Isolating (oscillator) circuits

Discussion in 'The Projects Forum' started by Welly, May 1, 2011.

  1. Welly

    Thread Starter New Member

    May 1, 2011
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    Hi
    A newbie question... I am making a small synthesizer that consists of an audio oscillator (basic 555 timer circuit), a Low Pass Filter (using a LM13700), and a Low Frequency Oscillator as a CV source (also a basic 555 circuit). These circuits are all run off the same power source -9V battery cell.
    The problem I am experiencing is that the LFO circuit which has its output tied to the filter, is creating unwanted interference to the audio even when the output is not connected to the filter. I understand that I need to isolate this circuit somehow. All I can gather by scanning the forums is that this can be done with an optocoupler or maybe a voltage regulator. Would someone be so kind as to enlighten me as to how exactly?
    It would be easy for me to insert something on any/all of these 3 circuits as they are on separate PCBs... Thanks in advance :)
     
  2. t06afre

    AAC Fanatic!

    May 11, 2009
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    You better post some pictures and schematic. Hard to give advice without more info
     
  3. Welly

    Thread Starter New Member

    May 1, 2011
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    Hi
    Thanks for the quick reply...
    OK, here are the schematics of the various circuits used in the project. All circuit's +9V and Grounds are connected to the same 9V cell.

    [​IMG]

    [​IMG]

    The Filter schematic is taken from Eric Archers great website www.ericarcher.net
     
  4. Audioguru

    New Member

    Dec 20, 2007
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    The 741 opamp is 43 years old and was designed to use only a plus and minus 15V supply. Many will not work from a single 9V supply.

    The pin 3 voltage of any opamp must be at half the supply voltage for it to be a linear amplifier but yours is at 0V so if the input works (but it probably doesn't) then the opamp becomes a rectifier.
     
  5. Adjuster

    Well-Known Member

    Dec 26, 2010
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    There are no capacitors shown across the supply input and ground of the 555 oscillator. Supply decoupling (bypass) capacitors are generally good practice, and in the case of your oscillator adding them may help reduce crosstalk through the 9V supply. You could try 100nF for a start, but you may find that bigger values give more benefit in your audio application.

    Supply decoupling capacitors should be fitted across the supply inputs to the other circuits. In each case, place the capacitors close to the supply terminals of the IC.

    Further improvement might be obtained by adding very small resistances (a few tens of ohms; more might drop too much supply voltage) between the 9V battery and the supply inputs to the 555s. Do not try this before fitting decoupling capacitors across the IC supply and ground pins.

    This "sticky" thread explains about decoupling capacitors:http://forum.allaboutcircuits.com/showthread.php?t=45583
     
  6. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    It is seldom mentioned, but simply placing 3 or 4 Nine volt batteries in parallel can often cure these power supply caused signal coupling problems. The increase current delivery capacity prevents device current draw from 'modulating' the voltage supply. They are a very poor battery choice due to high internal resistance and surge current capacity limitations. OH, and the supply bypass caps are something that should always be included.

    :)
     
  7. Welly

    Thread Starter New Member

    May 1, 2011
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    Hi
    Wow...Thanks for the quick replies.
    I added 0,1uF caps across the supply voltage of the 555 circuits.
    Now I get no sound from the oscillator at all, and the LFO no longer seems to send CV.?? There is a lovely white noise at the output, and filter still works fine.

    And Audioguru, thanks for the information about the 741. I got that circuit from the Engineer's Notebook by Forest Mims, and whatever its been doing to the signal, it seems to do what I needed it to ie. make the audio signal stronger.
     
  8. Audioguru

    New Member

    Dec 20, 2007
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    Everybody has seen Forrest's Notebooks. He also gets things wrong, you know.
    Don't believe everything on the internet because there are a lot of errors there.
     
  9. Welly

    Thread Starter New Member

    May 1, 2011
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    Hi
    Ok..The oscillator works again, and sounds much more stable with the added decoupling caps....I must have done something to the LFO, as it seems to have died.:mad:.. Will post an update once I have replaced the LFO...
    Thanks again for the help...:)
     
  10. Welly

    Thread Starter New Member

    May 1, 2011
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    Yes, thanks...I will go back to drawing board on that Pre-Amp. I have been learning the hard way. One of Forest Mims circuits recently caused one of my potentiometers to start smoking.
    I did build an amplifier using a LM386, but it seems to use a lot of current and was unnecessarily loud.
     
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