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Could you help on interpreting part of a circuit?

Discussion in 'General Electronics Discussion' started by TGV, May 9, 2013.

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  1. TGV


    May 9, 2013

    I'm studying the attached (part of a) circuit diagram. I interpret it as four cascaded filters, but I can't really get what it does. It is seated between a DAC and the output of a synthesizer and is followed by a few more resistors and a volume control. I did study electronics a bit, but that was decades ago and mostly theoretical.

    My first, rather dumb, question is: what is the unit of the capacitors? I'm interpreting 100P as 100 picoFarad, but 0.068, is that 68 milliFarad? That seems awfully large to me. My second dumb question is: what is BP 1/50 on the right? I've never encountered such a symbol before.

    The difficult question is: is this indeed just a sequence of four (three active, one passive) filters? It seems odd, because –when I try to simulate it using QUCS– I just get an amplification for the first one, and a drop-off of 10dB per octave for the last three, which seems way too much. So I think I'm not interpreting things properly.

    Can anyone shed some light on the function of this circuit and its components? Thanks for the help.


    BTW, IC56 is a TL072, the two IC54s are halves of a NJM4558DV.

    Attached Files:

  2. BobK


    Jan 5, 2010
    Those are Sallen Key active filters and should drop off at 12db / octave each.

    The capacitors without the p are in uF. 0.068 uF = 68nF or 68000pF

  3. duke37


    Jan 9, 2011
    The 1/50 could be a 1uF 50V capacitor but the blury symbol looks more like a diac which is not likely.

    BP = BiPolar???
  4. alfa88


    Dec 1, 2010
    1uF, 50V Non polarized electrolytic perhaps?
  5. davenn

    davenn Moderator

    Sep 5, 2009
    Yes exactly

    BP = Bi Polar ( non polarised) electrolytic

  6. TGV


    May 9, 2013
    Thanks for the answers, guys. So the first stage is just an amplifier, and then it's basically a rather steep low-pass filter.

    Thanks again!
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