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  • This subwoofer active filter circuit is a 24 dB octave filter with a Bessel character and cutoff frequency of 200 Hz. So, if you are interested in experimenting with audio circuits in subwoofer range, this circuit is for you.

    In subwoofer range, all audio frequencies below 200 Hz can be fed to a single speaker box since the human directional perception of sound diminishes at this frequency range. The normal stereo signals above 200 Hz can be fed to 2 satellite speakers.

    How does the subwoofer filter works: A1 and A2 buffer the signals coming from right and left channels. Opamp combinations A2/A4 and A9/A10 function as the highpass filters. The outputs are then connected to the final amplifiers of the battelite boxes. Signals from both channels are fed to A5. Opamps A6/A7 function as the lowpassfilter, A8 as the output amplifier for the subwoofer signal.

    The signal level can be balanced between the subwoofer and the satellite lines. The power needed for this filter circuit must ne a symmetrical power supply. The opamps can have either JFET or bipolar inputs.

    Active Subwoofer Filter Circuit Diagram

    subwoofer filter circuit schematic

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    46 Responses to "Subwoofer Filter Circuit"

    1. krishnakumar says: on January 3, 2010 at 10:00 am

      it is agood one.

    2. Mukesh Shania says: on May 7, 2010 at 9:52 pm

      Hi! Krishna Kumar
      Will you PLEASE arrange to send me the Bottom PCB Drawing to see the performance of you circuit on my mail.
      Your action in this regard would be highly appreciated.
      Mukesh Shania.

      • krishnakumat says: on November 13, 2010 at 7:14 pm

        dear mukesh shania
        Above circuit is in ELECTOR-ELECTTRONICS old issue with pcb design sir.
        with love

    3. super circit.I use this circuits in DTS cinema surround system in many theatres. Also i use it in my 5.2 ch “DOLBY DIGITAL 3D hOLOPHONIC SURROUND SYSTEM”.

    4. Senal Dulanjala says: on September 17, 2010 at 1:48 pm


    5. hello my name is joe, i need to a subwoofer circuit because my subwoofer is rotten.

    6. can you pls indcate the legs number for beter clearification?

      • Joe Perkins says: on July 9, 2011 at 1:49 am

        Dual OPAMPs, like almost all of them, come in an 8 pin package (DIP or SOIC or miniSOIC) with the same pinout.

        Pin 1 is OP 1 output. Pin 2 is OP 1 inverting input. Pin 3 is OP 1 non-inverting input. Pin 4 is VEE (most negative supply – ground can be used, but the virtual ground must be generated using two resistors and an electrolytic capacitor). Pin 5 is OP 2 non-inverting input. Pin 6 is OP 2 inverting input. Pin 7 is OP 2 output. Pin 8 is VCC (most positive supply).

        The LM833 is capable of 30VDC operation. The TLMC662 is capable of 12VDC operation, but has CMOS inputs and can swing closer to its rails. The LME49726 operates on 5V, buhas such low distortion commercial distortion analyzers have trouble reading it (VOUT = 3.5VP-P, VDD = 5.0V, THD = 0.00008%).

        Most OPAMP makers have the datasheets available on their websites and places like DigiKey that sell them have links to them as well.

    7. Is this circuit works? because i would like to build this one…..

    8. Which components do I change to allow me to choose a different crossover frequency? Also, what type and tollerence caps should be usef in the circuit?

      • Joe Perkins says: on July 28, 2011 at 4:53 pm

        The upper and lower sections are high pass circuits and they are set for about 200Hz. Changing the capacitors (C2, 3, 4, 5, 13, 14, 15, 16) is the easiest way to adjust the frequncy for those. Just make sure all capacitor values are the same. Double the value and you will halve the frequency. Cut the value in half and you will double the pass band. For the middle section, a low pass filter, adjusting the resistors (R9, 10, 11, 12) changes the frequency. The same rule applies. Keep all resistance values the same and increase to lower the pass band frequency and reduce the resistance to raise the frequency. The web page http://www.wa4dsy.net/robot/active-filter-calc has a great calculator for these filters.

    9. Thanks for the link. I still don’t understand the different values of resistors on each stage of the high pass filter and the different values of cap on each stage of the low pass.

