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Sound filtering circuit - will this work?

Discussion in 'Homework Help' started by 'r-e-s-i-s-t-o-r', Jan 9, 2014.

  1. 'r-e-s-i-s-t-o-r'

    'r-e-s-i-s-t-o-r' New Member

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    Hi!

    Thank you for the responses! MrAl, are you saying the original (post #1) circuit will work, assuming we use your formulas instead? Earlier, you said that it was better for us to divert away from using an inductor, and use just capacitors and resistors instead. Is this still the case? If so, could you explain how to replace it?

    You mentioned that we will need both an op-amp, as well as a power-amp? I have gotten lots of information regarding the op-amp (connecting it like we did in #1 will work, right?), but how should I connect the power-amp to the rest of the circuit? To be completely honest with you, I'm not even sure what a power-amp does. Any clarifications regarding that will be highly appreciated.
     
  2. audioguru

    audioguru Well-Known Member Most Helpful Member

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    A speaker is 8 ohms. It is driven with a power amplifier that provides enough current to drive it. It also amplifies the signal voltage.
    An opamp can drive a load that is 2000 ohms or more because it does not have much output current.
     
  3. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi again,

    Well usually we try to design with resistors and capacitors whenever possible because inductors are a little harder to stock, though you could do it. It's easier to find the falues you want with resistors using a more common capacitor, and you can make many kinds of filters out of just resistors and capacitors although the requirements of the filter have a lot to do with it too. So what do you really need here in terms of filtering. It sounds like you only need a basic filter.
    So the original circuit might work, but you probably want to go with resistors and capacitors alone and maybe an active device like transistor or op amp or both.

    For driving a speaker however another issue comes up as audioguru pointed out. That is the speaker impedance. A speaker normally has low impedance like 4, 8, or 16 ohms although you can also get speakers that have 32 ohms too. The problem with even that though is that the op amp can not drive it very well, so you need a transistor power stage or an IC chip that is made just for driving speakers.

    So an idea might be to use a passive RC filter and a couple op amps and a transistor or two or a audio power amp IC chip.

    I'll see if i can get a schematic together so you can take a look.
     
  4. dave

    Dave New Member

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  5. 'r-e-s-i-s-t-o-r'

    'r-e-s-i-s-t-o-r' New Member

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    Thank you very much for this answer!
    We found out early on that inductors were hard to find in the right values since we tried to calculate what values we needed before even posting here and we had a hard time finding the ones we needed. It would be very kind of you if you made a schematic witch circumvents that problem.

    You were talking about power amps IC chips. Are there any special models we should be looking at or are they all pretty much the same?

    Again, thank you for the very educational answer.
     
  6. audioguru

    audioguru Well-Known Member Most Helpful Member

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    In your post #6 you show an LM386 power amplifier IC but it is connected completely wrong. Its datasheet shows exactly how to simply connect it. It is small and cannot be connected to a heatsink for cooling so its maximum power output is only about 0.7W at room temperature.
    An LM3886 power amplifier IC has a metal tab that is fastened to a heatsink then its output power can be 85W and they can be paralleled and bridged for much more power output.
     
  7. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi again,

    Audioguru's last post reminded me that i must have forgotten to ask you how much output power you need. Can you get away with that 1 watt or a little less? That's usually good enough for many things like headphones and small speakers where you dont need a lot of sound projection like you would in a concert hall.
     
  8. 'r-e-s-i-s-t-o-r'

    'r-e-s-i-s-t-o-r' New Member

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    We haven't decided on what kind of speaker to use but it will not be anything big. Something like an old set of headphones or a tiny speaker. Audioguru's suggestion sounded like a nice and simple solution but judging from your post it sounds like it might be overkill?
     
  9. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi,

    Well you could use the LM386 (after reading the data sheet) or you could use a NPN transistor and op amp for the power amp since you dont need that much power.
    It's up to you.

    The schematic for the LM386 would be shown on the data sheet. The NPN transistor circuit is just a common collector with emitter resistor biased so it doesnt use too much quiescent current. The output is capacitor coupled to the speaker. The op amp(s) provide all the gain. Filter can be passive.
     
  10. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Don't you want to play sounds with your filters then play them without your filters to hear the difference?
    With a cheap little speaker then the difference will be very small because the speaker cannot produce all audio frequencies.
    You should use a high quality hi-fi speaker and a half-decent amplifier driving it to hear the difference.
     
  11. 'r-e-s-i-s-t-o-r'

    'r-e-s-i-s-t-o-r' New Member

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    Are you saying that we don't really need any sort of power amplification other than the LM386 if we do not want to drive our tiny speakers?
     
  12. audioguru

    audioguru Well-Known Member Most Helpful Member

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    The power from a little LM386 amplifier is about the same as the power from a cheap clock radio. The speaker in a cheap clock radio is about 3" in diameter and can fill a room with mid-frequencies. It cannot produce low frequencies and it cannot produce high frequencies something like your filters.

    Your tiny speakers might simply produce only low level squeaking sounds.
     
  13. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hello again,

    Did you mean to use the negative form of the verb there?

    I was saying that if the small speaker is the ONLY speaker you want to drive then you can use an LM386, but you need to read the data sheet for that part again.
     
  14. Bob_b

    Bob_b New Member

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  15. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi again,

    Here is a sample circuit that does the bandpass function. You can try this and see if you like it.
    The response curve is shown to the right of the circuit. You can see the -3db points occur at about 350Hz and 3500Hz.
     

    Attached Files:

  16. 'r-e-s-i-s-t-o-r'

    'r-e-s-i-s-t-o-r' New Member

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    I did not mean to come of negative. I just wrote a quick answer and I didn't think of I i came of but thank you for the answer.
    Thank you for ge example. We will try this out and see how it goes.
     
  17. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi,

    Sure, but note that the frequency points are approximate so the band span may be more like 300 to 4000Hz. If you want more accurate settings we could change the values a little.
     

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