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Op Amp HiFi Project

Discussion in 'General Electronics Discussion' started by lukesmith123, Jun 2, 2013.

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  1. lukesmith123

    lukesmith123

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    Jan 4, 2013
    I have built a nice little iPod dock HiFi with a small speaker cone placed inside a vase and connects to the headphone plug of an mp3 player.

    I wanted to improve on this by adding an Op Amp that could be powered by the iPhone. I read that the iPhone can output 3.3v, 160mA or more from the dock connector.

    My speaker is 4ohm, 5w

    Will an OpAmp like the LMV358 be suitable for this? For example: http://www.skpang.co.uk/catalog/opamp-breakout-p-641.html

    What will the audio quality and volume be like with something like that. Is there any thing else I could get that I could power with 3.3v, 160mA and get a better quality / louder sound from?

    Thanks in advance!
     
  2. BobK

    BobK

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    Jan 5, 2010
    You cannot use an op amp to drive a 4 Ohm speaker. You want an audio amplifier IC instead. And you won't get much power from 3.3V at 160ma. That is a little more than 1/2 W and an amplifier (unless it is class D) will be about 50% efficient, so about 1/4 W is all you can hope for.

    Bob
     
  3. lukesmith123

    lukesmith123

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    Jan 4, 2013
    Perhaps it would be a better idea then to add a power adapter to power the unit and only take the audio signal from the iPod dock.

    If I have power from the wall plug now, what kind of audio amp should I look for? 12v? what wattage?
     
  4. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    Yes, 12V is a reasonable supply voltage for a small audio amplifier. Since your speaker is rated for 5W, aim for an amplifier with a similar rating. A good way to get a good idea of the available components is to search on Digikey:
    http://www.digikey.com/product-search/en/integrated-circuits-ics/linear-amplifiers-audio
    Narrow down your selections one filter at a time. For example you probably want a through-hole device, not SMT, and you need a power supply range that includes 12V (plus a safety margin).
    Class AB or Class B are normal for small audio amplifiers like this. Class D has a higher efficiency but requires more external components and a more critical layout.
     
  5. poor mystic

    poor mystic

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    Apr 8, 2011
    You'll need at least 10W rated capacity in the wall wart.:)



    edited: ... and that doesn't mean 20W wouldn't be better.
     
    Last edited: Jun 3, 2013
  6. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    Right. An adapter rated at 12V DC and 1A or more will be fine.
     
  7. lukesmith123

    lukesmith123

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    Jan 4, 2013
    Ok thanks!
     
  8. BobK

    BobK

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    Jan 5, 2010
    You might be just as well off to buy an completed audio amp module from Ebay. They can be quite inexpensive.

    Bob
     
  9. lukesmith123

    lukesmith123

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    Jan 4, 2013
    Yeah I was thinking that too. Do you have any advice on selecting one?

    Also, the speaker cone I am using is a really old one from an old HiFi. Would it be possible to get more volume by buying a new speaker?

    The main function of this system is to be used without an amp. The speaker connects to the line out signal of the mp3 player like headphones and the amplification is passive. The amp will be there just so that it can get some more power into it optionally.

    So if I buy a new speaker cone to use what spec would be suitable for my needs?
     
  10. BobK

    BobK

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    Jan 5, 2010
    Not really, as I haven't tried any of them.

    Really old as in paper cone? Those speakers do not age well. You would be better off to buy a new one.

    You will not get much volume out of an MP3 player without using an amp.

    Bob
     
  11. lukesmith123

    lukesmith123

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    Jan 4, 2013
    Yeah paper one and I worked out it must be over 10 years old.

    The volume I'm getting is not bad as the vase the speaker is sitting in is amplifying it but I just want a little more volume.

    Should I get a 8ohm speaker rather than 4? What difference does it make?

    Can I get a speaker with a much higher wattage like 100w - 200w? will I be able to use it without an amp like I have been with the 5w speaker?


    Thanks so much for your advice.
     
    Last edited: Jun 3, 2013
  12. BobK

    BobK

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    Jan 5, 2010
    A speaker rated at a higher wattage is likely to be less loud. To drive it without an amp, you would want the most efficient speaker you could get.

    Bob
     
  13. lukesmith123

    lukesmith123

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    Jan 4, 2013
    Ah I suspected as much.

    How would the size of the speaker effect this?

    For example would a lower watt speaker that was only 5cm in diameter (vs 10cm that I am currently using) be louder in this setup or am I better off sticking with what I have?
     
  14. poor mystic

    poor mystic

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    Apr 8, 2011
    Hi :)
    Speakers are rated according to how much power they can handle. A 10W speaker won't be damaged by 10W signals, but a 5W speaker probably will be damaged.
    Because a higher-power-rated speaker is usually larger, it's common to find that it will also produce better bass. Any larger speaker might produce a better bass response.
     
    Last edited: Jun 5, 2013
  15. lukesmith123

    lukesmith123

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    Jan 4, 2013
    Ah ok thanks. I guess the speaker I'm currently using is a pretty good fit then.
     
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