Design optimisation - Schottky diode array

[mgd49]

so with 4851 you replace 4051 and schottky pairs.
another thing that is surprising to me is that there is no signal conditionning before the analog is converted to digital. in most samples i have found on the web, they quite all have this circuit sometimes with OpAmp, for peak detection or smoothing.
question : why megadrum is not conditionning the signal ?