Readers: If you'd like to build the project featured in today's post, please go to PCBWAY's Community pages--gerber file; KiCAD 9 project/pcb/schematic/library files, a B.O.M., and more are here.
You can also help out this site immensely by checking out PCBWAY using the link here. Thanks!
=====
When you put your Voltage Controlled Amplifier in series with a traditional ADSR you want the VCA to respond in a linear fashion, otherwise things can sound, well, crappy.
The Irwin dual VCA from this previous post used 2 VCA's in series to get reliable linear response.
Mike Irwin's design--I've built it many, many times now--sounds great, but always required 2 VCA's in series for every audio or control signal I wanted to amplify or attenuate.
But what about a quad VCA IC with linear response out of the box? And is affordable?
That's the Alfa RPAR AS3364.
I built a simple experimenter's board for this IC, with buffered and clamped inputs for CV, so I could hear what it sounded like and avoid the terrible breadboard experience.
 |
| Works! |
DESIGN
The secret weapon present in the AS3364 is the log to linear converters, seen in its datasheet, one per VCA:
The AS3364 datasheet recommends 0-2V for control voltages, which is not commonly found in modular synthesizers....but getting my 5V control voltage signals to this level was easy--I used a voltage divider and MCP6004 op amp configured as a unity gain non-inverting buffer:
 |
| One of four buffers |
To power the MCP6004 op amp I used a simple 78L05 voltage regulator with a diode drop to give me about 4.3V DC:
This got 0-5V control voltages into a useful range for each VCA. The MCP6004 clamped incoming CV a little bit higher than its supply rails; I got a nice buffer to boot.
I stole this clamping/buffering idea from Mutable (open source schematics here--take a look, highly recommended) who uses variations of this same circuit fragment in some of her designs.
THIS BLOG'S SPONSOR!
Before I continue, got to put in a quick word for this blog's patient and always helpful sponsor, PCBWAY. Once I get my circuit idea, I turn it into a .sch/.pcb and gerber file and send it off to PCBWAY. They get it back to me super fast....PCBWAY fabricates at low prices (5x 2 layer boards 99mm x 99mm PCB's for about $5 US.....).
PCBWAY provides a lot of other cool services as well: 3D printing and assembly, for instance.
Hats off to them, I am extremely grateful for their sponsorship of this blog. They do great work!
THE BUILD
The project was really easy. I used all through-hole because I had a bunch of 20K 1% resistors around; the BOM shows 30K but anything from 10K-47K would have worked as long as I used the same resistors for all but R9.
Piece of cake.
 |
| Happiness is always getting new bubble wrapped PCB's from this blog's patient and friendly sponsor, PCBWAY. |
 |
| Will it WFT (work first time?) |
 |
| Only one way to find out--build it... |
 |
| tested with +/- 12V to see if I have shorts/magic smoke--nope. |
 |
| Bench fu.... |
 |
| Ha! Worked and sounded great! |
WHAT NEXT?
Alfa RPAR, the guys who make the AS3364, have a bunch of other interesting chips, the AS3365 (panner), the AS3363 (ring modulator and other functions) and lotsa other chip dookie.
I would like to test all of those IC's out, but in the meantime the AS3364 might find its way into future designs due to its extreme simplicity. For instance, the EFM LFO2 (build is here) uses LM13700's as in-circuit VCA's, it would have been easier to have used an AS3364 IC.
Overall, an interesting chip for what we do. VCF's? Panners? Ring Mod? Sure. Lots of things I haven't thought up, yet, but, "you can never have too many plain-old VCA's."
See ya next time--Don't breathe the fumes.
No comments:
Post a Comment