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Filter Questions

Discussion in 'General Electronics Discussion' started by Yoa01, Jan 29, 2013.

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  1. Yoa01


    Jun 18, 2012
    Hi All,

    Yes, I am again having problems while trying to build filters for my synthesizers. I swear I should just... eh, nevermind, here's my problem:
    This schematic shows exactly what is on my breadboard, save the batteries being my external PSU and the OpAmp is a 741. The rheostat is a 1M pot set at 500k.

    In trying to create a filter (in this case just a simple 12dB butterworth lowpass), I'm finding that there are numerous problems with the physical circuit (LTSpice doesn't seem to like simulating it). Firstly, for whatever reason my 555's output is a pulse similar to that with what would come out of a highpass filter.

    Then, going into the filter, the first part (the RC setup) works flawlessly: I can adjust the cutoff and the signal is lowpassed, no problem. After the rest of the filter (the OpAmp, etc.), if I get any output it's a muffled pulse; adjusting the filter cutoff does nothing.

    What's interesting is that I've used this exact circuit before for a filter and it works great! I can't figure out what the issue is. You'd think that maybe my IC's are shot, but they work fine for everything else I've used them for. Any idea as to what might be wrong?

    Secondly (slightly different topic), I was talking with a friend of Nyle Steiner (creator of the famous Steiner Filter) and he showed me original schematics for Nyle's filter. One thing that I noticed was interesting was that, to make the filter resonant, Nyle simply used a potentiometer to increase the gain of the OpAmp. Given that my filter is currently non-functional and can't test, I must ask: does this really work? I'd assume so, but it seems too simple to work.

    Thank you for any help you can provide.

    PS: I'm using this filter circuit because it's the only one that worked when I was first trying filter designs. Standard Sallen-Key designs don't seem to work for me for varying reasons.
  2. duke37


    Jan 9, 2011
    The 555 will produce an output going from 0V to 9V.
    The gain of the op-amp is 2 so that the 741 would need a positive supply of at least 20V.

    Try attenuating the signal from the 555 by about 4.

    I am not used to dealing with a 555 but shouldn't there be a capacitor on the discharge pin?

    There is nothing wrong with a Sallen and Key filter.
  3. Yoa01


    Jun 18, 2012
    Ok, so attenuating the 555 OR decreasing the gain should help me out? Thanks! I can't get a 20V supply, so I have to work with what I can muster.

    Nope, not for this circuit. The 555 is actually flipped around; maybe that's confusing you.

    I know, I just can never seem to make one work. The signal always seems to run from the first resistor into the capacitor and to the output :(


    I decreased the gain of the OpAmp to (I think) .1 and now it works perfectly! And the weird pulse problem is gone. Seems like a lot of atteunation, but I suppose it makes sense considering the thing was trying to amplify the input to 18V! Thanks!
    Last edited: Jan 30, 2013
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