Circuit help

Discussion in 'General Electronics Chat' started by wolf08, Dec 14, 2009.

  1. wolf08

    Thread Starter New Member

    Dec 14, 2009
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    Hi Im new to circuits and there really interesting. i have been trying to make an amp using an LM386N ic. There is a web site on how to make an Amplifier with Gain = 200 (page 5) " http://web.mit.edu/6.115/www/datasheets/LM386.pdf ". I have some questions on what some of the stuff is in the circuit ( the symbols). The first is,
    1.Vcc means pos power in right?
    2.what does Vs mean and where does my input go from my ipod or radio.
    3.does it matter what lead i use from the audio jack if its positive or negitive.
    4.What does it mean when it shows vs connected to a 10k resistor then to ground with an arrow pointing to the middle of it?

    Please help! im really frustrated i have already fried my ipod!:confused::mad:
     
    Last edited: Dec 14, 2009
  2. VoodooMojo

    Active Member

    Nov 28, 2009
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    Vcc -- For TTL circuitry, this is the + volts which sits on the collector of the bi-polar transistor output circuit.
    Vs -- Supply Voltage. on the LM386N1 it is 4 volts min, 12 volts max
    LM386N4 it is 5 volt min, 18 volt max
    as the datasheet states. Pin 6 in either case.

    That is Vin with the resistor w/arrow to ground. Not VS. This is a 10K potentiometer. Volume control.
    Vin is where the imput comes in from the audio source.
     
  3. Audioguru

    New Member

    Dec 20, 2007
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    You want to make the amplifier with a gain of only 20 for a radio or ipod input. The gain of 200 is for sources that have a very low output level like a mcrophone.
     
  4. wolf08

    Thread Starter New Member

    Dec 14, 2009
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    where does my other input go from my ipod?
    my pos input gos to pin three correct.
     
  5. boriz

    New Member

    Jul 16, 2009
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    Look at the top of this webpage. Most of the information you seek is provided there.

    Vcc used to mean ‘voltage to the collector’ in 5v TTL circuits, but is commonly misused to mean ‘positive voltage source’. Equivalent to +VE on the battery.

    Vs means roughly the same thing.

    Audio signals vary above and below ground. They are AC and are therefore unpolarised by definition. However, disregarding the polarity can be a big mistake in some circuits. EG: A stereo audio signal has two signals, Left channel and Right channel. If you accidentally get the polarity reversed on one of them, any signals that are the same on both channels will cancel each other out. This usually manifests itself as a tinny sound with indistinct vocals. So with stereo systems, always play close attention to polarity, right through the system to the speaker terminals. For a simple mono amp, polarity isn’t important, but it’s always good practice to keep the same polarity throughout.

    A normal ‘audio out’ signal will peak at around 750mV. If you amplify that 200 times, you get 150volts! That’s obviously far higher than your Vs, so you’ll find that much of your signal is clipped causing a huge amount of distortion. I advise you to use the basic 20 gain circuit.

    The resistor with an arrow is a potentiometer. In this circuit it splits the incoming audio signal into two parts, part above the arrow, part below. Only the part below is sent to the amplifier and by varying the position of the arrow, (turning the potentiometer), you make this lower part of the input signal larger or smaller. It’s the volume control.
     
  6. boriz

    New Member

    Jul 16, 2009
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    EEK! Sorry, crosspost.
     
  7. wolf08

    Thread Starter New Member

    Dec 14, 2009
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    I don't no what amp to make i thought an iPod gave a week signal and a lot of gain was good for that what amplifier should i make for an iPod
     
  8. wolf08

    Thread Starter New Member

    Dec 14, 2009
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    I made the Amplifier with Gain = 20 and its really distorted.
    What does that mean?
     
  9. VoodooMojo

    Active Member

    Nov 28, 2009
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    think of it logically.
    The ipod has enough output to drive headphones, that is a sizable output.
    hook a microphone to your headphones and it is unlikely that you will hear anything through the headphones. that would be a weak signal.

    the 20 gain is what you want.

    "positive" from the ipod goes to the lug on the 10K potentiometer opposite the lug that goes to ground.
    The wiper from the potentiometer goes to pin 3
     
  10. Audioguru

    New Member

    Dec 20, 2007
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    If the sound from the speaker is distorted then turn down the volume control until it is not distorted. The LM386 has a low output power (0.45W at the beginning of distortion) so its max output volume is not very loud.
     
  11. wolf08

    Thread Starter New Member

    Dec 14, 2009
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    i think i have my circuit put together wrong because i have in on low volume and its still super distorted....
     
  12. wolf08

    Thread Starter New Member

    Dec 14, 2009
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    I gave up...... I tried everything and still got tons of distortion im really frustrated. im never going to figure this out.... AHH
     
  13. VoodooMojo

    Active Member

    Nov 28, 2009
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    remember the scene in African Queen where Charlie Allnut is dragging the Queen through all the reeds and rushes and being devoured by skeeters and full of fever and finally gives it up for dead?
    remember that?
    and then the camera pulls back and you see the big open waters just inches from where he finally gave in?

    yeah, it's like that!
     
  14. steinar96

    Active Member

    Apr 18, 2009
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    hmm, if its distorted it propably means your circuit isnt right. The sheet says the amplifier is supposed to be very little distorsion.
     
  15. boriz

    New Member

    Jul 16, 2009
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    Slightly confused by the diagram. The datasheet says that the input is ‘ground referenced’. With a single sided supply, wouldn’t that mean that only the positive signal excursions will be amplified, the negative just appearing as 100% clipping? Shouldn’t the ‘ground’ end of the input pot and the –ve input go to a virtual (50% Vs) ground?
     
  16. Audioguru

    New Member

    Dec 20, 2007
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    No.
    The input of the LM386 is at 0V DC and goes positive and negative with the AC signal. Its inputs are designed to work at 0V and its internal biasing allows the output to be at half the supply voltage. It has built-in negative feedback for a voltage gain of 20 or 200. It is not an opamp.
     
  17. VoodooMojo

    Active Member

    Nov 28, 2009
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    curiosity, what are you using for a speaker ?
     
  18. Audioguru

    New Member

    Dec 20, 2007
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    I assume that the circuit is built with a mess of long wires all over the place on a breadboard without a supply bypass capacitor. Then the LM386 is oscillating at a low radio frequency.

    I would normally ask for a photo of the circuit but the mess of wires won't show what connects to what.
     
  19. wolf08

    Thread Starter New Member

    Dec 14, 2009
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    Im using a small 8 ohm .2 watt speaker
    I use an a sorted jumper wires so its clean.
    I figured it out at first i was running the chip at max which will cause distortion, then i was using what i thought was a volume knob and that didn't work so i thought my circuit was wrong. I got a volume knob and im going to rebuild the circuit and i will post how it goes maybe with a pic ....... thank you
     
  20. wolf08

    Thread Starter New Member

    Dec 14, 2009
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    Every time there is a lot of bass it cuts out how do i fix this?
    How do you post pics here?
     
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