Hello I have once read in a book (which of course I can't find anymore) about a direct PWM volume controller. The idea is that you simply take the audio and chop it with a high frequency, variable duty cycle signal. Then you use some type of low pass filter, to get rid of the high frequency component, and that's it: the output signal equals the input signal (theoretically no distortions), scaled by the PWM ratio.
The schematic could be something like this:
4066 analog switch C / || input *-----/ ------------||---+----* output | || | | \ | R / 200khz | \ ---------------- | | PWM generator| GND ---------------- | | | | V | --\/\/\/-- Volume potentiometerThe advantage is that you can add any number of channels on the same PWM signal (I need 5), and they are completely synchronized. I would like to implement such a volume controller, do you think it could work? (if anyone knows a similar design, I would appreciate it, no point in reinventing the wheel.) I was thinking of using a 555 for the PWM generator, but I don't think it's a good idea, because I don't have the full range 0-100%. Especially the lower limit, there must be a setting on the potentiometer that gives 0 V on the PWM output, no audio is getting through, silence. Also, what do you think about the output filter? Choosing for the simple RC filter values that give no more than 3dB drop at 20 KHz, gives a 22 dB reduction of the 200 kHz signal. Strictly speaking, the 200 KHz signal would be exactly 22 dB below the audio signal. If the 200 KHz signal gets into amps and speakers, is there any danger? Because then I would have to use a more complex filter, and i want a low cost solution. Should I aply any bias on the 4066, to get low distortions? Thank you.