JFET Preamp Question

Discussion in 'The Projects Forum' started by tracecom, Aug 9, 2012.

  1. tracecom

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    Apr 16, 2010
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    I built this preamp on a solderless breadboard (less the zener diodes.)



    The input is .6 Vpp and looks like this.



    The output is 1.3 Vpp and looks like this.



    What's wrong?
     
    Last edited: Aug 10, 2012
  2. #12

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    Nov 30, 2010
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    You're picking up noise on the 3.3 megohms of input impedance?

    No. That's jitter in your scope?

    Move the signal frequency up and down and see if the sync locks in better.
    Go to a higher frequency scan rate and see if its a waveform riding on the intentional wave.
     
  3. MrChips

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    I don't see anything wrong. What do you see?
     
  4. tracecom

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    On closer inspection, it looks like the fault is with my sine wave generator (home made). The input is on top and the output on the bottom trace. It looks like the preamp is simply amplifying the distortion that's already there.



    Do you agree?

    Thanks.
     
    Last edited: Aug 10, 2012
  5. tracecom

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    I am not very experienced with an o'scope. And as you may recall, my scope is from a yard sale. But to me, it looks like the traces are showing some variation in frequency and amplitude.
     
  6. MrChips

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    Nah. Your scope has to pick a point on the wave to trigger.
    If it chooses a different point each time you will get trace jitter.
    So any bit of noise in the signal will cause this to happen. This is normal.
    This is not distortion.
     
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  7. #12

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    I entirely agree with MrChips. Your scope trigger is jittering on both traces in post #4. Nothing wrong with the signal generator. If you have some strange sync settings like, "TV" try them. You might get a better lock if it tries to sync on a different part of the wave.

    MrChips: I'm seeing a phase shift. It's betrayed by the black spaces inside the fuzz.
    If it was noise or bad focus, those black spaces wouldn't be there.
     
  8. MrChips

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    I think there is 60Hz noise added to that signal.
    How is the GND on the scope connected?
     
  9. #12

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    Yes. There is tiny variation in the gaps between the upper and lower trace. That shows that only one input has the poison. Likely picking up some hum in the preamp (but not very much).
     
  10. tracecom

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    The circuit is built on a solderless breadboard, and it's not too bad; it's powered from a single 9V battery. The sine wave generator is a separate circuit built on a perfboard and powered by two 9V batteries. Then, there are clip leads to common the grounds, plus the ground clips on the scope probes. So, yeah, there are lots of antennas out there for picking up 60 Hz.
     
  11. tracecom

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    I laid out a PCB today for a stereo version of the preamp, and ordered boards.
     
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