Active low pass/ active cut filter

Discussion in 'The Projects Forum' started by noingwhat, Jan 30, 2011.

  1. noingwhat

    Thread Starter Member

    Jul 23, 2010
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    I would like to build some sort of circuit so that it will pass freqs < 80hz at normal volume, and cut out freqs (-30db atleast) > 300hz. What kind of circuit would I use to achieve this? Low pass or high cut? What is the difference between the two? And would you suggest that I make it an active or passive (rc) filter?
     
    Last edited: Jan 30, 2011
  2. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    A fourth order Butterworth lowpass filter with 120Hz cutoff will do what you want.
    What sort of load are you driving?
     
  3. noingwhat

    Thread Starter Member

    Jul 23, 2010
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    I feel like this is a silly question, but since there are only 3 solder spot on the circuit diagram, but there are 8 pins on most of the opamps I am looking at, where do I solder the three points on the diagram?

    Another small question, would the values of the caps and resistors be dependent on which op amp I decide to use?
     
  4. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    I haven't seen the circuit diagram you are looking at. I assume you are talking about an op amp. Answer my previous question:
    Also, where are you getting your input signal, and what is the maximum amplitude of the audio, if you know.
    Do you have a DC power supply voltage(s) in mind?
    Knowing the answers to all these questions (there are 4 of them), we should be able to provide you with a proper schematic, including the answer to your question about all those extra pins.
     
  5. noingwhat

    Thread Starter Member

    Jul 23, 2010
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    I am trying to build a cheapy subwoofer using an old driver I found lying around, so this circuit is going to have the input from the TV, and output to the subwoofer amp. Not completely sure of the input voltage and current, but from what I understand it should not be a lot because it is just the RCA cables, and I am pretty sure there are only a few volts in there.
     
  6. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    Do you have a power amplifier for the subwoofer? Or is that what you are trying to build with this thread?

    There are two parts;
    1) Active Crossover/pre-amp (Your desired low pass filter with op-amp above)
    2) Power amplifier

    If you don't have the power amplifier, things get... difficult.

    You may want to read some of these Excellent introductions from ESP to understand what you want a bit better.
     
  7. Ron H

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    Apr 14, 2005
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    You didn't answer the question about the power supply(s).

    @Thatoneguy:
    He mentioned "output to the subwoofer amp". I assume that means one already exists. As you said, if it doesn't have one, "things get... difficult".:)
     
  8. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    My concern is that he may want to use an old receiver as his "sub amp" or something similar. Need the information, tried to re-word some of your questions, I guess.

    I guess we'll find out, knowing the make/model of the power amp will answer most all of the questions/concerns.
     
  9. noingwhat

    Thread Starter Member

    Jul 23, 2010
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    Yes, it's and old reciever as an amp, all I really need is the crossover. I had used a passive rc filter that I built for the speaker that I have used for a few months, but it never sounded all too great. It leaks too much up into the higher freqs (I messed up and made the Fc 300hz and for an RC filter, this doesnt provide much of a "subwoofer") and I am looking to get a bit better sound quality out of it. I'm not looking for much, but it sounds pretty good considering I only spent about $1 on it.
     
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