      • Joe Perkins says: on July 29, 2011 at 2:46 pm

        An “active” filter like this is just an RC filter with a way of feeding back some of the signal in a way that the impedence of the previous components can’t affect. This can help make the RC filter sections (you can see both kinds of stages are made with 2 resistors and 2 capacitors) more effective and the roloff at the chosen frequency sharper. This is where the filters Q comes into it. This is basically a measure of how sharp the filters roloff is. For each kind of filter, either the resistive or capacitive elements have a larger effect on the Q of the filter (yes, the Q stands for quality).

        For high pass filters, the different resistor values allow one to adjust the Q as well as setting the frequency. For low pass filters, the capacitors do this. Thus, each section either uses all the same resistor values or all the same capacitor values.

        A simple RC filter has a 6db per octave value calculated from f6db=1/2*pi*R*C (with R in ohms and C in farads and f in Hertz). To get better than 6db per octave, you would use an active filter and be able to adjust the Q for 15db per octave.

      • Thanks again for the help, but I don’t understand why there are 3 op amps in series. I understand the first is a buffer, but why 2 more, and how come the caps and resisters are different in each….

        The second opamp in the high pass has 10nF and 120K, when put into the equation the answer is 132.6Hz, the next opamp has 10nF and 220K, = 72Hz ???

        Also, where does the feedback resistor come into the equation?

      • Joe Perkins says: on July 29, 2011 at 7:52 pm

        As I tried to explain, these are not simple RC filters with 6db per octave rolloff. By using a buffer in each RC section you can keep the impedance (the resistance at a specific frequency) of each section from affecting those stages both before and after the op amps. By doing this, and adjusting the values the way they did, you can make the filters produce upwards of 30db per octave of roloff. Since using a lower frequency, as you noticed, reduces the gain a bit, you use an op amp with gain to make up the faloff in gain. This lets you add more WOOF to the subwoofer output :D

        Remember, you want a sharper roloff than 6db per octave, so you cascade stages and use slightly off frequencies to reshape the roloff.

        If you increase the R or C values, as appropriate, by any given percentage, you will change the crossover frequency by that percentage as well.

      • Please can someone tell me the gain of the last op amp in the low frequency circuit.

        I understand that the 5K pot will increase and decrease the gain, but what is the min and max gain level.


    10. i must try it,

    11. Joe Perkins says: on September 14, 2011 at 5:28 pm

      Sorry, Craig. Our conversation has reached the “Reply” depth limit for this forum.

      Gain of an OP Amp circuit is calculated by dividing the feedback resistance (Rf, the resistance from the inverting input to the output terminal) by the gain resistance (Rg, the resistance from either the input signal source (inverting circuit) or the virtual ground (the input signal is fed into the non-inverting terminal of the OP Amp) and subtracting it from 1.

      Thus: Gain = 1 – Rf/Rg

    12. Ankit Singh says: on January 14, 2012 at 12:26 pm

      it is required to connect above cct. to connect the power amplifier or not? i wating for this plz send it soon.

    13. what is the numbers of resistors and capistors

    14. shravanth says: on February 2, 2012 at 7:08 am

      hello every one i am going to make 2.1 channel using TDA2030 for speakers and TDA2050 for subwoofer i need good quality bass so, what to do? is this circuit will do or not please reply

    15. shravanth says: on February 2, 2012 at 11:17 am

      can any one mail me completed pics

    16. hey guys,thnx for uploading this circuit,may I know where to input power in this diagram and can I use same IC (IC no) for all?

    17. can any one tell me what are the voltages of the capacitors? I can;t find these values.

    18. Aditya says: on May 10, 2012 at 7:40 am

      Can i Use IC 741 ????

    19. hi,

      can anyone send me the complete schematic diagram of this circuit or you can give the link were to find one.

    20. Hi I am Alok.
      I want to learn how can I make a small woofer, which can play with my mobile phone…
      Plz help me..

    21. Joe,
      I would like to build your circuit, but I’m a little confused. Shouldn’t R8 be on the (-)input to the first stage of op #4? Also, if I just want to to combine my low voltage audio signals to feed my subwoofer, can I just ignore everything past the subwoofer inputs on both channels (c2 on and c13 on)? Approximately how many watts does this circuit produce?

