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.

Best Regards

 

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.

Best Regards

Something doesn't seem right here. Is there any chance that your 33 nF caps are supposed to be 33 uF (edit: even 33 uF seems to attenuate too much in the passband)?

The circuit you showed seems to have a peak response right at 1 kHz, while 33 uF caps would give you a (edit )100 dB attenuation at 1 kHz. This means that your 3.3 V signal would go down to (edit) sub mV levels.

If you actually built it with 33 nF, then ignore my response because I must have made a mistake in the calculations.

 

Hi

I built it with 33nF.
Actually, the sound is ok.
However, I want to know deeper about the concept and how this circuit work.
I got the circuit from here:
http://www.olimex.com/dev/pdf/ARM/ATMEL/SAM7-EX256.pdf
Could you please help me on this ?

I have read this
http://ww1.microchip.com/downloads/en/AppNotes/00538c.pdf
Why is the circuit using bandpass ?

What is the calculation of RC, after the op amp LM386?

Thanks in advance,
best Regards,

 

Hi

I built it with 33nF.
Actually, the sound is ok.
However, I want to know deeper about the concept and how this circuit work.
I got the circuit from here:
http://www.olimex.com/dev/pdf/ARM/ATMEL/SAM7-EX256.pdf
Could you please help me on this ?

I have read this
http://ww1.microchip.com/downloads/en/AppNotes/00538c.pdf
Why is the circuit using bandpass ?

What is the calculation of RC, after the op amp LM386?

Thanks in advance,
best Regards,

OK, ignore my previous post, since I was misunderstanding what you are trying to do. It seems you do want to pass the 1 kHz signal, in which case the 33 nF caps are appropriate to center the passband in the 1 kHz range. Although, I don't know why your measurement seems to show large attenuation.

Can you explain what you are trying to do in more detail. The output RC network seems to be relevant up in the hundreds of kHz.

You mentioned PWM. What is the frequency of your PWM? That might help shed some light on this. Is your PWM just the 1 kHz and you just want to amplify that and use it an an audio signal?