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OPAMP question regarding headphones

Discussion in 'Electronic Basics' started by Jacobe Hazzard, Jan 13, 2004.

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  1. There's a problem with this schematic - headphone A is fed by a
    potentiometer. This means the output will have some significant impedance,
    which will decrease the volume and increase distortion, depending on the
    volume setting. Also, connecting/disconnecting headphone A will affect the
    volume and sound quality at headphone B. Finally, wouldn't you rather have
    two volume settings? If you use different types of phones at the same level
    one might be intolerably loud while the other is too quiet.

    It would be better to use two buffers, one for each pair of cans (actually
    fit thay are stereo you need four buffers). Have the buffer outputs feed the
    speaker elements directly - set the volume before the signal goes in to the
    buffer and not after.

    Do you know anything about the headphones you want to drive, such as
    impedance or power handling capability? A 600ohm 3W load looks very
    different from a 16ohm 100mW load..

    Adam
     
  2. Hey.
    I have this problem with my samick mixer. Well its no problem when you are
    making music alone even then the output must be at a 75% setting.
    But when i need to have two headsets connected to the headphoneoutput the
    output is just to low.

    So Since i got some Opamps laying around. I exchanged the TL074 that was in
    the mixer with
    Burr Brown OPA134 OPAMPS. Now the noiseratio got MUCH better. Well anyway,
    since i had some
    opamps left i got this idee.
    First of all i can start with that electronics is just a hobby. I have no
    education whatsoever but i am planning to. Ok here we go:

    I want to have enough power to drive another headset. So if i take the
    headphone output and run
    it through the OPAMP witch is in a untiy gain configuration. I guess it will
    works as a
    buffer so it wont run down the orginal signal....
    So the question is: What am i missing? Will this work?

    I guess the headphone output voltage may be to much for the OPAMP.....?
    Is this just stupid?


    Rough schematic (dident bother to draw the opamp configuration)

    in signal (One channel)
    -------
    |
    |
    | |
    | |/ potentiometer
    | |--------------------- To headphone A
    | |\ |
    | |
    | | ----- To OPA134 (unity gain configuration) -- To
    headphone B
    |
    -----
    ---
    -
    OPA134 datasheet.
    http://www.elfa.se/pdf/73/731/07312010.pdf

    Anders Vinje
     
  3. Ian Bell

    Ian Bell Guest

    Anders N. Vinje wrote:

    snip
    Unfortunately this will not work for several reasons. First the OPA134 is
    not designed to drive headphones. Whilst it might work initialy, it will
    almost certainly exceed the dissipation of the device. Second, as others
    have noted, the potentiometer to headphone A needs to be low to drive
    headphone but high so as not to waste power. Better feed headphone A
    direct from the pones output and headphones B via a pot and a headphone
    driver chip.

    HTH

    Ian
     
  4. This is a fine opamp to drive headphones. One problem with adjusting the
    volume of a stereo signal is that you need to attenuate both channels - this
    means using a dual pot, or two separate pots for each channel. If your mixer
    already has a volume control, it might be best to just leave it out
    altogether, or use a binary level switch like in the link you posted
    elsewhere.

    And you can simplify things even more if you ignore my previous comments and
    drive both pairs of headphones at the same level (this depends on your
    intended use). In that case, you can probably get away with a single pair of
    buffers driving both sets. Either way, the circuit from the project in your
    link would be a good starting point, and will probably do exactly what you
    want. Just build one or two pairs of that circuit, and include whatever
    volume controls you want (suggestions are given there for the dual-pot
    method as well as the switched method).
     
  5. Don´t really know about the power handling capability. But i kinda rember
    the impedance was 32 Ohm.... Not sure..
    So do you got any suggestion how the potentiometer(s) should be connected in
    the circuit to overcome the impedance problem?
    I think i will go for the four buffer soulution since i got the dual
    versions(OPA2134) of the the opamps.

    This has been most helpfull.
     
  6. I want to have enough power to drive another headset. So
    configuration. .......
     
  7. Ian Bell

    Ian Bell Guest

    Jacobe Hazzard wrote:

    major snippage
    Sorry, but I do not understand this. Are you saying the OPA134 *is*
    suitable for driving headphones. If you are, how do explain the datasheet
    shows it is characterised only down to loads of 2K?

    Ian
     
  8. I think 'fine' is too strong an adjective for this case. The data
    sheet

    http://aes.sdsu.edu/documents/sbos058.pdf

    says the output current limit is about 40 ma, but the output swing
    graph shown that about +- 5 volts of head room is needed to get to
    that current. It current limits smoothly in both directions, so it
    will not distort terribly if you get close to the limit, assuming you
    have the supply voltage to burn, but it is not as good as some other
    types that have a higher output current capability.

    Take a look at a couple that do a lot better with low voltage supplies
    and high output current:
    http://cache.national.com/ds/LM/LM6171.pdf
    http://cache.national.com/ds/LM/LM675.pdf
    http://focus.ti.com/lit/ds/symlink/opa551.pdf
     
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