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Audio OpAmp Gain values

Discussion in 'Electronic Basics' started by [email protected], Oct 26, 2005.

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  1. Guest

    Hi all,

    I hope no one will mind if I jump in and ask a bit of a beginner's
    question.

    The circuit below is from the first stage of an audio distribution switch.
    Input will be 1V P-P over the audio range of 20-20K, but I am not well
    schooled, and do not know how to determine the values of the two
    resistors, the feedback, and the divider.

    Given the input values at 20K, how do I calculate the values for the other
    resistor pair in order to create a unity gain stage? I just want to put in
    the 1V P-P and get 1V P-P out.

    I know its rather a lame question for folks here who are wiser than I, but
    I am just getting started, the boss gave me the box and said the first
    stage gain is wrong, so I would like to fix it up quick, and learn
    something in the process.

    Thanks a lot for your tolerance and patience :)

    Mark

    ___
    |---|___|--|
    | |
    20K | +12V |
    ___ | |
    -|___|-+ | |\| |
    |--+----|-\ |
    | >--+---
    20K +-----|+/
    -|___|---+ |/|
    | MC33078
    | -12V
    .-.
    | |
    | |
    '-'
    |
    |
    ===
    GND
    (created by AACircuit v1.28.4 beta 13/12/04 www.tech-chat.de)
     
  2. John  Larkin

    John Larkin Guest


    20K!

    John
     
  3. Chris

    Chris Guest

    Hi, Mark. Welcome, the water's fine. This is where electronics newbie
    questions belong (unless it's somebody in an undergrad or tech college
    course who doesn't want to take the time to do their homework, in which
    case they'd better bring 20 zloty). Oh, and by the way, more knowledge
    doesn't necessarily mean more wisdom ;-)

    I took the liberty of putting labels on your resistors and input and
    output voltages above.

    Your circuit is usually called a "difference amplifier", or diff amp.
    For the case where R1 = R3 and R2 = R4 (which is usually the case),
    then

    V(o) = R2 / R1 * (V2 - V1)

    If you plug in your resistor value of 20K, you can see it would only
    work with a gain of 1 (R2 / R1 = 1) when your mystery resistors are
    also 20K, as Mr. Larkin said.

    Please download

    http://www.national.com/an/AN/AN-31.pdf

    The National Semiconductor appnote AN-31, "Op Amp Circuit Collection",
    is a great emergency cheat sheet to keep on your workbench. Your diff
    amp is on the first page, along with both the equation where R1 = R3
    and the more complicated equation for when they don't. It covers a lot
    of the basic op amp circuits, providing one typical schematic, and one
    equation (if necessary) for each circuit. This may be of great help to
    someone like yourself who doesn't do circuit design for a living, but
    will occasionally have to replace component values to tweak something
    or just wants to figure out what an op amp configuration is doing.

    Good luck in your education
    Chris
     
  4. Guest

    My great thanks to John and Chris for the answer to the question, and the
    help on how to figure it out myself in the future.

    Sure appreciate you both taking the time to bail me out and head me in the
    right direction. I was guessing 20K, but not knowing what kind of
    amp/buffer I was seeing, I just wasn't happy with just a guess.

    Much obliged for your assistance.

    Best Regards,

    Mark
     
  5. Paul Blitz

    Paul Blitz Guest

    If its of any help to you, I've designed a load of simple circiuts for use
    at Hospital Radios. One of these is a 1:4 audio distribution amplifier. The
    whole article is at:
    http://home.btconnect.com/woodgreen/tech_tips/techtip/art10.htm. If you need
    stereo, you build 2 of them. If you need fewer or more outputs, delete or
    add extra output buffers!

    You'll find both circuits and description there.

    If you go up a level, to
    http://home.btconnect.com/woodgreen/tech_tips/techtip/art_ind.htm, you'll
    see the list of other bits I've designed.


    (I have no problems with individuals using any of my circuits, but they may
    NOT be used for commercial purposes without contacting me!)

    Hope this helps!


    Paul Blitz
     
  6. John Larkin

    John Larkin Guest

    Incidentally, you can't prevent (at least in the US) anyone from using
    a circuit you designed, unless it's patented. All a copyright does is
    prevent them from using the literal image that you publish.

    And a 2N7000 is *not* a Fetlington (which was a fet-bipolar hybrid,
    now obsolete.)

    John
     
  7. John Fields

    John Fields Guest

     
  8. John Larkin

    John Larkin Guest

    Unfortunately, you're in a bad mood, again.

    John
     
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