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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; an online simulation of the current source for the 3080's "Iabc" pin 5 is here.
Laying this out was easy, but I had to make a few decisions that 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 so I used a few. If you build this LFO you'll need to find this IC. The DIP version of this IC is no longer made, but, as I am writing this post I see plenty for sale on Ebay, Amazon and elsewhere--it's not unobtanium.
- Inputs rails were +/- 12V, but the circuit should work fine with anything from maybe +/- 15V to +/- 9V.
- Inputs and outputs were put on 3 pin 100mil headers.
One modulation input was current limited by R5 and brought out to the "CV I/O" pin, but additional modulation sources could be easily added--wire up 100K resistor(s) between your CVs and the "Q1BASE" 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 transistor 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 'em 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 and the CA3080 got hot; once I cleaned up the sloppy solder everything was fine. Fortunately I didn't smoke the CA3080.
Using only the 10K trimmer board for modulation the LFO goes from < 1hz well into audio range. With +/- 12V CV added, I got a wide range of frequencies, from reeallly slllowww to well up into the audio range--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 I designed the LFO-MINI to be a daughterboard, I figure whatever I plugged it into could gain up or down these output amplitudes, so for now this was 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, a couple of these LFO's, and who know what else.
Until then, don't breathe the fumes.
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