Why am I getting cut off?

Discussion in 'General Electronics Chat' started by KCHARROIS, Aug 31, 2012.

  1. KCHARROIS

    Thread Starter Member

    Jun 29, 2012
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    Hello everyone I have a circuit that works great until i get to my final stage which is a class AB and I seem to be getting cut off, images attached show this... Why am I getting cut off it doesnt make any sense everything seems to be in order?

    Thanks
     
  2. #12

    Expert

    Nov 30, 2010
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    Could it be that R13 and R14 need to be connected to the -25 volt rail?
     
  3. KCHARROIS

    Thread Starter Member

    Jun 29, 2012
    292
    1
    But if I remove the class AB stage the second stage works fine... Whats up with that?
     
  4. ifixit

    Distinguished Member

    Nov 20, 2008
    638
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    Yes indeed it could.

    Also, C1 polarity should be the other way around when building a real circuit.
     
  5. KCHARROIS

    Thread Starter Member

    Jun 29, 2012
    292
    1
    #12You were right connecting to the -25V worked but why?
     
  6. KCHARROIS

    Thread Starter Member

    Jun 29, 2012
    292
    1
    Without the -25V at R13 and R14 my quiesent value was at half of my positive supply rail and my peak to peak voltage was under 10 V shouldnt it have worked anyways?
     
  7. #12

    Expert

    Nov 30, 2010
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    Theoretically, your first setup should work within the limitations of only having a drive voltage from zero to +25 volts, or, an output limitation of about +/-10 volts. I have to guess the simulator has a problem. There is no DC coupling to the output stage and it is self centering but the simulator doesn't seem to "get" that.

    Try disconnecting the drive transistor and simulate a signal generator at the left side of the capacitors and see what the simulator does.

    You need to be able to tell when the computer program is lying to you.
     
  8. KCHARROIS

    Thread Starter Member

    Jun 29, 2012
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    I think it should work aswell do you trust multisim?
     
  9. #12

    Expert

    Nov 30, 2010
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    I don't Sim.

    The first circuit I tried on a simulator was a jfet preamp with an opamp driving a constant current sink, and the simulator couldn't do it. No output. Does not compute.

    WTF good is a simulator that can't do a single stage audio amplifier?

    Oops, spelled "what" incorrectly.
     
    Last edited: Aug 31, 2012
  10. KCHARROIS

    Thread Starter Member

    Jun 29, 2012
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    Hahaha maybe I should trust in my calculations a little more did that circuit actualy work?
     
  11. #12

    Expert

    Nov 30, 2010
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    The self centering, single stage, jfet preamp is one of my best achievements. It's the only circuit I've ever seen that could accept any jfet with 5 to 15 milliamps of idle current (2n4416) and center the ouput exactly between the rails while you plug in any replacement jfet with that part number. The goal is to have a high impedance preamp, on the end of a long cord, and not have to sort jfets or adjust anything when you change the cord with amplifier attached to the far end of it.
     
  12. atferrari

    AAC Fanatic!

    Jan 6, 2004
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    Check what is the output at the emitter of Q4. That should tell you what is going on.

    Probably much faster than posting here.

    Please tell what you found out.
     
  13. #12

    Expert

    Nov 30, 2010
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    Wow, you must have a time lag in Argentina! The problem is fixed and we had time to make derisive remarks about the simulator program that failed to give the correct answers.
     
  14. KCHARROIS

    Thread Starter Member

    Jun 29, 2012
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    It definitly was the program I tested all stages and they work fine.
    Thanks for all of the replies #12
     
  15. Audioguru

    New Member

    Dec 20, 2007
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    I think audio amplifiers were capacitor-coupled like that about 55 years ago.
    Then and ever since they became DC-coupled with plenty of negative feedback.

    The amplifier shown has NO negative feedback so its distortion must sound awful.
    The differential input stage is perfect for negative feedback since its grounded and wasted input can be the negative feedback input.

    Frequently in these forums Multisim doesn't know anything about electronics.
     
  16. #12

    Expert

    Nov 30, 2010
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    It also doesn't need 2 capacitors to couple the output stage to the driver, but he will address these things as his skill improves.
     
  17. Audioguru

    New Member

    Dec 20, 2007
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    That horrible old schematic has been posted on the internet for many years originating from India I think.
     
  18. #12

    Expert

    Nov 30, 2010
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    It's obviously student level, each stage being able to stand alone. The result that there is no feedback loop to clean up the distortion is a symptom of the original purpose...to teach amplifier stages, one at a time. It makes me think he's a student. At least he's doing his own work, which is better than a lot of students that come here!

    I approve.
     
  19. Audioguru

    New Member

    Dec 20, 2007
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    He simply copied one of the very simple old circuits.
    No amplifier is made like that today.
     
  20. #12

    Expert

    Nov 30, 2010
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    While we're hijacking this thread, can I ask please, does it really need 2 capacitors to couple the output stage?
     
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