amp/pwr supply work fine seperate, but not together

Discussion in 'The Projects Forum' started by zionarbadon, Jun 17, 2011.

  1. zionarbadon

    Thread Starter Member

    Jun 10, 2011
    52
    0
    Hello,
    I have been working on a 2 stage amplifier for a project. I have set up the amplifier using a +15V and a -15V battery in multisim. I have it set up to give me the voltage I need across my load.

    I also have a power supply through a diode bride with a positive and negative rectifier that give me ~+/- 15V.

    However, when i take off the batteries and hook up the power supply, my load voltage drops way down. It reads as still putting out the correct voltage off of the power supply.

    Another thing to note is that with the batteries I get a very nice, non clipped AC waveform. With the power supply I get a very messy waveform that looks like audio waveforms (and since this load is supposed to be a speaker I didn't know if that was right, but I would imagine it isn't).



    -Brandon
     
    Last edited: Jun 19, 2011
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Without the schematic of your power supply AND the amplifier, about the only thing we can do is make wild guesses that go on for many replies, yet does nothing to help you solve your problem.

    So, to keep the thread relatively short and productive, please post schematics of both your amplifier AND your power supply.
     
  3. zionarbadon

    Thread Starter Member

    Jun 10, 2011
    52
    0
    Yeah, I didn't know if there was a common mistake or anything easy like that. I've spent so long trying to fix this xD
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    What is the current draw from the regulators?

    What does the voltage waveform look like on your bridge rectifier filter caps?

    It will probably take a number of cycles before the regulator output voltage stabilizes; and I don't see an indication of what the timeline is on your plots. battery voltage is available immediately; but when you are dealing with a simulated power supply, it may be many milliseconds before your output is stable.
     
  5. electronis whiz

    Well-Known Member

    Jul 29, 2010
    519
    27
    it could be that the transformers magnetic field is afaecting it.
    this could be because of induction into the amplifier circuit if there close together.
    frequincys interfering going from the transformer to the amp through the wires this could be eleiminated by using a filter cap near the transformer or wraping the wires around a toroid then to the amplifer.
    the filter caps needing to charge up when it gets turned on
     
  6. zionarbadon

    Thread Starter Member

    Jun 10, 2011
    52
    0
    Hmm, looks like the cap on the neg side isn't really doing anything. . .

    Not sure why though.
     
    • hmm.jpg
      hmm.jpg
      File size:
      209.3 KB
      Views:
      26
  7. zionarbadon

    Thread Starter Member

    Jun 10, 2011
    52
    0
    additionally i noted the cap charge time, and i have been letting it charge up.
     
  8. studiot

    AAC Fanatic!

    Nov 9, 2007
    5,005
    513
    With the figures shown I make the conditions at Q2 as follows:

    Base divider across 30 volts drops 24 across 4k and 6 across 1k making base at -9 volts.

    This sets emitter at -9.7 volts

    Thus emitter current = (15 - 9.7)/2 amps

    Ouch! :eek:

    One very dead 2n3904.

    just a question as I don't know Multisim, but you have both rails drawn the same polarity, but labelled one -15 and the other +15?
     
  9. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    A 2N3904 has a max allowed current of only 200mA so it cannot drive an 8 ohm speaker.
    An audio power amplifier usually uses two complementary power transistors operating in push-pull.

    The input transistors will not work because a resistor is missing that turns off the 2N3904 transistor. The emitter resistor value of only 25 ohms is much too small.
     
  10. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Although the 2N3904 transistor is rated for 200mA collector current, you really should de-rate it by 50%, or 100mA. 100mA is no where near enough for your load requirements.

    The 78xx and 79xx regulators are rated for a maximum of 1A output. They will shut down if the load exceeds 2A. Linear regulators are not such a good idea to use for high-current loads, as they dissipate a lot of power internally.
     
  11. zionarbadon

    Thread Starter Member

    Jun 10, 2011
    52
    0
    I changed it to 15 and rotated it 180 and had the same results.

