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Opamp Audio question

Discussion in 'Electronic Basics' started by andrew queisser, Jan 13, 2006.

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  1. I built myself a headphone amplifier for my bass. It's mono using one half
    of a single-supply dual opamp (TLC272A). I'm using a very simple circuit
    from the data sheet (AC-coupled non-inverting) and so far it's working ok.
    I'm not expecting high fidelity out of it.

    My question: can I simply bridge the inputs and outputs of the two amps to
    get twice the output power? I somehow feel that's the wrong approach, is it
    and why? If it's not ok, how much of the circuit do I have to duplicate? I
    basically have a voltage divider to offset the input to 4.5V (9V battery),
    10:1 resistors for feedback and caps everywhere to decouple input, output
    and the feedback divider.

    Thanks,
    Andrew
     
  2. Pooh Bear

    Pooh Bear Guest

    Depending on the load impedance you may actually be better off increasing
    current rather than voltage. Headphones may actually measure anywhere from 8
    ohms to 600 ohms IME for example.

    Without knowing that info it would be speculative to suggest anything.

    Graham
     
  3. Hi Graham,

    My several headphones are in the 25-60 ohm range. Not sure I understand your
    comment, though. I'm a relative noob, as you can probably tell. I thought
    that by running the two amps in parallel I'd effectively get twice the
    "available" current at a given voltage. I went with a 10:1 amplifier based
    on the schematics I was looking at and it turns out to be a good ratio for
    my bass, which has a pretty low level output voltage. Now I'm wondering if I
    can use a second parallel amp to get more output power for the lower
    impedance headphones.

    Thanks,
    Andrew
     
  4. If you google on "headphone-amp output level" you'll find a related thread
    in rec.audio.pro, May 2004. In that thread, an article by Douglas Self was
    mentioned, http://www.dself.dsl.pipex.com/ampins/webbop/opamp.htm.

    In that article's section on "driving heavy loads" he discusses paralleling
    opamps to achieve better current capacity. However, it's not a great
    technique. As Self's article makes clear, most opamps get pretty lousy with
    loads below 600 ohms; to get down to driving the 32 or 16 ohms typical for
    many modern phones and earbuds, you'd need to parallel quite a few opamps.
    For instance, I own a commercial headphone amp that uses four parallel
    opamps to drive each output channel. I have measured the distortion of this
    rig and it climbs steeply up with low (30 ohm) loads.

    Your TLC272 is a poor choice for this application; if you look on its
    datasheet you'll see that, characteristic of CMOS opamps, it is specified
    only down to 10k loads (so you're off by a factor of 500 or so), and it is
    not stable with capacitive loads (the typical capacitance of a long-ish
    headphone cord is enough to set it oscillating). Its current drive
    capability is very weak - although it's specified for a max current of 30mA,
    the amount of current it can drive while still having any sort of gain,
    bandwidth, and distortion spec is more like a tenth of that. You would be
    far, far better off with something like an OPA2134 (or even a TL072), though
    IMHO these still do not really have enough current drive, even with two
    sections paralleled.

    Rather than paralleling opamps, you might consider the common approach of
    adding a discrete class-B buffer to the output. Self gives an example
    circuit in his article. One caution: although Self says that he did not
    need any compensation to avoid oscillation, in my own experience I've found
    it necessary to put a small (47pF or so) capacitor in parallel with the
    feedback resistor.
     
  5. Pooh Bear

    Pooh Bear Guest

    Hi Andrew,

    I assume the TLC272 is a 'rail to rail' amplifier. That means it should be able
    to swing +/- 4.5V at the output. Into a 25 ohm load the load current would be
    +/- 180mA but the chip will current limit long before that.

    You'll therefore get more output by operating 2 sections in 'parallel'. You
    don't need more volts ( as you'd get by bridging ).

    Configure the second half of the op-amp as a voltage follower connected to the
    first half's output and connect the outputs together via a couple of low value
    Rs ( say 10 ohms ) in series with each output pin ( for current sharing ).

    Graham
     
  6. Hi Walter,

    Thanks for the info and link, guess I'll have some reading to do this
    weekend.
    I read some articles by Chu Moy and he used the OPA too. We have a lot of
    different opamps in our bin stock here but no OPA so I picked one with a
    decent output current but I didn't understand the relationship to distortion
    and gain. My plan was to learn about opamps with the stuff on hand and then
    order some parts that are more appropriate. I'm actually surprised that the
    sound is pretty good to my ears given that I've committed other sins like
    using cheap caps for decoupling.
    This sounds like a good idea - I might go for that.


    Andrew
     
  7. Hi Andrew,
    Ahh, I see, I was using the wrong terms. I meant to ask about paralleling
    the inputs and outputs but I said bridging instead. I'll try the voltage
    follower.

    Thanks,
    Andrew
     
  8. Rich Grise

    Rich Grise Guest

    Frankly, if you don't hear the sound clearly in the headphones with just
    the one amp, then you seriously need medical attention - you're suffering
    from hearing loss.

    Get thee to the clinic!

    Best of Luck,
    Rich
     
  9. Guest

    A hex invertor is another option.

    NT
     
  10. Google LM386 if you need a low cost easy to use amplifier to drive
    headphones down to 8 ohms quite easily.

    Jim
     
  11. Jasen Betts

    Jasen Betts Guest

    ["Followup-To:" header set to sci.electronics.basics.]
    bridge in the context of amplifier outputs usually means having both output
    terminals live but with opposite phase, (so while one is high the other is
    low...)

    This gives twice the power into twice the impedance.
    could be, what sort of phones are you using?

    if they're 32 ohm phones it probably is, if a much higher resistance it
    could work.
    maybe add an LM386 for the output stage.
    if that's not loud enough add two and bridge them.



