Hi chilest and welcome to WBF!
The mods are a little spread around this thread, but see http://www.whatsbestforum.com/showt...end-of-round-1&p=422685&viewfull=1#post422685 and http://www.whatsbestforum.com/showthread.php?2740-ack-s-system-end-of-round-1/page23 onwards.
The output filter modification in the analog section was meant to move the corner frequency from about 1MHz to about 193kHz, with the start of the rolloff at 176kHz (recognize that number?); the goal here was to reduce the amount of ultrasonic noise which the Spectrals would carry and amplify, and it was an eye-opener. The power supply modifications solidified the sound, and I verified its anticipated effectiveness with Berkeley before doing it. I have also found it effective to lift the chassis ground with a cheater plug. Elsewhere herein, you will find pictures of the output section which is now entirely shielded, and of mu-metal between the power supply and main board, to contain magnetic fields from the PS reaching the rest of the circuit. Sonically, the results are further validated now with the DMA-500 amps, and I get naturalness out of redbook digital as well.
The mods are a little spread around this thread, but see http://www.whatsbestforum.com/showt...end-of-round-1&p=422685&viewfull=1#post422685 and http://www.whatsbestforum.com/showthread.php?2740-ack-s-system-end-of-round-1/page23 onwards.
The output filter modification in the analog section was meant to move the corner frequency from about 1MHz to about 193kHz, with the start of the rolloff at 176kHz (recognize that number?); the goal here was to reduce the amount of ultrasonic noise which the Spectrals would carry and amplify, and it was an eye-opener. The power supply modifications solidified the sound, and I verified its anticipated effectiveness with Berkeley before doing it. I have also found it effective to lift the chassis ground with a cheater plug. Elsewhere herein, you will find pictures of the output section which is now entirely shielded, and of mu-metal between the power supply and main board, to contain magnetic fields from the PS reaching the rest of the circuit. Sonically, the results are further validated now with the DMA-500 amps, and I get naturalness out of redbook digital as well.