    Anyhow, I must be messin up my calculations big time cause that's not what I was getting, maybe cause I thought I had my beta around 100 (is there a way to check this in multisim?) Anyhow:

    Rinbase = B RE = 100x2 = 200
    200||1k=166.667
    166.667+4k= 4166.667
    30/4166.667=7.2mA
    7.2mA*4000=28.8V
    VB=30-28.8=1.2
    and from here i wasn't sure whether to add or subtract .7 o_O
     
  12. zionarbadon

    Thread Starter Member

    Jun 10, 2011
    52
    0

    So I need to start over? xD

    I'm confused still as to why it works with the multisim batteries.
     
  13. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Multisim is extremely STUPID!
    It doesn't know that the currents in your circuit are much too high and will burn out your little transistors.

    I think you need to learn about an audio power amplifier circuit.
     
  14. zionarbadon

    Thread Starter Member

    Jun 10, 2011
    52
    0
    Thats what's so difficult, I don't know anything about them. I just know basic transistor/amp solving, but the teacher gave us this extremely difficult project and I'm not really sure how to tackle it as we've always been given beta and resistances. I'm just kind of winging it.

    Do you guys have any suggestions? I have litterally been working on this for two weeks and its due monday as my final project. :(

    Any good sources, suggestions, etc would be extremely appreciated.
     
  15. studiot

    AAC Fanatic!

    Nov 9, 2007
    5,005
    513
    Looking at post#3

    R5 in the first schematic is 2 ohms

    R5 in the second is 20 ohms


    I purposely posted the guts of my back of envelope calculations. Beta has nothing to do with the static biasing conditions.

    I am using the first schematic.

    R1 = 4k
    R2 = 1k
    R5 = 2

    Vcc = +/- 15

    The voltage divider chain R1, R2 is set across the rails ie 30 volts.

    This forces the base to take up the quiescent point at their junction ie 24 volts below the +15 rail ie -9 volts.

    This forces the emitter to be 0.7 volts lower ie -9.7 volts.

    -9.7 volts is (15 - 9.7) = 5.3 volts above the -15 volt rail.
    This is therefore the voltage across the emitter resistor, R5.

    Thus the current in R5 is 5.3/2 = 2.65 amps.

    Of course you power supply cannot produce this amount of current so the circuit falls over and perhaps your Q2 is not (yet) toast.


    I have to knock off for a couple of hours now but will keep watch over the weekend. I'm sure other here will help meantime.
     
    zionarbadon likes this.
  16. zionarbadon

    Thread Starter Member

    Jun 10, 2011
    52
    0
    For example, any good parts to use would be useful and a good place to read up on audio amps and specifically the circuits/theory. My book is horrible.
     
  17. zionarbadon

    Thread Starter Member

    Jun 10, 2011
    52
    0
    I didn't know that we were allowed to do that as I understood it, the VB voltage is based off of how I posted previously because thats the only way our teacher ever did it. If you can get VB the way that you listed, that will make life a bit easier. That's the kind of information that I'm missing for this project xD
     
  18. zionarbadon

    Thread Starter Member

    Jun 10, 2011
    52
    0
    Please, anyone able to help, if I fail this class I lose my job. :( I could really use some good sources and parts.
     
  19. zionarbadon

    Thread Starter Member

    Jun 10, 2011
    52
    0
    Those were the regulators specifically suggested by my teacher :(
     
  20. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    If those regulators were suggested by your teacher, then you will need to limit your amplifiers' output current to stay within your power supply limitations.

    The Multisim "batteries" are "ideal batteries" - they will output exactly whatever you tell them to output, at any amount of current just shy of infinity. If you attempt to draw infinite current from a Multisim voltage source (like for instance, drawing a wire across the + and - terminals) the simulation will abort with an error.

    [eta]
    You can learn a lot about audio amplifiers over on Rod Elliot's pages, here:
    http://sound.westhost.com/
    Have a read through his Articles table of contents:
    http://sound.westhost.com/articles.htm
     
Loading...