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Amplifier question - output to 600 ohm headphones

Discussion in 'Hobby Electronics' started by Mick, Jul 22, 2003.

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  1. Mick

    Mick Guest

    I have built a small amplifier using an LM 386 which is connected to a pair
    of 600 ohm headphones which recieves an output from a gold detector.
    the problem im having is that there is only a small amount of improvement
    over the output without the amplifier.
    Any help would be appreciated.

    thanks in advance
    Mick
     
  2. The LM386 output is for a load from 4 to 16 ohms, probably the most
    efficient at 16 ohms.
    You need a transformer to put into the output that has a primary impedance
    of 16 ohms (on the output of the chip) and a secondary impedance of 600 ohms
    (for the phones). I imagine anything that has close to those values would
    give you extra performance as desired.
     
  3. Phil Allison

    Phil Allison Guest


    ** So you have a "gold detector" ????????????????????

    Care to share the design with the rest of the world ???????

    Have you got a patent ?????????????



    .............. Phil
     
  4. Trevor

    Trevor Guest

    Big deal, headphones don't require much power. High impedance headphones
    require more *voltage* than low impedance types, and that's where the
    problem lays.

    Trevor.
     
  5. I can see that 600 ohms would definitely be a massive drop in power which is
    exactly what is being complained about. I would like an explanation of how
    600 ohms would "eat up the power" though ???
     
  6. Mick

    Mick Guest

    Ha ha, my mistake. To have a gold detector which only found gold now that
    would be an wonderous device.
    In future i shall stick to calling it a metal detector which you find more
    often than gold.....

    But seriously if someone can direct me to a suitable coupling transformer
    for this application then it will help me a lot.

    thanks
    mick
     
  7. ok, easiest way to look at it. an 8ohm 100watt rms amp, when connected to a
    4ohm speaker will deliver near enough to 200watts rms. BUT going to 16 ohm
    will deliver 50 watts rms.

    4 ohm = 200
    8 ohm = 100
    16 ohm = 50

    double the resistance, half the power.

    the CHAMP (lm386 based jaycar amp kit) delivers near to 1 watt in 4 ohms.
    you are talking about driving this in to 600 ohms thats a stepping of 7 1/2
    times, so the reverse is 0.007 at best. , my sony 'style' headphones
    sensitivity rating is 96dB .01 watt 1mm so you can see the problem here.
    your headphones must have been designed to take a good output from a home
    amp.
     
  8. Sure, I have that on board :)
    My problem is how increasing the load impedance "Gobbles" the power. To my
    limited knowlege I would think that the power used would be much less with
    the higher impedance.
     
  9. the amp has only so much voltage / current it can distribute, when the ohms
    goes up, the resistance goes up, and when the resistance goes up, less power
    is getting through. think about it this way, resistance, the higher it is
    the smaller the water pipe, your tryna gravity feet water through the tip of
    a surynge when its only got enough pressure to feed through garden hose, Bad
    analogy but first thing to mind.
     
  10. P = V^2 / R, hence from 4R to 600R we have 150x decrease assuming V stays
    constant.
    Better leave the waterpipe analogies for highschool, they may be good for
    getting the feel of things but don't really wash in a quantitative argument.
     
  11. Mainlander

    Mainlander Guest

    600 ohms is a fairly high load for one of these which is designed to
    drive a 4 ohm or 8 ohm speaker. Try putting a 600 ohm resistor in series
    with a speaker and then without. I'm sure the difference in volume will
    be huge.

    I'm familiar with musical instrument speakers that have a wirewound pot
    of only 50 ohms in series with the speaker - that makes a BIG difference
    to the volume level.
     
  12. Mainlander

    Mainlander Guest

    Just get a chip with a higher power output. It will still not use a lot
    of power to drive the high impedance load.
     
  13. Mainlander

    Mainlander Guest

    Say you are running it on a 6 volt battery, Maximum voltage at the output
    6 volts. Using ohms law, the current through a 4 ohm speaker i = v/r is
    1.5 amps. The power in the load, v x i, 9 watts. Actually an LM386
    produces about half a watt so the voltage must be lower?

    Anyway if the load impedance is 600 ohms the current will be only 0.01 A
    and the power will be 0.06 watts. Pretty small.
     
  14. Mainlander

    Mainlander Guest

    The higher resistance reduces the current flow, because power is voltage
    times current the power will be dropped.

    It is correct, the power is dropped. You can't afford that.
     
  15. Phil Allison

    Phil Allison Guest



    ** An ordinary op-amp ( eg TL072 or NE5532) will drive a 600 ohm
    load - so a dual will drive a pair of headphones. Much gain as you like
    then.





    ................ Phil
     
  16. Mainlander

    Mainlander Guest

    Interesting. Is that because they are designed to drive balanced audio
    lines (600 ohm)? What is the output impedance of one of these, or does
    that depend on the circuit they are used in?
     
  17. Phil Allison

    Phil Allison Guest


    ** They can deliver about +/- 15 mA - limited by by heat
    dissipation mainly.



    What is the output impedance of one of these, or does


    ** Depends on voltage gain and frequency - the more of either the
    higher.

    With a gain of say 10 it is less than 1 ohm up to 10 kHz or so.



    ............... Phil
     
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