Amp with low speaker impendence

Discussion in 'The Projects Forum' started by Kontakt, Dec 7, 2012.

  1. Kontakt

    Thread Starter New Member

    Dec 7, 2012
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    I've built a coil for a speaker project, but the reading from my meter gives only 0.7 ohms. I know that this will burn up the cheap little amp I pulled from a portable ipod speaker. How can I make the amplifier work with the speaker?
     
  2. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Your meter can only read the DC resistance of that speaker. Any amp will drive it with an AC signal and is thus governed by it's AC impedance.

    You can check this by putting your meter across the speaker from that ipod. I will read about the same, near zero.

    If that "cheap little amp" is built on an IC then it most probably has internal protection seven ways to Sunday that keep it safe from anything you can put on it's output.

    Just give it a go, see if what you made works.
     
  3. Kontakt

    Thread Starter New Member

    Dec 7, 2012
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    I did give it a go. The amp got hot enough to sting, probably around 80 C. The internal thermal fault protection had the IC running at something like a 50% duty cycle, from the sound of the audio cutting in and out.
     
  4. crutschow

    Expert

    Mar 14, 2008
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    Why did you build a speaker coil with such low resistance?

    To drive such a low impedance, you may have to add an high to low impedance audio transformer to the output of the audio amp. Backdriving a 70V audio transformer such as this might work.
     
  5. Kontakt

    Thread Starter New Member

    Dec 7, 2012
    12
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    It's a direct speaking coil. An experiment in wearable electronics.
    Based on this design, my version uses magnet wire, which had the unanticipated side effect of much lower resistance.

    [​IMG]

    It works well, actually, despite being horribly inefficient and low impendence. I just need a small amp to drive it. Could I build an LM386 based circuit that could drive it without combusting? Do I need to put enough coils in series to drive the impendence up to 4 ohms? What do you guys suggest?
     
    Last edited: Dec 8, 2012
  6. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Use a thinner wire to increase the impedance of the coil, or more coils in series.

    Otherwise a quick trick for now, would be to put a resistor in series with the coil; 3.3 ohms or 4.7 ohms might be enough, you will need to test. This will limit the peak current from the amp chip and still give you an OK amount of current in the coil.
     
  7. Kontakt

    Thread Starter New Member

    Dec 7, 2012
    12
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    I'll give it a shot. I suppose I would have to do the same on an LM386 based amp circuit? There's not a high-power version of that design?
     
  8. tubeguy

    Well-Known Member

    Nov 3, 2012
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  9. Kontakt

    Thread Starter New Member

    Dec 7, 2012
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    Thanks for the link to the winder, but the amp is my focus at the moment. I'll work on a winder for my design once I can make the amp work reliably, and have a design for one I can build multiples of myself.
     
  10. tubeguy

    Well-Known Member

    Nov 3, 2012
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    The typical low wattage amplifier IC''s just aren't designed to drive impedance lower than 4 ohms - and then its tough.
    Side Note: Impedance of a coil is different than DC resistance - usually higher. But I would guess your small coil is still less than 4 ohms.

    The issue is the need for increased current to drive such a low impedance.

    The options are to increase the impedance (coil winder, smaller wire)
    matching transformer as suggested, or a current buffer.

    Edit: I suppose you could try paralleling amplifier IC's - but I've never done it.
    Don't use the LM386 - something bigger.
     
    Last edited: Dec 8, 2012
  11. Kontakt

    Thread Starter New Member

    Dec 7, 2012
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    Alright, I have it working with my cheap off the shelf amp by putting 4 10 watt 1ohm wirewounds in series with the two coils.

    What IC do you suggest to drive the coils? You suggested something bigger... What do you mean by that?
     
  12. tubeguy

    Well-Known Member

    Nov 3, 2012
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  13. Kontakt

    Thread Starter New Member

    Dec 7, 2012
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    My intended supply is a 7.2 lipoly, rated at 20C.

    I need that chip configured as a DC amp, right?
     
  14. Kontakt

    Thread Starter New Member

    Dec 7, 2012
    12
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    [​IMG]

    I've got the amp built into a tictac box, for my preliminary version. The quality is terrible, but it works. I want to build my own amp for the next revision.
     
  15. tubeguy

    Well-Known Member

    Nov 3, 2012
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    Thanks for the photo!

    DC amp ? Yes it requires a DC voltage power supply.
    But, audio signals are AC.
    Oh --- Build the amp as shown in fig. 3 for AC test.

    All components are required, especially C4 the speaker coupling cap. That could be even larger 2200 or 4700uf for better lo frequency response.

    Now the speaker:

    Very cool idea !!

    What are you using for a speaker magnet ?

    To greatly improve quality and let the speaker have a
    chance of reproducing lower frequencies it needs to be in an enclosure.

    Why ?? - Because with no enclosure the front and rear sound waves 'meet' and cancel each other out.

    Even mounting the speakers in the center of a large flat panel with no back will be an improvement.
    No - not discussing bass reflex - boom box etc. :):)
     
  16. Kontakt

    Thread Starter New Member

    Dec 7, 2012
    12
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    I have a neodynium magnet in there.

    [​IMG]

    It's an open speaker, bleeds a good bit to the outside, but it flexible nd not uncomfortable on my ear.
     
  17. tubeguy

    Well-Known Member

    Nov 3, 2012
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    DIY headphones - Very creative...
     
  18. absf

    Senior Member

    Dec 29, 2010
    1,492
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    It'd be too warm to be used in tropical countries.:D

    Allen
     
    Last edited: Dec 9, 2012
  19. Kontakt

    Thread Starter New Member

    Dec 7, 2012
    12
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    It works well enough here. I was really interested in this for applications while skiing and running; my headphones falling out of my ear was a constant nuisance. I just need to refine my design.
     
    absf likes this.
  20. Audioguru

    New Member

    Dec 20, 2007
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    Then its designers are bankrupt.
    A cheap IC amplifier burns up if its load resistace is too low.
    Why would it need to protect from a load that has an impedance too low when it will never see it like that?
     
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