Thanks for that Bill.
I think that the 2520 type oscillator sets its output to high impedance when enable is low, so I should be able to put in the missing link, drive MCLK_EN low, and then drive GCLK from the Pi. My only question is whether a 24MHz clock is feasible to send over the GPIO connector. I know that the AC108 has many possible input clocks. Could you advise on how to program its PLL for a lower frequency clock input?
Colin