Circuit: half works/half doesn't

Discussion in 'General Electronics Chat' started by Tony Elliott, Jun 22, 2016.

  1. Tony Elliott

    Thread Starter Member

    May 8, 2015
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    2
    Hi All,
    I have made this circuit, it is a voltage controlled amplifier.
    The signal in works but the voltage control (CV) doesn't. What is the best way of finding my error?
    I have checked the resistor values. All the connections and changed all the active components so far.
     
  2. dl324

    Distinguished Member

    Mar 30, 2015
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    What is the circuit doing?

    Start by checking the biasing of U1C. Do both CV inputs not work?
     
  3. Tony Elliott

    Thread Starter Member

    May 8, 2015
    140
    2
  4. dl324

    Distinguished Member

    Mar 30, 2015
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    Does the log/linear switch make a difference? Have you checked DC biasing for U1C?
     
  5. Tony Elliott

    Thread Starter Member

    May 8, 2015
    140
    2
    The switch makes no difference, and yes I put the trimmer to 0 volts. I've been looking at this circuit for about six hours!
     
  6. dl324

    Distinguished Member

    Mar 30, 2015
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    U1C is common for the log and linear switch setting, so make sure it's working. Check the DC biasing.
     
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  7. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    What do you mean, "I can just hear it". are you connected directly to a headphone / speaker or are you sending the signal through an amplifier?

    Also, turn your Bias Adjust up so it is at 1.2 to 1.4V (or more).
     
  8. Tony Elliott

    Thread Starter Member

    May 8, 2015
    140
    2
    There is an oscillator connected to the CV when I turn it up in frequency I can just hear it. The circuit is connected to a hifi.
     
  9. Tony Elliott

    Thread Starter Member

    May 8, 2015
    140
    2
    One thing I should of said was that the q4 transistor was around the wrong way when I first turned it on resulting in the 2n3906 transistor burning out. I then corrected this and replaced all the active components on the circuit. Could this have effected any of my passive components?
     
  10. Lestraveled

    Well-Known Member

    May 19, 2014
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    Lift pin 1 of the OTA (U2-A). Connect a 25K resistor between pin 1 and +V. The amplifier will then be at full gain so you can check it out.
     
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  11. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Measure the voltage at Vout when it is connected to your HiFi. The input impedance of your HiFi may be too low and attenuating the signal of the VCA. If so, you'll need a buffer (pre-amp) between your VCA and HiFi.

    Also, BiasA POT should be adjusted to have 0V at Vout with no signal.

    Are there people on the webs who have had success with this circuit, or has it garnered little enthusiasm because it doesn't work?
     
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  12. Tony Elliott

    Thread Starter Member

    May 8, 2015
    140
    2
    Im going to put this into practice when I get in the studio tomorrow morning, thanks for the advice! :)
     
  13. Tony Elliott

    Thread Starter Member

    May 8, 2015
    140
    2
    I got it working at last, I put a positive instead of negative into an IC:-( sorry for my stupidity.
     
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