I'm producing a signal which contains 7 frequencies using a microcontroller with a DAC. the frequencies are 16.5kHz 16.7kHz 16.9kHz 17.2kHz 17.4kHz 17.6kHz 17.8kHz. and sampling frequency (at the output) is 86kHz. The signal from the mc is at is then decreased by two resistors (600R/2600R)and fed into LM-386 (which increases the signal by 20) and then into a 4ohm Speaker.
Because of the generation method I get harmonics, aliasing and some parasitic low frequency of which I wish to get rid off. I've calculated that for RC-LPF with Fc=20kHz I need 220nF and 34ohm however, when I put them just before the the (600R/2600R) resistors the whole signal seems to be attenuated significantly!
Am I doing something wrong here? Thanks for the help!
EDIT: The microcontroller is pic16F1783
here's the schematic: