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Audio buffer output

Discussion in 'Electronic Basics' started by Michael, Jul 30, 2005.

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  1. Michael

    Michael Guest

    I have a multipart question for purposes of a self-tutorial.

    Some of this "esoteric" audio information can drive you nuts and often
    times, is contradictory.....hence, my questions.

    Ok....so I have this little USB DAC chip that I've breadboarded......the
    PCM2902 from TI.

    It's really a neat device, taking my computer's WAV files through it's USB
    port and ultimately outputing a stereo analog signal....all in one chip.

    Works great.....and for now I just have the output feeding (2) 47uf
    capacitors and some bleeder resistors right into my amplifier.

    First of all, the chip shows a Vout peak to peak of .6v and an center
    voltage of .5v and an impedance of 10k ohms.

    (Q) Does that mean the output swings between .2v and .8v? I'm not clear on
    that.



    Secondly, I'd like to learn more about the "importance" of a buffer
    stage.....("unity gain" or otherwise).

    Many of the circuits using this chip are taking the output into high quality
    opamps (analog devices and the like)....but I'm not clear on exactly "why".
    (This chip sounds damn good with just the caps...right into the amp).
    (and "maybe", less is better?)

    (Q) Based on the .6v p-p figure (low?) and the 10k output Z -- would that
    mandate a buffer stage? Is there some loading going on that is affecting
    performance...(that I may not hear?)


    (Q) Or are the opamps used to "tailor" the sound of the chip...using the
    various opamps available and coming up with your own preferred "recipe"?


    (Q) Or I suppose, a combination of the two?


    Thanks kindly for any education on this....I really appreciate it.
     
  2. Michael wrote:
    8<

    http://focus.ti.com/lit/ds/symlink/pcm2902.pdf
    It is a bit confusing, but there are the letters, VCCCI below the
    numbers, so I think that the .5 refers to half of whatever VCCCI is.
    Likewise .6 ,means 60% of VCCCI.

    So what it VCCCI?
    According to page 2 (Maximums) it can't be more than 4 volts.
    Near the bottom of page 5, we see it refers to the output of an
    internal regulator that produces a nominal 3.35 VDC.
    So, evidently, the DAC produces a signal based on this voltage, and
    using half that voltage as a zero signal value. And that makes the
    zero signal voltage a nominal 1.68 volts, and the peak to peak (full
    scale) swing a nominal 2.01 volts.

    The data sheet doesn't say what the output impedance is (I hate Texas
    Instruments data sheets for stuff like this), but only that he specs
    given apply with a load impedance of 10k attached. I would assume
    that thew output impedance is something quite a bit lower than this
    (but possibly not well controlled), so that if you keep the load at or
    above 10k, it will work as speced. But they don't say this, explicitly.
    I think this has to do with three things. One is the relatively high
    output load impedance spec (compared to cable capacitive impedance at
    the highest audio frequencies). If this thing acts strangely with
    loads below 10k ohms, a long cable could do that. Any ordinary buffer
    amplifier is likely to be able to drive a load with a lower impedance
    than 10k.

    The second thing is that there is not a very sharp low pass filter in
    the chip to remove the digital hash from the DAC output, to keep the
    device as general as possible. If you add some low pass filter to
    your buffer, you can get a cleaner signal. So, you decide what
    bandwidth you need, and include filtration that removes stuff higher
    than that and include it in your buffer.

    And then there is the issue of levels. If you require a signal with
    peaks higher than +-1 volt, you will need to add voltage gain to your
    buffer.
     
  3. Michael

    Michael Guest

    So, evidently, the DAC produces a signal based on this voltage, and using
    I'll remember that and for kicks I'll look at it on my scope.
    I thought anything below a volt was too low. I need to make a chart with
    datasheet "abbreviations" like VCCCI.....(I gave up when no one could make
    their mind up on Vss and Vcc.)


    Most amps inputs are usually higher so that's good.

    I think this was the concern I saw when it came to these opamps on the
    output. I see the "stop band" attenuation is only -43db....pretty lousy, I
    guess.

    I'll just have fun playing with it......it sure sounds good to me....best
    audio I've heard really, I'm hearing detail in some songs that I've never
    heard before.....and I don't really "hear" any high freq digital noise
    whatsoever.

    So many audio nuts seem to design for test equipment rather than the
    (limitations?) of the human ear...so maybe just raw caps will sound better
    to me and that (should be) the bottom line.

    I think when it comes to audio....it's all about how your brain processes
    it.

    John......thanks very much.
     
  4. JeffM

    JeffM Guest

    (I gave up when no one could make their mind up on Vss and Vcc.)
    Vss is the Source supply for a Field-Effect Transister (FET).
    Vcc is the Collector supply for a Bipolar Junction Transister (BJT).
    These specify not only the terminal,
    but the technology involved as well.

    Are you not finding this to be the case?
     
  5. Michael

    Michael Guest

    Vss is the Source supply for a Field-Effect Transister (FET).

    Ok thanks.

    I've run into documentation in the past that had IC's marked with Vss pins
    as well as Vcc and Vdd used and even seen some newsposts as to what it all
    means. (Of course, maybe the IC actually had an FET in it, so they
    designated Vss?).

    I've even seen an IC marked with Vss that was really a
    ground pin.

    Like I say, I'm going to make a list of what they "should be".
     
  6. Michael

    Michael Guest

    Vss is the Source supply for a Field-Effect Transister (FET).

    Ok thanks.

    I've run into documentation in the past that had IC's marked with Vss pins
    as well as Vcc and Vdd used and even seen some newsposts as to what it all
    means. (Of course, maybe the IC actually had an FET in it, so they
    designated Vss?).

    I've even seen an IC marked with Vss that was really a
    ground pin.

    Like I say, I'm going to make a list of what they "should be".
     
  7. JeffM

    JeffM Guest

    I've even seen an IC marked with Vss that was really a ground pin.
    That is quite normal. Look at some more spec sheets.

    Having Vdd (Drain supply) and ground called out is more common,
    but it's possible to have:
    Vdd and Vss (both being non-zero potentials)
    Vdd and ground
    or
    a negative-value Vss and ground.
     
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