Can you tell us anything more about what kind if input your synth needs? I beleieve the PWM output can change quickly enough to trigger your synth or to control the ADSR envelope.
The thing is, I'd like to be able to change the voltage very quickly and as I understand it this requires that I build a rather complex high order LP filter.
An LP (low-pass) filter actually makes the voltage change more slowly
. But, it could be true that you need a high-order low-pass filter for this application.
...and I'm wondering if there are any ready made DAC's that I could use instead?
That might be a good idea. Do you think 8-bits (256 different output levels) is enough for you needs? I can't recommend one, but here
are a few you can look at.