[sdiy] Blackmer VCA cell

[Mike Bryant]

Really depends what frequency you want your CVs to be able to move at.   In a piece of HP test equipment I designed in early 1980s I multiplied a 4kHz bandwidth telephony signal this way using a 1MHz clock.  TRW introduced commercial versions of their military digital multipliers soon after so that was as far as I ever took that design, and indeed that was when I stopped designing analogue synths as well.


-----Original Message-----
From: Brian Willoughby [mailto:brianw at audiobanshee.com] 
Sent: 24 October 2021 00:09
To: Mike Bryant
Cc: *SYNTH DIY
Subject: Re: [sdiy] Blackmer VCA cell

On Oct 23, 2021, at 15:50, Mike Bryant wrote:
> A standard comparator based PWM circuit fed from one voltage drives a MOSFET switch fed with the other voltage and that feeds the averaging filter.   Just one FET more than a normal PWM.

That makes sense. So, an op-amp (comparator) plus a FET per PWM channel?

How difficult is it to select a capacitor for that averaging filter that will work well across the full range of both CV inputs? ... or does the frequency of the PWM determine the capacitance and that's it?

Brian