low pass filter for audio frequencies

Discussion in 'The Projects Forum' started by tzitzikas, Jun 25, 2010.

  1. tzitzikas

    Thread Starter Active Member

    Jun 3, 2007
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    hi, i am looking for an audio frequencies low pass filter.
    i would like the filter to cut-off the audio frequencies above 5Khz.
    do you have any schematic?
     
  2. ifixit

    Distinguished Member

    Nov 20, 2008
    638
    108
    Hi tzitzikas,


    A simple RC filter would do.
    1. The roll-off slope would be 20dB/decade. Is that good enough?
    2. What are the input and output impedances?
    Regards,
    Ifixit
     
  3. tzitzikas

    Thread Starter Active Member

    Jun 3, 2007
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    the output is an mc1496 modulator. look the schematic http://tzitzikas.webs.com/modulator-1.jpg
    the input is an equalizer. i don't know the input impedance
    can you design something?
    i am thinking to design an RC lpf but i don't know how to choose the R.C values
    i have find this calculator http://sim.okawa-denshi.jp/en/CRlowkeisan.htm
    and i put C=330n and it find R=120ohm for 4khz=fc. what do you think about these values?
    (i choose Fc=4khz and not 5khz because if i choose 5khz at the frequencies little above 5khz the filter cut-off little)
     
  4. ifixit

    Distinguished Member

    Nov 20, 2008
    638
    108
    Hi tzitzikas,

    Your calculation seems to work for a 4KHz cut-off, however, I would prefer if it didn't effect the source impedance, so I suggest different values of 1200Ω and 33nF.

    Formula: Fc=1/(2∏RC)

    Are you trying to filter the audio out of a Amplitude Modulated RF carrier?:confused:

    Regards,
    Ifixit
     
  5. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    It looks to me like you want your filter to go in front of P1, the 5k pot. Is this correct?
     
  6. tzitzikas

    Thread Starter Active Member

    Jun 3, 2007
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    hi. Thank you for your answer. I would like to connect this filter before the 5K pot. I must tell you, that i have change the 100uF capacitor after 5k pot with an 2.2uF capacitor, because the audio had high low frequencies and the equalizer can't helps more.
    i have combine this am modulator with this pll: http://tzitzikas.webs.com/pll-1.JPG
    and with this oscillator: http://tzitzikas.webs.com/oscillator-1.JPG
    the problem is that, when this unit operates, sometimes the pll have problems with the lock. Every 30 minutes approximately, the lock (from lcd display) disappears for 1sec and the carrier disappears from the receiver, and at this time i am hearing a very strong noisy signal in the receiver.
    This unit is driving an 1w linear amplifier with mosfets. At this 1sec the linear's dc current goes from 1.5A up to 4A. Someone tell me, that the problem is the high audio frequencies above 5khz. Because of these frequencies, the pll have problems with the lock. I tried to ground the crystal's shield. It helps but the problem didn't disappear. What do you think about this??

    ifixit where do you connect the collector and the base of transistor? The base has series electrolytic or MKT capacitor?What value? Can i use this circuit after equalizer and before the modulator?
     
    Last edited: Jun 26, 2010
  7. ifixit

    Distinguished Member

    Nov 20, 2008
    638
    108
    The transistor was to reference your circuit. Just ignore it then. The R and the C are the filter, place the filter in between whatever circuits you want.
     
  8. tzitzikas

    Thread Starter Active Member

    Jun 3, 2007
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    0
    hi. i tried the filter but the pll had again problems with the lock. after that i tried to operate only the pll with oscillator and modulator without the linear. The pll had not problems with the lock this time. So, the linear 1-2w effects to the pll.
    The pll, modulator and oscillator pcbs are in separate R.F metal grounded boxes. I can't find witch is the problem. What do you think?
     
  9. flat5

    Active Member

    Nov 13, 2008
    403
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    Pics might be useful.
    coax and feed-through caps are being used?
    Is one power supply being employed for all modules?
     
    tzitzikas likes this.
  10. tzitzikas

    Thread Starter Active Member

    Jun 3, 2007
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    hi. I am using coaxial cables 50hm (between modulator and linear, for linear's output and for pll's r.f sample from oscillator). I am using one power supply with lm338 , 12-28volt dc/5A for all modules and i reduce the voltage at 12vdc for pll,oscillator and modulator by using LM7812.
    I don't have used feed through capacitors.
    this is linear's schematic: http://tzitzikas.webs.com/irf640-1.JPG
    i must use feed through capacitors??where i must connect them??only at linear ??
    i am thinking to replace C6=100n capacitor with a feed through capacitor.
    2 photos:
    http://tzitzikas.webs.com/pll_new02.jpg
    http://tzitzikas.webs.com/pll_new01.jpg
    In the first photo you can see 2 small metal boxes. In one of them is the pll and in the other one is the oscillator-modulator.
    In the center of the big box is placed the linear 2w and at the right you can see the power supply.
     
    Last edited: Jun 29, 2010
  11. flat5

    Active Member

    Nov 13, 2008
    403
    17
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