Reply 20 of 30, by Tronix
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After a year, a small update here. A user from one russian old-hardware forum trace the new PCB and recently I soldered it. Started checking without an amplifier:
Digital f***n noise still present, because SAA1099 has an open-collector PWM output, which we pull-up to +5V power rail and collect all the interference. So, i make new +5V analog path with help 78L05 linear regulator from +12V, as is done in many sound cards:
The interference is all gone! The sound immediately improved and became clear. Finaly, replace 78L05 TO-220 package with TO-92 and mount ampliefier. Sound quality still good without noise. Waiting for variable 10k resistor....