Help with making circuit

Discussion in 'General Electronics Chat' started by w a n, Feb 9, 2010.

  1. w a n

    Thread Starter New Member

    Aug 25, 2009
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    As a little side project, I decided to make an audio amplifier for an iPod or similar mp3 player out of the LM386 op amp. Looking at the datasheet, I found that they already have designed a circuit that amplifies a signal by 20 (top left corner on page 5 of the pdf). Well, it seems detailed enough, but I just wanted to make sure my plans are sound so I don't somehow short circuit something and waste my precious, precious money :D.

    Anyway, this is my plan: I'm going to hook up a 9v battery to pins 6 and 4 (Vs and ground, respectively). Then, I'll connect the the input from a 3.5mm headphone jack (probably connected to an iPod), to pin 3 (- input), and then back to the ground of the headphone jack. By the diagram in the datasheet, it looks like I'm supposed to connect pin 2 (+ input) to the ground on the 9v battery. I'll leave pins 1, 7, and 8 open (gain, bypass, and gain, respectively). Finally, I'm not quite sure what I'll connect to pin 5 (output). I searched around for speakers and came up with these 8 ohm, 2 watt (4 watt max.) from digi key. In the diagram in the datasheet, it says I'm supposed to connect a 250μF electrolytic capacitor (low pass filter?) between the output pin and the speaker, and a 0.05μF (ceramic?) capacitor between the aforementioned electrolytic capacitor and the output pin, leading to a 10Ω resistor and the ground of the 9v battery. Well, I couldn't exactly find any 250μF electrolytic capacitors (maybe I wasn't looking in the right places), so would it be okay if I used this 220μF capacitor from Radio Shack? If not, could anyone suggest an alternative capacitor?

    Other than that, I mainly just wanted to know if my above plans were solid. If anyone sees a flaw in them, PLEASE tell me before I waste time and money and become extremely frustrated like the last time I tried a "fun" project like this.
     
  2. flat5

    Active Member

    Nov 13, 2008
    403
    17
    Use a 470uf.
    Use another 470uf or a 1000uf across the power supply.
    You should show us a full schematic of your circuit. That way there will be less chance of a misunderstanding and you will be less likely to destroy the IC or the Ipod.
    Don't expect a lot of clean sound from a 386 at 8-9 volts. It may be good enough for a quiet hotel room.
     
  3. Audioguru

    New Member

    Dec 20, 2007
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    You don't have a volume control but it can be fed from the headphones output of the ipod that are controled by its volume control.

    Your cheap speakers will produce no bass, low midrange and have a peak at higher frequencies. They will sound awful.
    The datasheet confuses the sound level from them so it might be very quiet.
     
  4. w a n

    Thread Starter New Member

    Aug 25, 2009
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    @flat5: here's the schematic of the circuit from the datasheet:

    [​IMG]

    also, if the lm386 isn't a good amplifier, do you know of any better ones? I couldn't find anything else on National's website for my purpose. Maybe another manufacturer. For the record, I found this thread on head-fi that says that the output voltage of an iPod at max volume is 1.110v. Any suggestions on some other type of amplifier? preferably 8-pin DIP?

    @Audioguru: Would just a potentiometer work as a volume control? Also, I noticed that problem with the speakers as well, but I couldn't find anything better. Any suggestions on speakers?
     
  5. Audioguru

    New Member

    Dec 20, 2007
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  6. w a n

    Thread Starter New Member

    Aug 25, 2009
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    Well, I'm looking for just the speakers, not an entire speaker system. Of course I could just buy a speaker system and take the speakers out of it, but I'd prefer not to spend any extra money.
     
  7. w a n

    Thread Starter New Member

    Aug 25, 2009
    8
    0
    Nevermind. I found this old portable speaker system lying around my house but the batteries in it had leaked into the inside and destroyed the pcb. I was able to salvage the speakers though. They are 4Ω, 2W speakers and I'm guessing they can hit some pretty low frequencies since they were once used to play music.

    So. Back to the diabolical plan.

    Just so that I'm clear, I should put a 470μF instead of the 250μF electrolytic capacitor (see diagram above) and another 470μF capacitor instead of the 0.05μF capacitor? That doesn't seem right. I know capacitors are used as filters to get rid of high or low frequencies, but what exactly is the difference between 250μF and a 470μF? I know it can hold more charge, but in the application of a filter, does it allow higher frequnecies of something? Also, 0.05 is considerably smaller than 250. Wouldn't I want one capacitor to be much smaller than the other to keep the same proportion? Then again, I don't really know much about capacitors.

    Other than that, I wouldn keep the circuit as it is in the diagram. Would it be able to drive two of the speakers I mentioned above in series? parallel? For the record, I have four such speakers, but I probably won't use them all on this.
     
  8. Mike33

    AAC Fanatic!

    Feb 4, 2005
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    As you increase the capacitance, it will allow more bass through - I think that's what he's getting at. I don't think you'll get a whole lot of volume out of the thing, but hey, try it out! Experimenting is part of the fun. You can run 2 of the 4 ohm speakers in series, for a total of 8 ohms. 386's are pretty forgiving, anyway.
    If you don't get the volume you want, try the TDA2822M mentioned above.
     
  9. Audioguru

    New Member

    Dec 20, 2007
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    The LM386 is designed to drive 8 ohms. With a 4 ohm load its output power is slightly more but it makes a lot more heat. Connect your two 4 ohm speakers in series to make an 8 ohm load. Then the output capacitor can be half the value calculated below for a 4 ohm speaker. The max output will be at half power at 43Hz using a 470uF output capacitor feeding 8 ohms.

    The output of the LM386 must have the 0.05uF (or 0.047uF) capacitor in series with a 10 ohms resistor from the output to ground as shown on every circuit in its datasheet.

    If you use a 250uf output capacitor feeding a 4 ohm speaker then a simple calculation shows that the max output at 160Hz will be at half power and lower frequencies will be less. No bass.
    If you use a 470uF ouput capacitor instead then the max output at 85Hz will be at half power.
    If you use a 1000uF output capacitor then the max output at 40Hz will be at half power which will sound better if the speakers are good enough.

    The capacitor across the power supply voltage can be 100uF to 1000uF.
     
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