Low amplitude from a LM13700N.

Discussion in 'General Electronics Chat' started by gface83, Jan 19, 2017.

  1. gface83

    Thread Starter Member

    Jul 16, 2016
    74
    1
    Hi guys, I,m back again! I'm not sure if I am posting this in the right section, If it's not, let me know and I'll try to figure out how to move it.
    Anyway, I have built a VCA using a TL074CN as a CV mixer and a LM13700N transconductance op amp. The circuit is performing as it should but I'm getting very low volume on the output. I have checked to see what is going on with an oscilloscope and everything seems to be fine with the TL074N and the signal is fine going in to the input of the LM13700N. The signal going in is around 4 volts, but it is in the millivolts on the output, resulting in a very quiet signal. I have also checked the signal on the oscilloscope after the input goes through the 47K resistor, but that doesn't give me any sort of reading I can understand. It just looks like static, but as I say the circuit is working and coming out of the output so I don't think this is the problem.
    This circuit is from the MFOS website and Ray mentions; "I attempted to get a gain of one at maximum control voltage although due to variation between LM13700 chips your mileage may vary." Could the chip be the problem? I have already tried 2 different ones and I'm getting the same result, so I'm not so sure it is the chip. Any help on this matter would be greatly appreciated. Thanks.
    Here is the schematic:
    [​IMG]
     
  2. hp1729

    Well-Known Member

    Nov 23, 2015
    2,306
    283
    Design 1060 LM13700 a.PNG
    You are losing the signal out of U5-A? What is the gain of that circuit? I would have expected to find something more like the attached.
     
  3. gface83

    Thread Starter Member

    Jul 16, 2016
    74
    1
    Yes, I am losing gain out of U5-A. The signal going in is as expected, but, very low on the output. The circuit has been designed to provide a gain of 1. The circuit has been built by Ray himself and many others on the internet, so it should work as is, but for some reason mine isn't and I can't figure out why.
     
  4. hp1729

    Well-Known Member

    Nov 23, 2015
    2,306
    283
    Ray would certainly be hard to disagree with. I'm no engineer. All I know is what I read in the data sheets.:)
     
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