[sdiy] Deep thoughts on Old drum machines: the "VCA"

Benjamin Tremblay btremblay at me.com
Sat Jan 22 19:29:01 CET 2022

I just remember some summer in the early 1980s building a number of PAiA circuits, hoping it would open the doors to being able to make some good sounds. 
I built the surf synthesizer and it was really wonderful. I ran it for days in the garage. 
I tried the wind chimes synthesizer, and it sounded like someone shaking a giant glass jug fill of bamboo sticks.
Then I tried some of the modular circuits. 
The 1-transistor bandpass filter, which was pretty much the same circuit used in the surf and chime circuits. 
When I finally got to the “lowpass filter” circuit from the 2700 series, it was a dud. It was a filter in name only. 

The next summer, I hit the university library and found the Moog 904 patent diagram. After building that, I was no afraid to build the Moog VCA. 
But making a stable, reliable VCO was for the big guys. To this day I have never made a decent VCO, but the older I get the more pointless it seems.

> On Nov 26, 2021, at 2:16 PM, Mattias Rickardsson <mr at analogue.org> wrote:
> Brian Willoughby <brianw at audiobanshee.com <mailto:brianw at audiobanshee.com>> skrev:
> One interesting side-effect of the swing VCA is that it creates a DC offset in the signal that a real VCA mostly avoids. This potential problem is removed by capacitively coupling the different audio stages together, so that any DC offset is removed. However, even an AC-coupled system can pop when the DC offset changes abruptly. For percussion sounds, though, this abrupt pop might actually be an improvement to the sound.
> Probably also for synth sounds. Among all potential cases of people saying that "the old synth sounds better than the new version/clone/emulation/whatever", offset thumps might be one of the unknown secrets that reverse engineers often miss.
> /mr
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