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Hello again:
I needed a small dual LFO with edge connectors for a few upcoming projects.
Done--fabricated--worked first time. Joy!
To review: I found a cool OTA-based LFO design on EFM's site, here. I built a single really small CA3080 version, see this post, but then realized for some projects I needed--more?
I redesigned two LFO's in one small PCB--part I post is here.
The Revision1 design had some stupid mistakes, which are corrected in the version you see in today's post. Everything now works. Go A's!
THE PCB
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| Thanks as always to PCBWAY for sponsoring this blog and coming through with perfectly manufactured PCB's. They do great work! You can help out the blog by checking 'em out, here. |
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| PCB remains pretty small, 48 x 63mm. Which means: You can get 10 of these for $5USD from PCBWAY. |
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| This rev worked great the first time! |
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| ....doesn't happen often, but when it does I'll take it. |
DUAL LFO SETUP AND PINOUTS
As with the earlier designs, the 10K trimmers set the minimum LFO frequencies when CV = 0V.
On the bench I could adjust this trim to make the slowest frequency REAALLLLY slow--didn't figure out exactly how slow but maybe one full cycle every 5-10 seconds, something like that.
Because I want to use this design as a drop-in to future projects I broke the inputs and outputs of each LFO to 100mil edge pins:
- TRI1, SQR, TRI2 and SQR2 are outputs.
- 11,12 are ground;
- 13 is Vcc (circuit should work with different rail voltages; I have only tested with +/-12);
- Bias (Pins 4,5,9,10) set the voltage offsets for each output independently, with negative voltages relative to ground driving the offset up. I used "inverted CV" to avoid needing another op amp stage for each bias offset.
You shouldn't have to do a lot of tweaking, except for R9 and R5. These 2 resistors set the overall frequency based on incoming CV. I tried values from 10K to 100K, what you use here depends on your needs.
On the bench 20K worked for me, but if you are adding more external CV you should put a 0 ohm resistor in there then follow the guideline laid out in step 3 in "ideas" section below.
IDEAS FOR PROJECTS USING THE DUAL LFO AS A DAUGHTERBOARD
First idea: output of LFO1 feeds input of LFO2. Put a digitally controlled switch or VCA in here?
Second idea: Have bias controls for each output. As mentioned above, to save op amp stages the bias is wired "backwards" so plan accordingly. Also, the output of each waveform is about 12V P/P so to not clip a voltage divider at each triangle output may be needed.
This bias offset could be clamped and then controlled by CV. Why not?
Step 3: add more CV. I found the LFO's respond well to wide swings in CV, from maybe -10V to 10V, going from EVENN SLOWWER to way into the audible range. Cool! If I set R9 and R5 to 0 ohm resistors the CV input op amps become mixers, so it's easy to add as many CV's as needed....maybe a "master CV" pot from -12 to +12 and a few CV inputs with a B100K in front. The diagram above shows you how to set this up.
OUTTRO
Overall: glad this design is finally working. I will use this and the 3080 miniLFO in upcoming projects. Until then, modulate slowly, and don't breathe the fumes!