    Bye.
    Jasen
     
  12. Jasen Betts

    Jasen Betts Guest

    connecting outputs directly together is generally a bad idea.

    ok try this.

    connect the non inverting inputs together and use separate feedback
    sections for the non-inverting inputs

    connect both outputs to 4.7 ohm resistors and the other end of both
    resistors to the headphones,

    drive the headphones in series.

    (drive the left channel via a capacitor, ground the right leave the common
    terminal unconnected)
     
  13. I would recommend that a considerable amount of skepticism be applied when
    reading articles by anyone who writes about audio without the benefit of
    either a distortion analyzer or randomized double-blind testing.

    On the other hand, it is also true that some of the flaws introduced by
    running an opamp into a too-low impedance are of the sort that is not always
    easily heard by untrained ears. Hearing different sorts of distortion is a
    trained skill; one has to learn what to listen for, and in fact some kinds
    of distortion are often perceived as positive (as being more pleasant than
    the undistorted sound) by naive listeners. For instance, a little bit of
    low-order distortion of low frequencies can make bass sound more "rich" and
    "full". Clipping (what happens when the volume exceeds the voltage that the
    opamp can supply, thus turning sine waves into sorta-square waves) is easily
    audible, and ugly-sounding, when it affects more than a few percent of the
    wave; things like intermodulation distortion and slew-rate limiting are less
    easy to hear unless you have appropriate source material and know what to
    listen for.
     
  14. The input impedance of an LM386 is too low for it to be directly driven by a
    bass (the OP's signal source). He would need an input stage of some sort;
    his existing opamp stage would probably be appropriate for this.
     
  15. Blake

    Blake Guest

    Not necessarily. The LM386 has an input impedance of 50K and a voltge gain
    of 200. Keeping in mind that the guitar can put out as much as 1Vpp (at
    least my one does), there's pleanty of room for a 500K to 1meg series input
    resistor.

    I built a headphone amp for my guitar using an LM386, and I find it has all
    the volume I could ever want, with both my bass and six string. But you do
    need to avoid those inefficient 99c store headphones.
     
  16. Guest

    LM386s are acceptable for answerphones, but not much else. They sound
    horrid. You cant get much simpler than a 386, but you can certainly do
    better.

    NT
     
  17. I checked out RadioShack's catalog, since there's a store next to my work
    and I did pick up a LM386 on Friday. Haven't gotten a chance to play with it
    yet but it will be one of my next experiments. My bass has active
    electronics so it's probably going to be ok.

    My current amp works at low volumes but starts distorting rather severely
    when I turn up the output of the bass OR the volume control on the
    headphones (these are cheapo 40ohm headphones with volume pot built in.)

    Thanks,
    Andrew
     
  18. Thanks for the tips - I'm definitely getting a lot of distortion when I turn
    up the bass or turn down the impedance of the phones (turn the volume pot in
    the headphones to max). The distortion is very unpleasant, it sounds like a
    bass played through a cheap fuzz box.

    I've tried the amp with my old Sennheiser headphones which have 600ohms and
    I also get distortion when I turn up the bass. The volume level seems about
    the same as with the 40ohm phones which is a bit confusing to me. Maybe I'm
    already current limited with the higher impedance.

    So far I haven't had the scope and the bass in the same room (one is at
    work, one at home) so I can't really tell what's going on but headphone
    impedance isn't the only thing.

    By the way, I'm a total beginner on the bass so I'm sure I won't have
    "appropriate source material" for quite a while yet.

    Cheers,
    Andrew
     
  19. [snip]
    Hi Walter,

    Speaking of caps - how does the size of the decoupling caps come into play.
    I know that the capacity affects the frequency response but I don't quite
    understand how it affects distortion. When I had very small decoupling caps
    in my signal path I got a clean signal on the scope but when I hooked up the
    phones the amplitude dropped to near zero and I hear a faint, highly
    distorted, high-frequency signal. When I placed a much larger cap in the
    path I got a much cleaner signal.

    I understand that low frequencies pass through the large cap but I don't
    quite understand whether the small cap introduces distortion by clipping. I
    was, probably incorrectly, assuming that the lower frequencies are
    attenuated but if I think about it in terms of charge I imagine that with
    high amplitudes the cap being charged up quickly and then saying "hey, I'm
    full, I can't get the remaining 50% of your signal". That would result in
    the kind of fuzz I'm hearing.

    Thanks,
    Andrew
     

  20. Although caps do introduce distortion, it's much subtler than what you're
    hearing.

    Don't try thinking of the caps' effect in time domain ("charging and
    discharging"), it'll just confuse you. Think of it in frequency domain: the
    cap is a resistor, whose resistance is different for low- and high-frequency
    signals. The resistance is Z = 1/(2 * pi * f * C). So if you combine that
    with the resistance of the headphones (which is reasonably constant for all
    frequencies in the audio range), you'll see that you get two things: first,
    a voltage divider which passes more or less signal to the 'phones depending
    on frequency, so a small capacitance means the 'phones see less signal;
    second, the total resistance that the opamp sees depends on frequency as
    well, so a small capacitance means the opamp isn't loaded as heavily.

    As a quick rule of thumb: the capacitance you want, for coupling between
    stages (or to a load), is C = 1/(2 * pi * R * f) where R is the load
    resistance and f is the lowest frequency you want to pass. So for 16 ohm
    phones, to pass signals of 40Hz and above, C = 250uF. Notice that it has
    nothing to do with how much power is involved: it's the same for a milliwatt
    or a thousand watts. (In truth, there are some issues there; this is just a
    first approximation.)
     
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