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Using a PWM controller as audio AMP? (self.askelectronics)

submitted 5 days ago* (last edited 4 days ago) by einfach_orangensaft to c/[email protected]

6 comments fedilink hide all child comments

After playing around with some greeting card modules to modulate sound onto a LED i found out that the driver part of said greeting card modules are not audio drivers but PWM drivers (oscilloscope confirmed).

This made me wonder if normal PWM controllers could be used to drive more power full LEDs.

My idea was to just remove the potentiometer and feed in music from Aux at that point.

Is there a chance of this working?

Update: I just tryed if it works and sadly it dosent, LED gets modulated somewhat but there is not audio to recover from it afterward, its only noise.

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[–] [email protected] 2 points 4 days ago

[–] litchralee 7 points 5 days ago (1 children)

are not audio drivers but PWM drivers

They can be both! A Class D audio amplifier can be constructed by rendering an audio signal into a PWM or PDM output signal, then passed through an RC filter to remove the switching noise, yielding only the intended audio.

That said, in this case, using the unfiltered PWM output would work for greeting cards, where audio fidelity is not exactly a high priority, but minimal parts count is.

This made me wonder if normal PWM controllers could be used to drive more power full LEDs.

What exactly did you have in mind as a "normal PWM controller"? There's a great variety of drivers that produce a PWM signal, some in the single watt category and some in the tens of kilowatts.

Whether they can drive "more powerful" LEDs is predominantly a function of the voltage and current requirements to fully illuminate the LEDs, plus what switching frequency range the LEDs can tolerate. Some LED modules that have built-in capacitors cannot be driven effectively using PWM, as well as anything which accepts AC rather than DC power. You'd need a triac to dim AC LED modules, and yet still, some designs simply won't dim properly.

My idea was to just remove the potentiometer and feed in music from Aux at that point.

You'll have to provide a schematic, as I'm not entirely sure where this potentiometer is. But be aware that the output current needed to drive a small speaker is probably insufficient to light up a sizable LED, nevermind the possibility of not even having enough voltage to meet the required forward voltage drop of the LED.

Is there a chance of this working?

It might, but only if everything just happens to line up. But otherwise, it's likely that it won't work as-is, due to insufficient drive current.

[–] einfach_orangensaft 2 points 5 days ago (1 children)

They can be both! A Class D audio amplifier can be constructed by rendering an audio signal into a PWM or PDM output signal, then passed through an RC filter to remove the switching noise, yielding only the intended audio.

I did not know that! Very interesting for sure :D

You’ll have to provide a schematic

[–] litchralee 2 points 4 days ago

Based solely on this drawing -- since I don't have a datasheet for the PWM controller depicted -- it looks like the potentiometer is there to provide a DC bias for the input Aux signal. I draw that conclusion based on the fact that the potentiometer has its extents connected to Vref and GND, meaning that turning the wiper would be selecting a voltage somewhere in-between those two voltage levels.

As for how this controls the duty cycle of the PWM, it would depend on the operating theory of the PWM controller. I can't quite imagine how the controller might produce a PWM output, but I can imagine a PDM output, which tends to be sufficient for approximating coarse audio.

But the DC bias may also be necessary since the Aux signal might otherwise try to go below GND voltage. The DC bias would raise the Aux signal so that even its lowest valley would remain above GND.

So I think that's two reasons for why the potentiometer cannot be removed: 1) the DC bias is needed for the frequency control, and 2) to prevent the Aux signal from sinking below GND.

If you did want to replace the potentiometer with something else, you could find a pair of fixed resistors that would still provide the DC bias. I don't think you could directly connect the Aux directly into the controller.

[–] [email protected] 2 points 5 days ago (1 children)

It might work if the six I put is digital, as PWM is a digital signal. It is my understanding that aux jacks are analog, and this require a waveform signal. If you drive a PWM signal into an analog aux jack it would sound broadly like static with a regular interval.

But if you’ve got the parts, just try it.

[–] einfach_orangensaft 1 points 5 days ago

no i dont try to drive into aux, i try to drive from aux