Small Audio Amplifier

Discussion in 'The Projects Forum' started by ljplum12, Apr 26, 2007.

  1. ljplum12

    Thread Starter New Member

    Apr 26, 2007
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    0
    Hey guys, I'm new to the forums, and I just thought I'd say hi before I started in with my questions.

    I'm looking to build a small audio amplifier, something that will amplify a signal straight out of a guitar to play through a 3 watt or so speaker. What I want to do is be able to hook it up to a 9V DC power supply, and I can't seem to find a schematic that will allow me to do so, they all need a positive and negative voltage. The one I was looking at primarily was this:
    http://www.allaboutcircuits.com/vol_6/chpt_6/10.html

    Can anyone help me with the biasing so I don't have to use a negative voltage supply, and so I can run it off of 9 volts, or can someone direct me to a schematic that will do what I want? Thanks in advance.

    Lloyd
     
  2. Gadget

    Distinguished Member

    Jan 10, 2006
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    0
    Something like a couple of LM2002 chips (or a dual amp chip etc etc) in Bridge mode would be quick an easy way to get reasonable power from 9 volts with a single rail supply.
    Many circuits used in car audio would probably also do..(making allowences for the reduced supply). (google car audio amps).
    A "non" bridged output would only give you about 1 watt for an 8 ohm speaker or double that for a 4 ohm speaker when using 9 volts. Using a bridge config would double the output voltage swing effectively quadrupling the potentual output power...(i.e. 4 watts into 8 ohm, or 8 watts into 4 ohm) and make the use of large output decoupling capacitors unnecessary.
     
  3. ljplum12

    Thread Starter New Member

    Apr 26, 2007
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    I've got a 072BD which is a dual op-amp chip. I'm not quite sure what you mean by bridge mode. Could you elaborate on that a little? Thanks for the help.
     
  4. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    An op amp will not put out enough current to drive a speaker. You need an audio amp chip, or chips in the case of bridge mode.
    Bridge mode is where both terminals of the speaker are driven, one input being inverted with respect to the other.
     
  5. ljplum12

    Thread Starter New Member

    Apr 26, 2007
    3
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    I was hoping to stay away from an audio amp chip and just build it discretely. Any advice on that? Thanks for the replies.
     
  6. gootee

    Senior Member

    Apr 24, 2007
    447
    50
    ljplum12,

    The single 9V supply might be a bit of a pain. What sort of supply is it? If you're talking about a 9v battery, you might want to try to calculate how long it might last, at your intended output levels. And if it IS a battery, you might want to consider using two of them, to get a split supply, to make life easier and expand your range of options. If it doesn't have to be portable (or maybe also as an option), think about "wall wart" supplies, which are plentiful and cheap, and cheaper than batteries, by far, in the long run.

    As mentioned, you could search the web for some car audio amplifier circuit information, since they are often designed for a single relatively-low supply voltage.

    You "might want to" (i.e. "definitely should") go to some IC manufacturer's websites and look at their listings of "Audio" amplifier ICs. There might be a chipamp IC available that would fit your needs and make the job very easy. Make sure you download the datasheets and any related application notes, for any likely prospects.

    Just as an example, go to http://www.national.com and find the LM380 datasheet. That might not be the best candidate for your intended application, since it's only spec'd for a supply down to 8v. But it has quite a few example schematics, including bridged amps.

    If you can use a higher-voltage supply, and especially if you can have two of them, you might want to check out some of National's other chipamps, such as the LM1875, or even the LM675. There are many, many LM1875 amplifier circuits, on the web. But make sure you get the datasheets and application notes from national.com, too.

    You might also want to look around on the TI website, at http://www.ti.com .

    If you're determined to do it with an opamp with a push-pull transistor booster, that should work, too. I've had good luck using BD139/BD140 transistors, for those. And your TL072 opamp will probably be just fine, for that. (However, I've only designed them for for +/- 18v supplies.)

    But everything would probably be easier and better if you had more voltage, and/or a dual-voltage power supply.

    Good luck!

    - Tom Gootee

    http://www.fullnet.com/~tomg/index.html

    -
     
  7. gootee

    Senior Member

    Apr 24, 2007
    447
    50
    In the example "bridged" amps circuit topologies you find, if they're done with chipamps you can visualize your opamp+booster circuit as a chipamp.
     
  8. mtcw

    New Member

    Apr 26, 2007
    6
    0
    http://www.diyaudio.com

    Go there and you will find dozens of designs and hundreds of people to answer questions about them.

    Good luck!
     
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