      • R8 is shown correctly–it provides a gain of -1 for this op amp. Yes, if all you need is a subwoofer, you can eliminate everything including C2 and C13 and beyond. This is not a power amp–you must provide an additional power amp for the subwoofer.

        Also, since the L & R high-pass filters are not used, the two input voltage follower op amps may be eliminated along with R1 & R19–simply increase the values of R2, R8 & R17 to 47K and tie directly to the input coupling capacitors. Increase C1 & C12 to 1uF to move the low frequency cut-off down to 6hZ (from 34hZ)–surprised to see this deficiency in a subwoofer.

    22. HI,

    23. hello every one i am going to make 2.1 channel using TDA2030 for speakers and TDA2050 for subwoofer i need good quality bass so, what to do? is this circuit will do or not please reply

    24. I’m glad to have found a circuit that does everything I need. LP/HP at 200HZ and 24DB/octave. I was surprised that there wasn’t a kit already out there that does exactly this. There are many kits on eBay with filter/amp combined, or we find just the active filter for the subwoofer, but not the combination for HP and LP.

      I don’t have the time to try or much experience to draw out the PCB for this. If you have a working unit and you can send me a PHOTO COPY of the bottom of the PCB unit, I will pay you $10 USD for the layout. Thanks.

    25. What is the voltage which is being used to run this circuit. Thanks

      • Jim Keith Jim Keith says: on August 15, 2014 at 2:54 pm

        The power rails are not indicated, but are generally ±12V. In quad op amp packages, +12V goes pin 4 and -12V goes to pin 11. Check out the datasheets…

      • Thanks Jim where about’s can the datasheets be found

    26. Also the capacitors state 0.12 are these micro or nano or are the capacitors stated with no value on the end just 0.12F. cheers.

    27. is capacitor valies are in uf ?

    28. good circuit

    29. pramod manuja says: on November 25, 2014 at 11:40 am

      is the all capacitors are ceramic ?

    30. Joseph Perkins says: on November 25, 2014 at 2:20 pm


      A general note about capacitors and the expected values. Forgive me if you already know this stuff. This is for the beginners.

      Coupling Capacitor Values:

      In audio circuits you should expect to see values as low as 0.001UF to as high as 10UF. In radio circuits you’ll see values from 0.5PF to 0.01UF

      There are always exceptions, but this is the common trend based on resonance and impedance.

      Bypass Capacitor Values:

      These tend to range from 0.001UF to 100UF with values from 0.001UF to 0.1UF being common in RF circuits and 0.01UF to 1UF being common with digital circuits. Audio circuits generally use 1UF to 100UF.

      In the above circuit, if you use a split supply (+6V and -6V at lowest and +12V and -12V at highest), then use 2 capacitors of 10UF per supply rail as near to the VCC pins of the OPAMPS. Space them out a bit. Think of the supply rail like a clothes line and the capacitors are the poles that hold the line up.

      Capacitor Values:

      There are several ways to denote capacitor values. If the qualifier is not given, such as PF or UF or NF, then all caps listed will have the same qualifier. In this schematic, only C11 has the qualifier, but all capacitor values listed only make sense if they also have the UF qualifier.

      The five qualifiers are, in decreasing order of magnitude,

      F = Farad – Common with SuperCaps used for clock chips or CMOS memory.

      MF = Millifarad – Rarely used, but would apply to any capacitor from 1000UF to 999999UF. Milli means one thousandth, in this case one thousandth of a Farad.

      UF = U is based on the greek letter Mu and means micro, or one millionth.

      NF = Nanofarad – Not as common as PF and UF, but used in professional circles. This is thousands of picofarads. Thus 1NF is 1000PF and 22NF is 22,000PF.

      PF = Picofarad – One trillionth of a Farad. Below about 0.1PF, the spacing of conductors on a PCB would swamp this you have entered the stripline zone where PCB tracks can be inductors and parallel tracks either a capacitor or a transformer.

      Another thing to watch for in capacitor values is the marking on the part itself. In general, small ceramic discs and some mylar capacitors list their values in the same manner as resistors, but with numerals instead of color stripes. That is, two significant digits and a multiplier of tens.

      Thus, less than 3 numbers the value is likely to be in PF, 100 means 10PF, 151 means 150PF, 104 would be 0.1UF or 100,000PF.

      There are exceptions to this, but this is common.

      Good luck and happy DIY’ing :)

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