[sdiy] Chris Synths polysynth

[Mike Bryant]

Totally agree a Butterworth is better, but he did ask for very cheap  :-).   

How many control voltages need 15kHz ?   I would think 100Hz or so for most surely, maybe 1kHz for a few where a Butterworth would be helpful, but even less for envelopes ?

My studio lock PLL has a 10 Hz rollover and PWM clock is 96kHz so it's well killed by the filter.

-----Original Message-----
From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On Behalf Of Gordonjcp
Sent: 20 October 2021 17:43
To: synth-diy at synth-diy.org
Subject: Re: [sdiy] Chris Synths polysynth

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.

-- 
Gordonjcp

_______________________________________________
Synth-diy mailing list
Synth-diy at synth-diy.org
http://synth-diy.org/mailman/listinfo/synth-diy
Selling or trading? Use marketplace at synth-diy.org