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Re: Uni-vibe LFO questions

2/2/1999 6:18 PM
moocow Re: Uni-vibe LFO questions
A twin-T oscillator also works as a sine-wave LFO. It uses only a single NPN transistor, and it is less sensitive to transistor variations than the phase-shift oscillator. However, its speed range is only about an octave. Op-amp oscillators have a much wider range.  
What is the speed range of the Uni-Vibe, anyway ?
2/2/1999 6:21 PM

Yeah, that's the way I chose in my experimental LFO that I posted here a ways back. It's at  
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but I'll also stick it here. I chose for this one to use an opamp variation of the diode sine shaper with two breakpoints and only shape one side of the triangle to approximate the exponential sweep in the LFO itself as opposed to the voltage driven LED variation. I also stuck some other musically interesting junk on it  
like half/full speed switching and a manual sweep.  
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2/2/1999 8:01 PM
Dave B.

Hey Mike,  
Do you know of any schematic that has an example of the "classic 2-op amp triangle-square wave generator"???  
Will this be suitable for a correct Uni-vibe sweep??
2/2/1999 11:14 PM
Mike Irwin

Hi Dave,  
One of the best places to go for op amp info is the National Semiconductor website. Go to and download AN-20 (see page 10) and AN-31 (see page 6). This is the "classic" circuit-almost foolproof... Don't worry about the op amps shown-just about any single or dual op amp can be used here. Both op amps can be identical. TL072's are fine. Put in a much larger capacitor than the 0.1 microfarad shown. Use a 4.7 ufd film cap if you can get one, or connect two 10 ufd 16 V electrolytics in series (+ of one to the - of the other, this will function as a larger non-polar cap). The circuit as shown will work over a wide range of supply voltages +/- 15 V to +/- 5 volts should be fine. Note that a bipolar supply is used. For single supply operation you will need to make a "fake" ground to connect to the + input of the integrating op amp and to the - input of the comparator op amp. The amplitude and frequency pots interact, so fix it at the max useable amplitude and use a 10K pot connected to the triangle output and ground for the variable amplitude triangle output. Then adjust the frequency to taste. Experimentation is required!
2/1/1999 4:09 PM
Dave have you read RG's univibe article yet?

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