I have a simple audio circuit that needs to be amplified.
The circuit is a CMOS based circuit, running on 9V coming from a 12V battery pack stabilized with a 7809 IC. I have also an amplifier circuit based on the Texas Instruments LM4950. The circuits are on separate PCBs.
I'd like to power the amplifier with the same voltage source as the audio circuit (the 12V battery pack.)
Now my problem is: when the circuits are powered with two different sources (two 12V battery packs) all is fine and the amplifier circuit sounds perfectly. When I connect it to the same battery pack (directly to the 12V) of the audio generator circuit it sends out only loud, interrupted noise.
I tried to opto-isolate the amplifier voltage source to eliminate the distortion and to filter the voltage with capacitors but it does not work. I also measured current to check if the current absorption of the amplifier is too high for both circuits but I measured only about 100 mA.
Maybe is a silly thing but I really can't find out to resolve it, hope that someone can help me.