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Carrying on with the latest "keep it simple" and "just say no to AI" posts: this time I designed, laid out and populated a small CA3080 based LFO PCB. Works!
The basic OTA > op amp core was stolen from EFM's LFO2; see the previous post here.
Laying this out was easy, but I had to make a few decisions that now seem a bit arbitrary:
- I used Through Hole Technology not SMD. I don't do a lot of through hole work any longer, but have so many THT parts lying around, I felt like I had to use them?
- I had a tube of NOS CA3080's. The DIP version of this IC is no longer made, but, as I am writing this blog I see plenty for sale on Ebay, Amazon and elsewhere.
- Inputs were +/- 12V, but the circuit should work fine with anything from maybe +/- 15V to +/- 9V
- Inputs and outputs were put onto 3 pin 100mil headers.
One modulation input was current limied by R5 and brought out to the "CV I/O" pin, but other CV sources could be easily added.
For this, I put a 100K resistor between the CV and the "Q1 base" wirepad:
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| Be careful! If you leave off the 100K resistors and plug your additional CV directly into the "Q1base" wirepad you risk blowing up Q1. Make sure to current limit the additional CV with a series resistor as shown. |
From here this was an easy build:
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| I got 10 LFO minis from this blog's loyal Sponsor, PCBWAY, for $5USB (excluding tariff and import fees). You can greatly help this blog by checking them out here. |
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| Testing.... |
The LFO worked first time--almost.
I had a short between two of the bypass cap pins (C1) initially; once I cleaned up the sloppy solder everything was fine.
Using only the 10K trimmer board for modulation the LFO goes from < 1hz well into audio range. With +/- 12V CV added, I got a pretty wild range of frequencies out of the board--this is a the hallmark EFM design: cool and simple.
Output for triangle is about 6V P/P, for the square it's close to the rails. Since this is designed to be a daughterboard, I figure whatever I plug it into can gain up or down these values, so for now this is good enough.
What's next? It would be easy to make this a dual super small LFO using SMD parts and an LM13700. Overall, this will find its way into some "quad mod" type modules swirling around in my head, that combine random voltage sources, audio inputs, and who know what else. Until then, don't breathe the fumes.
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