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Help with audio amplification

Discussion in 'Electronic Design' started by [email protected], Nov 4, 2005.

  1. Guest

    I hope you'll forgive a simple query.

    I have an audio amplifier module that takes 12v and an audio input, and
    in turn drives a loudspeaker. Specifically, it is the module shown

    ....and for what it's worth, I've wired ina 10k log potentiometer as

    I'm using this to amplify the audio output from a Garmin Quest ---
    a small in-car GPS navigation unit. The audio output I'm using is
    intended for use with an earphone. Using a multimeter I see that for
    the duration of the announcements, the DC voltage across the output
    rises from 0v to about 1v.

    The problem I have is that <1s before and after the GPS makes an
    announcement, the louspeaker makes a loud and irritating pop, and
    I'd really appreciate some advice on what to do. By experimenting I
    discovered that putting a 10 or a 100 micro-Farad capacitor across
    the audio input to the amplifier removes the final pop and most of
    the first one. It also reduces the volume. Since I don't really know
    what I'm doing I'd be grateful if anyone could let me know the best
    way to approach this.

    Thanks and regards,

  2. Chris Jones

    Chris Jones Guest

    Probably you will have more success if you put a capacitor in series with
    the audio input, between the GPS and the pot. This would form a high-pass
    filter. You would still hear a click, but with less bass. You could
    adjust the cutoff frequency, fc=1/(6.28 * R * C) where R is roughly the 10k
    of your pot, and C is the capacitor you put in series. Try maybe 100nF or
    1uF to start with.

    Do you hear the click when the earphone is connected instead? If there is
    no click with the earphone, then you might have one of those capacitorless
    headphone amplifiers which are so trendy these days. They work by biasing
    the "ground" of the headphone jack up to some DC voltage so that the other
    terminals of the headphone socket can swing above or below the "ground" of
    the headphone socket, without the expense of a 5 cent capacitor. If it is
    one of these chips then you could use an audio isolation transformer.
    (should cost $1 or so as a component, or more with connectors already
    attached, but you might be able to salvage one from an old transistor radio
    or similar) You could also use a differential amplifier but that would be
    more work.

  3. Mac

    Mac Guest

    You can probably get away with a series resistor and a shunt capacitor.
    This forms a low-pass filter. By tweaking the capacitor and resistor
    values, you can probably reduce the pop to an almost inaudible level
    without attenuating the voice too much.

    You want your cutoff frequency to be around 5 kHz, I guess. That should
    let the voice through. You have a 10 k pot, so you want the R to be around
    100 Ohms. Now we can just ignore the pot.

    C = 1/(2 * pi * 5kHz * 100 Ohms) = 318 nF

    So use the next lowest value, 220 nF. This will move the cutoff up from 5
    kHz, but the exact value isn't critical, so this should be OK.

    I am infamous for making arithmetic mistakes in this forum, so double
    check my arithmetic. Hopefully I got equation right. ;-)

    Oh, since you say you don't really know what you are doing, I'll draw an
    ASCII art schematic (view with courier or other fixed-width font):

    | |
    | \
    | 10 k /
    = 220 nF pot \<--- amp input
    | /
    | \
    | |
  4. Mac

    Mac Guest


    Heh. I just finished telling him to try a low-pass filter. Would that not

  5. Not quite.
    You have proposed a low-pass filter. Ideal for getting rid of the signal
    and leaving the pop.
  6. Pooh Bear

    Pooh Bear Guest

    He needs a high pass filter.

    Having looked at the product description though, I wonder if the amp isn't
    simply defective. I'd expect the amp module to be already ac coupled.

  7. Mac

    Mac Guest

    So I have now gathered. For some reason I thought the pop was mostly high

    Oh well.

    Nice thing about usenet is that bad information will usually get corrected
    by followups.

  8. Mark

    Mark Guest

    A HIGH PASS filter might change the pop into a click.
    If you want to get rid of the click too , you'll need something more
    complicated, like a gated attenuator.

  9. GPG

    GPG Guest

    Terminate the GPS output with a resistor equal to the normal headphone
  10. Guest

    Thanks very much for all the replies. If nothing else I have
    a lot of ideas to keep me busy with!

    It was interesting to see the dissent regarding
    high/low pass filters. Having a physics background,
    I had assumed that the click was essentially a square
    pulse reflecting the bias of the GPS output. Thinking
    of the corresponding Fourier series, I would have thought
    that blocking high frequencies would remove the
    sharp corner of the square pulse and round it out more.

    Thanks once again,

  11. Rich Grise

    Rich Grise Guest

    Actually, that would turn a 'click' into a 'thump'. What you
    need to do is provide a leakage path from the amplifier side
    of the coupling capacitor, to drain off the DC bias from the
    cap. Or mute it completely, which is another project. ;-)

    Good Luck!
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