PWM to Analog conversion.

[NewGuy458]

Hi,

I'm building an amplifier with the TAS5518 processor.
I wish to use also the PWM output from it for the headphones.
Can somebody tell me how to convert the TAS5518 Differential PWM Output to analog to be used with a standard OpAmp?
Thank You very much.

 

[nigelwright7557]

Resistor and capacitor is simplest into non inverting input of op-amp.

IR2113 is another option.

 

[Pafi]

Well... yes, a low pass filter can convert "PWM to analog", but for high quality DVDD and DVSS must be perfectly clean, wich is not automatically true!

 

[toptip]

PWM to analog

I am curious to follow up on this.

There are some nice and inexpensive home theater system out there, many with back channels connected wirelessly, like the Sony HBD-N7100 series.

These are convenient but they have no preamp outputs, only speaker connections. For instance if one wanted to use more powerful amplifiers for the front channels or a powered subwoofer, the only option is to connect those the speaker outputs. Well, that is kind of uncool.

Looking at the service manual, these are all digital devices, where eventually the signal is converted to PWM to drive the output amps, presumably some sort of Class D.

I was able to pull out the PWM leads just before the power amps but I am kind of stuck there. Whatever I read, the suggestion is to use a simple RC filter to convert that to analog. I suspect that may be unsatisfactory. On eBay there are some PWM=>Analog converters but specifications say they top out at 3kHz.

I am wondering if there is something better? Many thanks.

 

[abraxalito]

LC filters are normally used at the output of classD amps. They give more rejection than RC, or alternatively a wider passband.

 

[jjasniew]

One could use an op-amp set up as a 1st order filter - as already suggested. That would provide PWM to analog - and a low Z driving source for the rest of your connection - cables, amplifier input Z. So that's a little bit better than just an RC filter.

 

[MorbidFractal]

Or... second order low pass set to 40KHz.

 

[toptip]

PWM to analog with op amp

If I use an op amp IC, does that simply act as a buffer for the following stage following a RC or LC filter, or does the filtering happen in the op amp’s feedback loop?

(Separate from this tread, I always wanted to know if passive vs feedback loop filtering are alternative ways of building anything from RIAA to tape playback preamps...need a tutorial on the subject.)

 

[b_force]

Resistor and capacitor is simplest into non inverting input of op-amp.

IR2113 is another option.

I vote for 2 resistors with 2 capacitors, which gives a passive 2nd order filter (Q = 0.5 if you choose the values right)

 

[rayma]

If I use an op amp IC, does that simply act as a buffer for the following stage
following a RC or LC filter, or does the filtering happen in the op amp’s feedback loop?

Could be either way. there are many, many variations.
http://sites.bu.edu/engcourses/files/2016/08/ActiveFilterNotes.pdf

 

[toptip]

Thank you great, well-written text.

 

[toptip]

I found this TI write-up. I cannot tell if it is actually a PWM-to-analog converter / buffer amp (with 4th order filters) or for some other purpose. The giveaway is, it has a -3dB point of under 4kHz.
https://www.ti.com/lit/ug/tidu027/tidu027.pdf?&ts=1589095714829

 

[Pano]

I'm curious as why you'd want to run the output of the TAS5518 into an op-amp. Isn't the TAS5518 already an amp? Just low pass it and attenuate if for headphone use.

 

[toptip]

That is probably right. But won’t the low pass filter have an impedance that should be buffered before it is fed to a low impedance load?

 

[Pano]

No, it would have a low pass filter designed for 4 or 8 ohm loads. For higher impedance loads it would need tweaking. But that could be solved simply by using an L-Pad on the output to get to headphone levels.

 

[nigelwright7557]

If I use an op amp IC, does that simply act as a buffer for the following stage following a RC or LC filter, or does the filtering happen in the op amp’s feedback loop?

The op amp buffers the output of RC network.

 

[Pano]

Ah ha, I see my mistake. The TAS5518 is just the front end processor, not the power section. And you want to use those outputs meant to go to the amp chip. You'll still want some sort of low pass filter.

 

[toptip]

PWM output

Yes. These HTIB systems do not have preamp outputs. And yes to the low-pass. I had two questions —

1. Does a first order filter suffice? The TI write-up has a fourth order
2. Do I need a buffer op amp before I can feed an outside power amp?

I suspect a higher order low pass filter will have a higher output impedance. In the TI example, they increase resistor values by 10X at each stage not to load the prior stage.