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

[Brian Willoughby]

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.

Unfortunately, even real VCA circuits can bleed some of the CV through as a DC offset.

Brian


On Nov 23, 2021, at 10:24, Benjamin Tremblay via Synth-diy <synth-diy at synth-diy.org> wrote:
> I learned about drum machine technology in a random order, and here I am at the age of 54 studying the disco-era machines. I’m looking at the Keio/Univox snare and cymbal sound generators, and I’m stunned. They don’t even try to be a “VCA”; they just bias the amplifier to pass sound, and cut off the bias to make it stop.
> This seems to be common in most transistorized drum machines from the 60s and 70s. Same for the PAiA drum sounds, and the drum machine in the old Engineer’s Notebook. I guess the philosophy is, “it’s just noise, what does it matter if it’s grossly distorted?” But that means a cymbal or brush sound must have be built lite this: NOISE—> GATE —> [Resonant High-pass Filter] because they often have a sweet and mysterious decay, which must be smoothed by the filter. 
> 
> The Minipops 5 I have been working on is as old as I am, but in much better condition than me. Both the Minipops 5 and the Univox JR-5 have this weird, obnoxious snapping snare drum sound, and it’s growing on me. It sounds like an IC is exploding on the board. I’m going to try to hack these tone generators and expose some knobs, but a slow decay using the bias-driven gate circuit will probably sound… different. 
> 
> Roland’s “Swing VCA” is just barely a real VCA: it’s like half of a current-controlled differential amplifier. It still distorts the sound a lot if you want to have a decent dynamic range, but at least it tries. Nowadays folks build drum tone generators using real VCA chips or OTAs, and that seems gluttonous, or glutinous.