MonoXover 2-way Pre-amp Audio Crossover

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.

Features:

• 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

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

Here are all the parts you need, and example part numbers available from digikey.com:

• 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.

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)	Fc	R7, R8 (ohm)	R5, R6 (ohm)
100	0.120	456	1453	1000	840
100	0.100	500	1592	1000	1000
100	0.082	552	1758	1000	1233
100	0.068	606	1930	1000	1541
100	0.056	668	2127	1000	2014
100	0.047	729	2322	1000	2695
100	0.039	801	2549	1000	4018
100	0.033	870	2771	1000	6720
100	0.027	962	3064	1000	25552

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

OUTPUT:

 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.