[sdiy] Chris Synths polysynth

[Gordonjcp]

On Wed, Oct 20, 2021 at 10:37:00AM +0000, Mike Bryant wrote:
> If you really want cheaper DACs consider using PWM followed by RC smoothing.  And not an FPGA but something like the new Pi 2040 MCU which has lots of I/O programmability.  I suspect you could get about 20 to 24 8 bit resolution voltages for about a $1.  And when if you need more resolution combine two PWM outputs with different summing Rs.  12 or 13 bits linearity is easily attainable this way, maybe a little more.

I've never seen anyone else do it but I've always had super clean results from PWM by having a 12dB/oct lowpass filter at half the sample rate followed by a notch filter at the sample rate.

It's super easy to calculate a Sallen-Key filter for this.  For a Butterworth response with equal resistors the "ground" cap needs to be half the "feedback" cap, and if you pick 1nF and 470pF then 15kΩ resistors will give you 15kHz-ish and a Q of about 0.73, close enough.  Add in a transistor and a couple of resistors for the emitter load and biasing and you've got a lot more components than a simple RC filter, but far far far better performance.  An RC filter ought to give 6dB/oct rolloff with a Q of 0.5, which is such a slow transition and gentle curve that the Butterworth filter seems almost precipitous by comparison!

A notch filter is harder to do but it'll completely scrape off the last of the PWM "carrier breakthrough" leaving a super clean output.

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Gordonjcp