sine oscillator for audio/lfo

Discussion in 'General Electronics Chat' started by 1680sean, Jul 4, 2012.

  1. 1680sean

    Thread Starter New Member

    Jun 24, 2012
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    I have been working on some op amp oscillators lately and really struggling with the sine. I had a wien bridge running but I want to control frequency with one dual ganged pot (or just a single pot, if possible). Can this be achieved by simply putting another integrator on a triangle oscillator and connecting the output to the negative input of the schmitt trigger? It works in this simulator applet but not on my breadboard, although I'm using tl082's so not a big surprise.
    Any suggestions on how to make this work i.e. opamps, virtual grounds, different circuits altogether?
    I've also got 741's, 324's, npn's, and pnp's.
     
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  2. upand_at_them

    Active Member

    May 15, 2010
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    The TL082 needs a dual supply. Try a LM324.
     
  3. absf

    Senior Member

    Dec 29, 2010
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    Or you can use the circuit attached....
     
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  4. 1680sean

    Thread Starter New Member

    Jun 24, 2012
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    I've been using the leftover half of an op amp as a virtual ground (found it here http://tangentsoft.net/elec/vgrounds.html) and it seems to work for the basic triangle and square circuit. Trying the 324 but now the triangle and square don't even work. Is there a special way to attach the batteries?
     
  5. daviddeakin

    Active Member

    Aug 6, 2009
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  6. t06afre

    AAC Fanatic!

    May 11, 2009
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    This is how you can connect two 9 volt batteries or some other voltage sources to get a dual supply
    [​IMG]
    I will also recommend reading this ebook
    http://focus.ti.com.cn/cn/lit/an/slod006b/slod006b.pdf
     
  7. Audioguru

    New Member

    Dec 20, 2007
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    ANY opamp will work from a single-polarity supply. Instead of using two batteries to make a dual-polarity supply simply use two resistors to bias the opamps at half the supply voltage like shown in the project that was posted.
     
  8. t06afre

    AAC Fanatic!

    May 11, 2009
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    You can find a lot of free software on the net also. That turn your PC sound card into an audio oscillator
     
  9. DeanHuster

    New Member

    Jul 4, 2012
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    The problem with using a pot to vary the frequency is noise and linearity. A truly linear pot is needed to make a decent frequency dial with good repeatability and freedom from goofed-up dial markings if the pot has to be replaced. And a truly linear pot is a wirewound pot which, good though they are, have jumps from value-to-value as they're rotated and unless you have a "fine" frequency adjust pot with a much lower value (with a "calibrated" position), it would be difficult to adjust to an exact frequency to null out notch filterd or peak out bandpass filters with narrow bandpasses.

    A great solution to this is to use a FET-input op amp with high value timing resistors (switch THESE for range switching) and use an air variable capacitor to adjust the frequency instead of a pot. No, it's not linear. But is doesn't have "steps" as you adjust the frequency and it doesn't get "noisy". Hewlett-Packard (Agilent) used this technique on their 200CD audio oscillators they made in the 1960s. They worked great and you could bend plates on the cap to get the frequency dial to track dead-on.
     
  10. Audioguru

    New Member

    Dec 20, 2007
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    I have a Wien Bridge oscillator I made using a TL071 Jfet-input opamp and a dual 365pF variable capacitor used in old AM radios (does anybody make an AM radio anymore?).
    It uses a Jefet for amplitude stability and its distortion is fairly low.

    I have an extremely low distortion oscillator that makes a sine-wave with 10 steps then a switched-capacitor lowpass filter IC that smoothes the steps so there are no harmonics.
     
  11. 1680sean

    Thread Starter New Member

    Jun 24, 2012
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    Thanks for the suggestions. Yeah i'm starting to realize how terrible the pots from radioshack are. Has anyone had any luck building their own variable capacitor? All I have are some variable inductors in an old vcr. Are their any oscillators with just inductors and resistors?
     
  12. 1680sean

    Thread Starter New Member

    Jun 24, 2012
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    also, could one use an additional TL082 to stabilize the amplitude? I'm going to go scavenge for J-FET's.
     
  13. DeanHuster

    New Member

    Jul 4, 2012
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    They're getting harder to find, but there are still variable caps to be had. Try Antique Electronic Supply (http://www.tubesandmore.com ) and grab some 365pF versions (they had two sections -- 365pF and another around 150pF and you can parallel them) that were used in a lot of AM radios. I have a communications receiver that uses a triple 365pF main tuning cap and that would be even better, but harder to find.

    I checked to make sure the link worked -- it does -- and I notice that AES has a triple 365pF unit. Of course, it ain't cheap.
     
    Last edited: Jul 6, 2012
  14. 1680sean

    Thread Starter New Member

    Jun 24, 2012
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    So if the sections are different doesn't that mean it wouldn't work in a wien bridge oscillator? I imagine the three section has the same values. That would be great for a phase shift oscillator, right? I think I found one in a radio. It's a square box, about 3cm with a big wheel operated by a belt attached to the tuning knob.
     
  15. daviddeakin

    Active Member

    Aug 6, 2009
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    A pot works perfectly well for basic applications. You may be over thinking this, depending on what you're actually trying to do (?).

    Variable capacitors are great for high precision instruments, but if you just want a low distortion sine oscillator and you're not too worried about long-term stability, then a pot is fine.
     
  16. 1680sean

    Thread Starter New Member

    Jun 24, 2012
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    I just want a sine wave for my ring modulator that's symmetrical and doesn't clip. I haven't yet found a triangle-to-sine diode network type circuit that looks or sounds right so I'd prefer something else.
     
  17. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    I'll apologise as this may be off topic, but if you are mainly interested in building a sine generator that will work well (rather than building one for a learning experience) here is a microcontroller based one that does a pretty good audio quality sine and has the benefit of being xtal locked so once set to a frequency you will get an exact and stable frequency until you change it.

    Also the frequencies are xtal locked in actual Hz steps, so you can set it to exactly 1000Hz, 1010Hz, 1020Hz etc.

    [​IMG]

    http://www.romanblack.com/onesec/SineDDS.htm

    (edit) Sorry I forgot to mention it also does square, triangle, sawtooth waves as well.
     
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