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Analysing Power Amplifier circuits-General discussion

Discussion in 'General Electronics Chat' started by fantabulous68, Feb 27, 2010.

  1. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    It's not a spilt supply amplifier, with a DC coupled output, so a little offset isn't a problem - you don't add an axtra resistor on these designs, there's no need for it. What you do is design the front end potential divider correctly, and not just half and half as it is now, in order to give the maximum possible output voltage before clipping - some designs even include a preset so you can tweak it.
     
  2. fantabulous68

    fantabulous68 Member

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    just clarifying........does the positive terminal of the capacitors all face ground whils the negative terminal of the capacitor faces the negative supply rail?
     
  3. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Mostly. Simply look at the voltages at each wire of each capacitor.
     
  4. dave

    Dave New Member

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  5. fantabulous68

    fantabulous68 Member

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    in the last circuit diagram i posted of the power amp im building,

    THE VOLTAGE SOURCE (V1) TERMINALS are the wrong way around?

    the positive terminal of V1 should go to gnd?
     
  6. fantabulous68

    fantabulous68 Member

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    yeah i will try that 2morrow. Thanks
     
  7. audioguru

    audioguru Well-Known Member Most Helpful Member

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    What?
    Why are you feeding a signal as high as 120VAC to the input of the amplifier?
    It will blow it to kingdom come.
    The input to the amplifier is AC, not DC and is about 0.07V, not 120V.

    The 120VAC connects to the primary of a power transformer. The 36VAC secondary of the power transformer connects to a bridge rectifier then to a big filter capacitor to produce -50VDC.
     
  8. unclejed613

    unclejed613 Well-Known Member

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    V1 is an AC source. it doesn't matter much. if you reverse the terminals, you still get the same waveform, voltage and current, but with a phase inversion, which might matter some in a circuit sim.

    another note about running this in a sim, or troubleshooting with an oscope.... troubleshooting the circuit may prove to be a bit "counter-intuitive", in the sense of what happens in the "top" half of the amp will be seen in the bottom (negative) half of the waveform on an oscilloscope (or sim) and vice versa. it's not a real problem once you get used to seeing it that way. with the sim you can set the scaling reversed if you wish, and some oscopes have an invert function if you wish to use it. so just keep in mind, when you see the waveform "hanging" on the negative peak during clipping, it's the bottom transistor that's saturating.

    audioguru wrote:
    No.
    You want node 14 to be close to the DC output voltage for it to be biased at half the supply voltage. The emitter current for Q7 is in the feedback resistor R10.

    you're right.... it's been about 20 years since i've troubleshot an amp of this type... a Dynaco Stereo 40 if i remember correctly.....
     
  9. fantabulous68

    fantabulous68 Member

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    I am not feeding a signal as high as 120VAC to the input of the amplifier. I said before, ignore details on voltage source 1, but you are forgiven:D you read alot of threads a day & must have forgotten

     
  10. fantabulous68

    fantabulous68 Member

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    GrrrrrrrrrrEaT News!!!!!!!!!!!!!
    i changed the amp a little BUT BUT BUT it worked:D

    Im going to take pics tomorrow and upload it here:)

    I was real excited when it worked lol.

    "KILLED" some transistors along the way because I didn't check if i was given the exact transistor I asked for.
    :)

    I will also upload the circuit i used and worked.

    I learnt alot & had alota fun
     
  11. unclejed613

    unclejed613 Well-Known Member

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    as a friend of mine who is a construction framer always says "measure twice, cut once".... always double-check your parts when building or repairing.
     
  12. mneary

    mneary New Member

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    You have been posting unattributed, inaccurate, and incomplete schematics throughout this thread, expecting us to refer back and forth while reading your mind and trying to distinguish between transcription errors, design errors, and tricks.

    I'm glad that you are quick to forgive when one of your volunteer tutors fails to be perfect.
     
