headphone amp powered by 19v adapter

Discussion in 'The Projects Forum' started by juggler73, Apr 10, 2010.

  1. juggler73

    Thread Starter Member

    Apr 10, 2010
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    hi
    i have a headphone amp that i got as a kit from ebay it works well for what i need it for but the problem is that the batteries dont last long and i was hoping to use a spare 19v 4740mA laptop adapter i have

    the amp runs off two 9v batteries and you can see by the picture 1 it not a straight forward connection.

    i was thinking that i would need to make a voltage divider seen in picture 2 and connect it as the batteries would connect.

    does anyone have any advice on this working

    thanks
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    Oops - no, that's not going to work.

    Fortunately, there's a pretty easy solution to your problem; it involves using an inexpensive power opamp.

    But, first need to correct the rather non-standard way they decided to wire the power switch. I'm rather curious why they decided to switch both of the "virtual grounds" instead of the +V/Vcc and -V/Vee leads.
     
  3. juggler73

    Thread Starter Member

    Apr 10, 2010
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    hi
    i forgot to say im only starting out in electronics so help is needed

    how do i correct the wiring and whats an power opamp and then how would i connect it together

    i would get a mains powered headphone amp but i cant find one thats a kit or a list of parts

    thanks
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    OK, first off it's very helpful if you include your country in your profile - that way we can better suggest parts that you might be able to get easily. Click on the "User CP" link below the forum link to get to your profile settings.

    Parts that are easy to get in the States can be rather expensive in Europe. For example, Asia has a very different system for transistors than we have in the USA.

    I have various things going on, so this is going to be rather a step-by-step process.

    First of all, see if you can find available an L272 or L2720 power opamp, and is it reasonably priced. These opamps are very inexpensive in the States, as they are widely used in automotive applications.
     
    Last edited: Apr 10, 2010
  5. juggler73

    Thread Starter Member

    Apr 10, 2010
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  6. SgtWookie

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    Jul 17, 2007
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  7. juggler73

    Thread Starter Member

    Apr 10, 2010
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  8. SgtWookie

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    Jul 17, 2007
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    I clicked on your link; a whole list came up.

    This one?
    http://gb.mouser.com/ProductDetail/...GAEpiMZZMtCHixnSjNA6Gd6XD%2bkxe1piyzEnUqf3fI=
    Mouser Part No: 512-L272AM, £0.382 each?
    That should do the trick nicely.

    When you go to place your order, it wouldn't hurt to have a couple extra on hand if you might be interested in doing some small electric motor projects in the future. However, don't order straightaway, as you will need a few more bits.

    I need to finish making some changes to your original drawing that you posted, but I have to leave for a hour or so.
     
  9. juggler73

    Thread Starter Member

    Apr 10, 2010
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    i was just looking at the delivery and its £12 so it must be coming the US

    so im looking elsewhere hopefully i can find some in the UK

    thanks
     
  10. juggler73

    Thread Starter Member

    Apr 10, 2010
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    Last edited: Apr 11, 2010
  11. SgtWookie

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    Jul 17, 2007
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    Good find - you're set with that ;)

    Sorry I'm a bit slow; have many irons in the fire, and many people want some help.

    Here's how you can use the L272 as a "rail splitter":

    [​IMG]

    For the unused opamp, you would simply wire the - input to it's output, and the + input to the output of this one. That way, it won't cause problems by oscillating on it's own.
     
  12. juggler73

    Thread Starter Member

    Apr 10, 2010
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    hi
    no worries dont apologize im glad for your help

    what is this circuit actually doing is it dividing the 19v adapter but im not quite sure how to connect to my headphone amp.

    pictures would be good

    is R5 1ohm
    how many volts is the C1 .22uf cap

    thank you again
     
  13. SgtWookie

    Expert

    Jul 17, 2007
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    R1/R2 form a voltage divider. At their junction will be 9.5v, when referenced to ground, or 19v/2 = 9.5v.

    R3 and R4 are the feedback path. The output of the opamp will follow the noninverting input (9.5v) very closely.

    R5 is 1 Ohm. C1 is 0.22uF, and should be rated for 25v or higher. R1 & R2 form a Boucherot cell, which helps to eliminate any tendency to oscillate. Boucherot cells are very commonly used in audio amplifier circuits.
    http://en.wikipedia.org/wiki/Boucherot_cell

    I haven't finished modifying the image you posted.

    The battery connections to the switch/board will need to be changed a bit.
    It was goofy how they switched the two battery grounds rather than the + and - power rails.

    Are you planning on leaving the batteries in while using your external power? If so, that will complicate things a bit.

    Pictures? Well, I don't have a unit like yours. You'll have to make do with drawings.

    Yes.
    25v minimum.
     
  14. juggler73

    Thread Starter Member

    Apr 10, 2010
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    nope get rid of the batteries

    drawings will be very helpful
    thanks
     
  15. SgtWookie

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    Jul 17, 2007
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    OK, compare this to your original.

    I've changed the switch connections so that V+ and V- are switched instead of the grounds.

    The board only needs one ground connection. You can see that the traces are connected.
     
  16. juggler73

    Thread Starter Member

    Apr 10, 2010
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    hi

    wow that looks great :)
    ive got the stuff ordered and it come this week so i'll do it then

    just one thing i need to check that i got the pin places right

    thanks
     
  17. flat5

    Active Member

    Nov 13, 2008
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    Your pin 6 should be pin 2.

    The Vcc is the + power.
     
  18. SgtWookie

    Expert

    Jul 17, 2007
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    Nope - you have the Vcc pin labeled 6. That should be pin 2.

    Don't forget that you will need to connect up the other opamp to keep it from oscillating.

    Pin 5 to Pin 3
    Pin 6 to Pin 7 (it's OK to use the same reference as the other opamp)

    You should also have an 0.1uF cap across the Vcc/GND pins of the L272. Having a 100uF cap across the +19/GND inputs will help to keep things quiet.
     
  19. juggler73

    Thread Starter Member

    Apr 10, 2010
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    hi

    i'll get back to yer when i got it going and if i have any problems when i got it all soldered together

    thank for your help :)
     
  20. SgtWookie

    Expert

    Jul 17, 2007
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    Are you going to be using Veroboard, stripboard, what?
     
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