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low pass filter questions...

Discussion in 'Electronic Basics' started by Alan Adrian, Jan 20, 2005.

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  1. Alan Adrian

    Alan Adrian Guest

    I come and go in this group over time... been away for a while as I've had
    no time to play with my electronics.

    I have a little project that I'll try to explain briefly.

    Using an "fm modulator", I have taken the headphones output from my digital
    music player, and gotten it to play on my Van sterio, through the Antenna
    cable, which is a CD/radio unit, so my tape adapter doesn't work on this
    vehicle. It does work fine on the car though, even with the noisy DMP
    described below...

    The input to this fm-modulator is so weak that I built (from a Velleman Kit)
    a pre-amplifier that boosts normal headphones output to a decent Line-In
    level, which it likes.

    When I was using a Mini-Disk as my music player, this system worked great.

    I've since upgraded to an iRiver HD based digital music player and the above
    system sounds like crap. The player works great with headphones, and
    through my casset tape adapter though.

    I put my Computer digital scope on the system, and as I expected there is a
    bunch of noise centered on 50khz, starting at 30khz and going to 70ish... spectrum1khz tone.jpg
    here's a spectrum plot of the noise on a 1 kHz tone after it's amplified by
    my pre-amp...

    The pre-amp is based on a TLO72CP chip and the input signals pass through a
    1uF cap and 10K resistor with a 1 meg feedback resistor tapping off just
    before the input.

    I tried making an RC low pass filter to put on the front of this thing, but
    it doesn't seem to have much effect on the digital noise. Though I'm a bit
    weak at this audio stuff, and rusty to boot...

    Any suggestions as to how to proceed to clean up the high frequency noise
    would be greatly appreciated.

  2. Jamie

    Jamie Guest

    Put a Load resistor equal to what the put expects of your device..
    for example, lets say your head phones are 32 Ohms. put in some
    common R like 47/50 Ohms etc..
    in other words, make it look like there is a real load connected.
    then i would drop the ratio of your preamp.
    use 1K ohm for the series R and 100k for the Loop back.
    its also possible that you don't need all the gain that you have for
    this device, this would make a problem of saturation with upper pass
    freq's being induced.
    my idea would be to lower the gain of your Op-amp and put in a loop
    back cap to filter out the high freq.
  3. off topic but what kind of scope do you have?
  4. This may sound like a naive solution but why not just put an inductor
    in the input to suppress high frequency noise?
  5. Alan Adrian

    Alan Adrian Guest

    I've had it for a few years now. It's a Picoscope ADC 200.

    The software is capable of making a lot finer bins on the spectrum, but I
    didn't feel I needed to...

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