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

[Benjamin Tremblay]

That article is fascinating yes. 
You can tell that Ace/Roland always had more care for how the sounds were made than Korg and other manufacturers of console organ rhythm boxes. 

I tend to look on ebay for to see what weird old rhythm boxes are out there for sale. But I have learned to watch the demo videos before placing a bid; most of these boxes are not that interesting to listen to!
Some of these have 4 or even 3 voices and a handful of rhythms only suitable for the Lawrence Welk show of the era. And most of them have perfunctory voice circuitry. 

Last night I was looking at the Korg Minipops 7 schematic. What’s amazing is basic drum sounds are passive circuits. And what’s also amazing is the more advanced sounds are just conventional circuits being triggered by multiples of the tempo clock. 

But Roland did stuff! Roland analog drum synths make you scratch your head and take note. 

In 1985 I had a few hundred dollars to buy a drum machine. The local music store tried to unload a Korg KPR-77 on me. I liked it, and thought it would work well with the songs we wanted to perform. But my colleagues put some pressure on me to move into the future and go digital. The dealer then tried to unload the Korg PSS-50 on me. I played it for a day and could not figure out how to program it. Nobody told me it could not be programmed! Finally I settled on the Korg DDM-110, with its freeze-dried staccato samples.  I used that thing until around 1989, with individual outputs and a knob to slow down the instrument pitch. By putting a capacitor on the raw low-impedance output for the bass drum, I was able to get some gritty kick sounds when I underclocked. But ultimately I had to accept that no matter what I did, the only interpretation of the sound of the “Korg Super Drums” was bad sampling.  Even the modern lo-fi sound uses smoothing and compression to achieve mellowness. 

It’s not just genres that change how we hear sounds. Its also having convenient, effortless access to DSPs to turn your found sounds into whatever you want. The “synthesis without sampling or processing” approach in that article is interesting, but what’s missing for me is to fulfill their wish: Make me a Roland machine with ALL THE KNOBS to control the synthesis process. I don’t care if it’s digital, but expose the controls. Let me build those 909-style synthesizers from scratch and set up the knobs. 


> On Nov 23, 2021, at 1:24 PM, Benjamin Tremblay <btremblay at me.com> 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. 
> 
>