MonoXover 2-way Pre-amp Audio Crossover




About: Dan Goldwater is a co-founder of Instructables. Currently he operates MonkeyLectric where he develops revolutionary bike lighting products.
The monoXover is an audio crossover.  It is designed to be used for building integrated active (powered) speakers.  That is, a speaker with a built-in amp and crossover.  The monoXover is a pre-amp crossover - it sits before the amp.  This means it can be small and low cost, and it allows a higher overall power efficiency in the system.  Check the diagrams below for the intended uses.

You can purchase a fully assembled monoXover from me here.

  • 2-way 2nd-order crossover (ie, it can be used with any speaker that has a woofer and a tweeter)
  • compact low cost design using easily available parts
  • easy to adjust the crossover frequency
  • adjustable equalizer for the woofer and tweeter
  • can convert stereo to mono, or can select either left or right channel only
  • published design you can use and modify as needed

Step 1: How It Works

check the schematic below.  (click the [i] in the upper left of the image to get to the higher resolution version)

the input jack is on the left and the output jack on the right.  the headers above the circuit explain what each section does.

If you want the stereo-to-mono function, install both R3 and R4.  if you want just the left channel or the right channel, install just one of them.

The relative output volume for the woofer and the tweeter is set by the pots R6 and R7

Step 2: What You Need

Here are all the parts you need, and example part numbers available from
  • 100mH inductor (2 pcs) with under 100 ohm internal resistance.  example: J.W.Miller 5900-104-RC
  • 3.5mm stereo jacks (2 pcs).  example: CUI SJ1-3515N
  • 1k potentiometer (2 pcs).  example: Bourns 3386F-1-102LF
  • 22 to 33 ohm resistors (2 pcs).
  • film capacitor (2 pcs).  see next step for value
  • resistor (2 pcs).  see next step for value
  • experimenter pcb to solder to, such as a SchmartBoard

You can purchase a fully assembled monoXover from me here.

Step 3: Choosing Your Crossover Frequency

The crossover frequency is set using the C1 and C2 capacitors.  R5 and R6 resistors also must be changed depending on the chosen caps.  What frequency do you want?  For a larger 2-way PA speaker (for example, with a 12" or larger woofer and horn tweeter), a frequency around 2khz is usually good.  With smaller speakers, it seems common to increase the frequency, maybe 3khz for a 6" woofer.

If you are modding an existing speaker, normally it has a crossover built in already.  The frequency is often in the spec sheet.  However, the built-in crossover often uses a low cost 1st-order circuit that requires setting a higher frequency crossover.  Often it is desirable to lower the crossover frequency when using a higher order crossover.  The limitation on this is that the tweeter can be damaged by sending too much low frequencies to it.

I'm not an expert on crossover frequencies, search the web if you need more info on this topic.

The table gives correct values for a 2nd order Linkwitz-Reilly crossover.
All values are based on a 100mH inductor, because this yields the highest load resistance, and thus the largest output signal given a fairly weak mp3 player output.
I've attached an excel spreadsheet with live formulas so you can calculate any crossover you like.
  • Choose the crossover frequency you want (Fc)
  • Read across to the C1/C2 column for the correct cap values
  • Read across to the R5/R6 column for the correct parallel load resistors (the total load is R, but since we use a 1k pot, we put another resistor in parallel with it to match R)
L (mH)C1, C2 (uf)R (tot)FcR7, R8 (ohm)R5, R6 (ohm)

Step 4: Putting It Together

I made a pcb for this circuit.  however, it is not hard to build on a generic experimenter board.


 woofer output is on the left channel (channel 2).  
 woofer level control is R7: CounterClockWise for maximum.

 tweeter output is on the right channel (channel 1).
 tweeter level control is R8: ClockWise for maximum.




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    16 Discussions


    Question 11 months ago

    As the kits are no longer available, is there a reasonably priced alternative?


    3 years ago

    What happened to buying kits?

    Link is dead... :'(


    3 years ago

    Can i ask what is crossover frequency and what is formula to calculate lets say if I have coil with different induction ? Thanks!


    3 years ago on Introduction

    Would it be possible for you to make an instructable for a 3 way crossover? Highs, mids/lows, and sub?


    4 years ago

    Hi Dan.
    very interesting instructable you have made here.
    i am pretty new in the xover making, and have some questions.
    can see you have a speciel made pcb, where did you Get it?
    and what about the quality of the components, can i just order a good bounce at ebay or should i go for some els? anything better then other?

    best regards.


    6 years ago on Introduction

    Just got mine as well. Check underneath the board. It's pretty clear that the 1/4" jack is the input and the 1/8" jack is the output.

    At first I thought we'd solder the resistors in the empty slots labeled c3 and c4, but after inspecting with a DMM I'm not so sure now.


    6 years ago on Introduction

    So, just got mine in the mail. Looks different (1/4" jack) & cannot clearly tell which is input and which is output. Also, confused about the comments above on how to use as a mono converter only (I don't need crossover function). Do I still need to solder some parts on the board?.. R3/R4? Also, would be helpful to have a diagram of the board to show where things are.


    6 years ago on Step 2

    hi there. myself and a friend want to make a powered speaker from a surround sound centre speaker that has a 1" tweeter and two 4 1/2" woofers. just wondering if you had suggestions for the resistor and capacitor values? we're pretty new to this stuff so any advice would be greatly appreciated. cheers -b


    7 years ago on Introduction

    Might it be a good idea to use the 10 watt amp that you mention bridged that you for the tweets and a TA2020 bridged for the woofer?

    1 reply

    Reply 7 years ago on Introduction

    Tripath amps (at least the TA2020/24) will fry if you bridge the outputs, from what I understand.


    8 years ago on Step 4

    After a few tries/tests I found R7 is controlling Right Channel (Tweat,Ch1,Red): Clockwise (maximum)
    .... and R8 is controlling Left Channel (Ch2,Woofer,White): CounterClockwise (maximum).

    I tried the monoXover to both a Sure Electronics TA2024 ( and a smaller 16 watt Sure Amp,

    Can you confirm this? thanks,

    3 replies

    Reply 8 years ago on Step 4

    gah did i botch it? its also possible that some cables may end up swapping things somewhere along the line, so maybe the method to figure it out is more useful. i think you want to do it like this:

    hook up just the woofer first. put your ipod volume fairly low. turn both dials (R7/R8) until it is as loud as it gets and you know which dial is controlling it. play something with a low, heavy beat - do you hear the beat? if not then you got the woofer on the tweeter channel. now you know which channel is which, put the woofer on the correct channel. the woofer will also be way louder on the correct channel. set the woofer's dial to maximum. now hook up the tweeter. adjust its dial until things sound ok - on all the speakers i tried the tweeter dial is set at about 50% to get good equalization.


    Reply 8 years ago on Introduction

    You can now buy a fully assembled monoXover from me here


    9 years ago on Step 2

     I am interested in one of your kits