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Help designing analog filter designs for an audio activated 8x8x8 LED cube

  1. Dec 1, 2012 #1
    I am working on designing a music visualization system using LEDs. I would like to create an analog design that uses op amps to filter, amplify, and add DC bias to an audio signal so it can be read into the A/D of a 5V microprocessor. The input signal is a +/- 1.5V volt audio signal coming from an MP3 player. From this signal I need to produce three output signals. This means that I need to design three filters (a lowpass, a bandpass, and a highpass). These filters also need to amplify and bias the input signal so it will be a 0 to 5V signal that can be read by the processor's A/D at the maximum resolution. So basically what I have pictured is to first bias the signal, then amplify it, then filter it:

    MP3 Player Audio signal --> Bias --> Amplify --> Filter --> uP

    I am familiar with building filters with amplification, but I am not sure about the correct way to go about adding a DC bias to the filters with amplification.

    Lowpass (bass range): cutoff f @ ~500 Hz
    Bandpass (mid range): center f @ ~2250 Hz
    Highpass (high range): cutoff f @ ~4kHz

    The parts I have available for use are: LT1632 dual op-amps, resistors, 100nF capacitors
    Measuring the output signal from the iPhone: V(peak-to-peak): ~2.85V centered around 0V
    Required Gain: ~1.75
    Available Power supply units: +5V and +12V DC

    I've tried designing these filters by hand, and using TI's FilterPro software, however I have not been able to achieve the DC bias affect. I have been simulating the filters using LTSpice. I have the design files for the three filters though if it would help in describing my current problem. Any help in this area would be greatly appreciated.
  2. jcsd
  3. Dec 1, 2012 #2


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    Science Advisor
    Homework Helper

    Change that to

    MP3 Player Audio signal --> Filter --> Amplify --> Bias --> uP

    You should be able to combine the Amplify and Bias steps into one op-amp stage. Look up "level shifting op amp circuits" if you are still stuck.
  4. Dec 2, 2012 #3


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    Staff: Mentor

    Hi mp4nerd, http://img96.imageshack.us/img96/5725/red5e5etimes5e5e45e5e25.gif [Broken]

    It seems that you are provisioned with only single ended power supplies. Rather than modify circuits to power op-amps off a single ended supply, it may be more expedient for you to employ a switching regulator module to produce a negative supply rail for the op-amps in your filter, amplifier, etc. There exist active filter designs that can provide all of LP, BP, and HP outputs from the one circuit, e.g., http://mysite.du.edu/~etuttle/electron/elect65.htm

    Follow this active filter stage with a separate buffer stage incorporating the required gain and adding some DC bias there.

    Good luck with your colourful project! http://img571.imageshack.us/img571/7470/rainbowf.gif [Broken]
    Last edited by a moderator: May 6, 2017
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