Vintage

While both the inaugural AD 800 and later AD-844 CD players were all based on a Sanyo chassis with Sony DSP and DAC components, the AD-812 marked Fisher’s switch to a mix of Sanyo DSP and Yamaha oversampling and DAC technologies. The 16-bit mono YM3015 DAC – also seen in the Crown CD-110 [HFN Sep ’23] – includes a sample-and-hold function that can multiplex the L/R channels at 176.4kHz. This is required to support the 2x oversampled (88.2kHz) output of the partnering YM3805 processor, the combination responsible for a 5.67µsec interchannel delay [see L/R impulse, Graph 2] and reduced phase lag of 45° (from 90°). The asymmetrical ringing seen on the impulse is a function of the digital filter, a limited-tap linear phase type, and the series analogue filter which bites steeply above 20kHz, increasing the stopband rejection to 53dB.

The 2x filter also produces the +0.3dB/–0.4dB ripple at the top of the player’s response [Graph 2] while the impact of the analogue filter is witnessed in the effective suppression of HF distortion harmonics. This is the reason why THD at 20kHz is up to 20dB lower than at 1kHz over the top 30dB of the player’s dynamic range [see Graph 1]. At the player’s 1.9V maximum output, delivered from a ~1kohm source impedance, the A-wtd S/N ratio is a sub-16-bit 92dB while distortion is >0.1%/1kHz, and remains so over the top 20dB of its range. Linearity is skewed at low levels so that signals at –90dBFs appear some 5.5dB higher in amplitude (500Hz-20kHz) while jitter is very high at 7845psec. The latter comprises sidebands at ±10Hz and ±30Hz, linked to specific processes within the AD-812, but rectifier modulations at ±100Hz otherwise dominate. To be fair to this 40-year-old digital veteran, that result may simply reflect a tired PSU! PM