  13. audioguru

    audioguru Well-Known Member Most Helpful Member

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    It is clearly shown on your schematic that is covered with Chicken-Pox dots.
     

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  14. unclejed613

    unclejed613 Well-Known Member

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    you want that input transistor regular or extra crispy?

    must have been a severely clipped output when he simmed it.....:confused:

    i've heard of speakers that can withstand being directly plugged into the wall outlet, but never an input stage to an amp....:eek:
     
  15. fantabulous68

    fantabulous68 Member

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    What is also shown is what i said previously..............IGNORE DETAILS OF VOLTAGE SOURCE V1.
     
    Last edited: Mar 25, 2010
  16. fantabulous68

    fantabulous68 Member

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    Results

    The circuit was now tested with the load and had the following settings:
    Frequency =1.5 KHz.
    Input signal was set at 50mV/DIV for pic 1 ---->100mVp-p
    Output signal was set at 1V/DIV for pic 1---->3Vp-p
    The input signal is the top waveform. The output signal is the lower waveform.

    The signal at the output had an amplitude of 3Vp-p. The gain was calculated to be 30V/V while the predicted value was 30V/V.

    --------------------------------------------------------------------------------------------------------------------
    The input signal was then varied from 10 kHz to 20 kHz. The amplitude of the output waveform decreased. The gain also decreased as the frequency was varied from 10Hz to 20 KHz. Hardly any distortion was seen.

    At 10 KHz the following output was observed on PIC2:
    At 10 KHz the amplitude of the output waveform was 2Vpk-pk. The gain was 20V/V.


    At 20 KHz the following output was observed on PIC 3:
    At 20 KHz the amplitude of the output waveform was 1.2Vpk-pk. The gain was 12V/V.

    More results to come(class B crossover distortion etc:D)

    Please feel free to comment on the oscilloscope results :).
     

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  17. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    I would do if you'd posted them! :D
     
  18. fantabulous68

    fantabulous68 Member

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    grrrrrrrrrrrrr.......its the net, it was taking long to attach the pics- i added numbers to the same pics
     
    Last edited: Mar 25, 2010
  19. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    2 & 3 are rather distorted - if you can see it on a scope it's really bad.
     
  20. fantabulous68

    fantabulous68 Member

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    PIC 4

    To observe a class B output on the oscilloscope, the Vbe multiplier transistor was shorted out between its collector and emitter. The circuit was now tested without a Vbe Multiplier and the oscilloscope had the same settings as before.

    Crossover distortion was viewed on a scope in the form of a ‘kink’ in the output curve as seen above. This is a characteristic of class B Mode.
    When the circuit was tested with a Vbe multiplier and the potentiometer was adjusted, the output curve smoothed. By adjusting the potentiometer in the VBE multiplier, the base bias voltage VBB is changed. This changes the quiescent current that flows. By adjusting the quiescent current, the class of the amplifier changed. β is not exactly known, so there is no way to choose specific resistors to bias the transistors for a certain class. Therefore a potentiometer was used.

    Any clue why the input signal is distorted? Does it have to do with feedback and slew rate?

    PIC 5

    The next test was to measure the total harmonic distortion (THD) of the amplifier. Total harmonic distortion is measured in percentage terms and is a representation of the distortion of an audio signal at full output level. A clean sine was required to be inputted into the circuit. A function generator will not be able to generate a clean sine wave therefore a spectrum analyzer used. The input sine wave was set to a fundamental frequency of 1.5 kHz & 25mVpk. Thereafter harmonic markers were placed on the signal. The THD function that the spectrum analyzer had was implemented. The THD measured was 0% as seen in the figure below. This was an acceptable value since no distortion was visible on the output waveform. THD should be around 0.01-0.1%.
     

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  21. fantabulous68

    fantabulous68 Member

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    Is it distorted more at higher(20KHz) or lower frequencies(10KHz)?
    I would say 20KHz since the gain lowered, right?
    I should put my glasses on
     
    Last edited: Mar 25, 2010

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