Extracts from John Atkinson's measurements of the Reference version:
The manual says that the preamplifier output impedance is "a few ohms." I measured 96 ohms at high and middle frequencies and a higher 1426 ohms at the bottom of the audioband. However, I had problems measuring the behavior of the 404 Reference’s preamplifier output due to the presence of high levels of ultrasonic noise (footnote 1). I believe this noise, which had a center frequency around 1.7MHz, was confusing the input of the Audio Precision SYS2722. This noise was present with both samples of the amplifier, and could be the cause of a "buzz" Jason Serinus told me he experienced with the preamplifier output.
Though the 404 Reference’s maximum power is specified as 50W into 8 ohms (17dBW), using our definition of clipping, which is when the output’s percentage of THD+noise reaches 1%, fig.4 indicates that the amplifier clipped at 39.5Wpc into 8 ohms (14dBW). The Moonriver clipped at 60W into 4 ohms (14.8dBW, fig.5). (Both graphs were taken with both channels driven. The first sample’s right channel behaved identically.)
The manual says that the preamplifier output impedance is "a few ohms." I measured 96 ohms at high and middle frequencies and a higher 1426 ohms at the bottom of the audioband. However, I had problems measuring the behavior of the 404 Reference’s preamplifier output due to the presence of high levels of ultrasonic noise (footnote 1). I believe this noise, which had a center frequency around 1.7MHz, was confusing the input of the Audio Precision SYS2722. This noise was present with both samples of the amplifier, and could be the cause of a "buzz" Jason Serinus told me he experienced with the preamplifier output.
Though the 404 Reference’s maximum power is specified as 50W into 8 ohms (17dBW), using our definition of clipping, which is when the output’s percentage of THD+noise reaches 1%, fig.4 indicates that the amplifier clipped at 39.5Wpc into 8 ohms (14dBW). The Moonriver clipped at 60W into 4 ohms (14.8dBW, fig.5). (Both graphs were taken with both channels driven. The first sample’s right channel behaved identically.)