Active crossover for loudspeakers - Linkwitz Riley 2 or 3 way 24dB/Oct

Every multi driver loudspeaker system needs a crossover filter to supply the correct frequency band to the individual drivers, Bass to the woofer, mid to the midrange and high to the tweeter. In commercial loudspeaker systems this is usually done by means of a passive crossover filter with large coils, capacitors and resistors. This however is only done for commercial reasons and certainly not because this is the best and most optimum way to do the filtering, it is just the commercially cheapest way to this.

 

The best and most optimum is to apply an active crossover with power amplifiers for each driver in the system. Expensive but unbeatable in dynamic response and options to tweak the system and optimize for your listening space.

 

In this article we will introduce our approach with the electronics we designed to get the best from your setup. 

In the schematic on the right you can find the basic setup for a 3 way active crossover.

In case of a 2 way setup the band pass section can be omitted.

 

Note how the incoming signal is filtered into frequency bands for each driver before amplification. This way of filtering is preferred because the actual slopes of the crossover points are very steep, 24dB per octave in contrast to the 12dB per octave in most passive crossover networks build into commercial speaker systems.

 

Also the amplitude at the crossover point is at -3dB that guaranties a flat frequency response at the crossover point, no bump or dip in the response where the filter crosses over from one driver to the next.

 

Please note : NEVER drive the tweeter directly form the amplifier output this will destroy it sooner or later and ALWAYS add a good quality capacitor in series with it to block DC current. Usually 10μF in a 2 way system and 22μF in a 3 way system will be sufficient. This capacitor is in the signal path so spend some money on it !

Our implementation differs somewhat from this approach because we wanted to be able to configure the circuit for use as a 3 way, 2 way or 2 way filter with subwoofer output. 


The input signal is first fed trough a high-pass section for the tweeter and a low-pass section that acts as the input of a bandpass section.

In case of a 2 way setup the output of this low-pass is fed to the woofer amplifier and the input of the low-pass for the subwoofer.


In a 3 way setup the output from the band-pass if fed into the midrange amplifier and the last low-pass output is used to drive the woofer amplifier.

The figure on the right represents the actual filter circuit.

 

The crossover points for the filter stages can be easily set by plugging in two resistor banks that set the crossover frequencies, easy to change and experiment to get the correct settings for the drivers used.

 

In a 3 way setup, one for the first high and low- pass that sets the crossover point between the tweeter and midrange (e.g. 6000Hz)  and one for the second stage that sets the crossover point between the midrange and the woofer (e.g. 600Hz).

 

In a 2 way setup, one for the first high and low- pass that sets the crossover point between the tweeter and woofer (e.g. 2500Hz)  and one for the second stage that sets the cut-off point for the subwoofer (e.g. 150Hz).

 

2 function board headers per stage are available to add notch filters or delay correction to correct for driver alignment. 

 

In addition a 0 to -12dB attenuation circuit is implemented for each output and balanced and non-balanced outputs to drive the power amplifiers.

On the left you find a complete stereo filter system setup including power supplies, transformers and system controller. Note that we have designed it as a dual mono system. 


To support the actual filter boards and make it into a usable system we have designed an additional system controller and power supply.


The system controller handles switching power to the filter system and also controls the power amplifiers by 12V triggering. 3 individual 12V trigger lines fire with 1 second interval so they will not be turned on at the same time. This prevents tripping the mains main fuse when 6 amplifiers turn on at the same time. 


The system can be turned ON/OFF manually with a single push button but also from the  pre-amplifier by the 12V trigger input. It also includes an audio detect input to automatically switch ON  when an audio signal is detected and OFF again when there is no input for about 15 minutes. This function can be turned on and off with the power button.


The power supply has separate output stages for supplying power to the driver stage and filter stage of the filter board individually. This ensures that the filter stage always has a ‘clean’ supply voltage regardless of the load on the output stage.


Two large 50VA torroidal transformers complete the setup.

LR24-3 – The crossover filter circuit board.

PSP05-PICTURE01

PSP05  – Power supply.

System control stage.

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