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Circuit PWM to Audio

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pistole89

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Hi everyone,

I found a circuit which convert PWM into audio signal.
First they used 2 cascade low pass filter and high pass filter, then LM386 audio amplifier with RC.

Could anyone please explain in more detail about the circuit ?

As I know, the first bandpass filter is used to attenuate the PWM.
I have tried to play the duty cycle from digital 0x00 to 0xFF.
The output from these band pass (before Opamp LM386) is shown.
The value is only about mV, although the PWM is 0-3.3V

Thanks for your help.
It will be very appreciated.
 

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The break frequency of your low pass filter is 4kHz. From your schematic, the input PWM is 1kHz, so the LPF will not be doing what you expect. To smooth PWM to a DC voltage, you need the break frequency of your low pass filter to be much lower than the PWM frequency.

The best way to play with this sort of stuff is in LTSpice. it will save you a whole ton of time. Here is an LTSpice tutorial if you are new to it:
http://www.simonbramble.co.uk/lt_spice/ltspice_lt_spice.htm
 
PWM audio requires the PMW to be modulated by the audio signal. For this the PMW switching frequency must be much higher (10 times at least) above the highest audio signal frequency. The PWM filter corner is then set at the highest audio frequency. That way you can readily filter out the PWM switching frequency, leaving the audio signal.
 
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