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Amplifier for unknown loudspeakers?

Discussion in 'General Electronics Discussion' started by TomOlsson, Mar 12, 2013.

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  1. TomOlsson

    TomOlsson

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    Mar 12, 2013
    Hello! I'm not sure where this should go, projects or homework or whatever else could fit,so I put it here.

    I sort-of inherited a pair of old stereo-speakers originally belonging to a Denver MCA-50 unit from the late 90's (I believe) from my parents. Each unit has three drivers, with one being a "dedicated" tweeter (40mm dia, ish. ~1cm deep), as well as one 'sub' (80mm, ~1,5cm deep) and one mini-drive (20mm, ~0,5cm deep). Despite their age they all seem to work when given an adequate voltage.

    However, there's a catch. I want to hook these into my external soundcard/mixer, which only has professional-level lineouts (+4dB) that require a high impedance load, in the area of 10-100k ohm.

    I've included a sketch of the loudspeaker setup below. This is the info I have about the components
    • L1: 80mm, marked as dynamic 4 ohm, 3-5W
    • L2: 20mm, no marking
    • L3: 40mm, no marks.
    • C: 47uF, Umax=16V

    I believe that these are intended to operate at 2W (all 5 models that came after ours had 2W outputs -- ours isn't marked) However, the L1 driver is marked as 3-5W. I've never worked with drivers before, so I've got no clue how to approach that. If anyone knows anything about these markings I'd be in eternal debt.

    From this info (with a more certain wattage for the loudspeaker), is there any way to correctly estimate (incorrectly guess?) the 'black box' characteristics such that I can gain the output from the mixer to a good level?

    My though on the amp is that I use two linked inverters, one to load the lineout properly, and then one to invert again and drive the speakers. I've done a sketch of what this might look like (EC2) -- the gain for the second amp is merely a guess, and probably wildly above what is needed. Would this be a working solution? Anything I should think of?
     

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  2. BobK

    BobK

    7,682
    1,688
    Jan 5, 2010
    Any 2W audio amp would do.

    Bob
     
  3. TomOlsson

    TomOlsson

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    Mar 12, 2013
    I didn't know that there were IC's dedicated to sound amplification -- I've looked upon those now, and I'm a bit confused. The input voltage for the amp's I've learned to use at school are a +- x V, whereas these seem to expect a -0.3 to +- to VDD +0.3 volt range.

    My input has 1.228 Vrms , so in order to fit this inside the operating range, I need at least Vdd equal to 1,738 (Vpk) *2, and a DC offset of 1,738. This means that when Vac = 0 the Vtotal = 1,738. I might also need to increase my Vdd upto the needed output voltage.

    Am I understanding this correctly?
     
  4. BobK

    BobK

    7,682
    1,688
    Jan 5, 2010
    No. Those are the absolute max inputs to avoid damaging the device. Let us know what device you are looking at. A link to the datasheet would be good.

    The supply voltage generally depends on the output power desired, not the inputs.

    Bob
     
  5. TomOlsson

    TomOlsson

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    Mar 12, 2013
  6. BobK

    BobK

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    1,688
    Jan 5, 2010
    You can get complete boards for less than that. Try Ebay. It does not have to be exactly 2W. A higher wattage will not hurt anything as long as you do not turn it up too high.

    Bob
     
  7. TomOlsson

    TomOlsson

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    Mar 12, 2013
  8. BobK

    BobK

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    1,688
    Jan 5, 2010
    Yes, that is the kind of thing I was talking about.

    Bob
     
  9. TomOlsson

    TomOlsson

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    Mar 12, 2013
    Many thanks, I shall order one of those, and then it should simply be a case of soldering connectors etc.Hopefully I shall manage that on my own.

    Again, thank you for the help.
     
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