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Circuit Design: Two Way Active Audio Crossover Filter

There are different kinds of loudspeakers like Tweeters, Mid-range Speakers, Sub-Woofers and Woofers which can reproduce the voice in their own particular frequency bands only. In an audio play back device, the entire audio signals are separated into different bands and are applied to the corresponding type of loudspeaker. The Tweeters are normally fed with frequencies above 5 kHz, Mid-range speakers are fed with frequencies in the range of 300 Hz to 5 kHz, and Sub-Woofers with 300 Hz to 40 Hz and Woofers are fed with frequencies below 40 Hz.
The entire audible voice spectrum stretches from around 20Hz to 20 KHz and there is not loudspeaker design which can reproduce all these frequencies with the same effect. The Woofers are made to produce subsonic sounds (below 20 Hz) and there are musical instruments which can produce frequencies above 18 KHz. To reproduce all these sounds different types of loudspeakers are fed with their own band of frequencies extracted from the music.
The filter circuits that are used at the output side of the audio device which filters out different band of frequencies and use them to drive different type of loudspeakers are called Audio Crossover Circuits. Three-way cross over circuits are very common in output side of the audio devices which filters out frequency bands for Tweeters, Mid-range Speakers and Sub-Woofers. This tutorial discusses the design and implementation of Two-way audio cross-over circuit using Active filters for quality filtering.
 
Two Way Active Audio Crossover Filter Circuit on Breadboard
Fig. 1: Two Way Active Audio Crossover Filter Circuit on Breadboard
 
The Two-way audio cross-over circuits are used to drive the Mid-range Speakers and Sub-Woofers separately. Mid-range speakers are fed with frequencies in the range of 300 Hz to 5 kHz, and Sub-Woofers with 300 Hz to 40 Hz. Since the musical sound normally falls around the maximum frequency of 5 to 8 KHz, for driving the Mid-range Speakers a high pass filter of cut-in frequency around 300 Hz is enough. Bass-beats of the songs appear in the Sub-Woofer range and a Band-pass filter can be used to separate out these frequencies from the entire audio signals. The Two-way cross-over circuit can be considered as a combination of HPF (High Pass Filter) and BPF (Band Pass Filter) as shown in the following block diagram;
Block Diagram of Two Way Active Audio Crossover Filter Circuit
 
Fig. 2: Block Diagram of Two Way Active Audio Crossover Filter Circuit
 
The HPF and BPF can be realized using the passive components like inductors and capacitors only, but here Active Filter circuits are used to improve the quality of the filters. Again for HPF an active synthetic inductor based filter is used to avoid a bulky inductor from the circuit. The BPF is designed based on a MFB filter for a good quality yet simple circuit.

Synthetic Inductor Circuit

The basic concept of the Synthetic Inductor circuit is to use a capacitor and inverse its properties so that it behaves like an inductor. The advantages of this circuit over the actual inductors are the very low internal resistance, easily varying the inductance value in wide ranges, possible to design high quality filter circuits etc.The circuit diagram of a Synthetic Inductor circuit is given below;
 Circuit Diagram and Equivalent Circuit of Synthetic Inductor
Fig. 3: Circuit Diagram and Equivalent Circuit of Synthetic Inductor
 
Here the property of the capacitor ‘C’ in the above circuit has been inverted with the help of the unity gain op-amp circuit. The value of the inductance depends on the values of the resistors R1, R2 and the capacitor C also. The inductance of the Synthetic inductor circuit is given by the following equation;
L = R1 * R2 * C
The high frequency filter can be realized using a single capacitor and a synthetic inductor connected in series in which the one end of the inductor is grounded, input is fed from the free end of the capacitor and the filtered output is taken from the point where the capacitor and the inductor are connected together in series. The circuit diagram and the equivalent circuit of the Synthetic inductor based High Pass filter is given in the following diagram;
 Circuit Diagram and the Equivalent Circuit of the Synthetic Inductor Based High Pass filter
Fig. 4: Circuit Diagram & Equivalent Circuit of Synthetic Inductor Based High Pass filter
 
The circuit contains a capacitor in series ‘Cf’ which forms a High Pass filter with the Synthetic inductor circuit. If the inductance of the Synthetic inductor is say ‘L’, then the pass band of the High Pass filter starts from the frequency given by the following equation;
 
Most of the audio frequencies other than the Bass frequencies appear above an average frequency of 700 Hz and hence the High pass filter is designed for a cut-in frequency of 700 Hz.
  Circuit Diagram of High Pass Filter Designed for Cut in Frequency
Fig. 5: Circuit Diagram of High Pass Filter Designed for Cut in Frequency
 
The MFB filters are very commonly used in circuits due to the fact that they provide reasonable performance with the simplest circuit. They can be designed to obtain narrow bandwidth and high gain. They are suitable for the design of Band Pass filters since the bandwidth, mid-band frequency can be easily adjusted or varied. These circuits has an amplifier with more than one feedback and hence the name. The circuit diagram of a MFB band pass circuit using a single resistor and capacitor feedback is given below;
 Circuit Diagram of MFB Band Pass with Single Resistor & Capacitor Feedback
Fig. 6: Circuit Diagram of MFB Band Pass with Single Resistor & Capacitor Feedback
The equations relating the value of the components to the gain, Q factor, bandwidth and mid-band frequency are given below;

Since this filter will be used to drive a loudspeaker a buffer circuit needs to be added at the output of the MBF band pass filter which will drive the loudspeaker without affecting the filter characteristics. The buffer can also be made easily with another op-amp. An MFB band pass filter with Fm = 70, Q = 15, Am = 100 has been designed and the complete circuit is given below;
 Circuit Diagram of MBF Band Pass Filter with Buffer Circuit
Fig. 7: Circuit Diagram of MBF Band Pass Filter with Buffer Circuit
 
A synthetic inductor based HPF and a MFB based BPF has been designed separately and to make an Audio Cross-over circuit one has to connect the input of both the circuits together and fed the audio to that common input point of both the circuits. The complete circuit diagram of the Audio Cross-over circuit is given below;
 
 Circuit Diagram of Two Way Active Audio Cross-over Filter
Fig. 8: Circuit Diagram of Two Way Active Audio Cross-Over Filter
 
The audio input is fed from a PC and the filtered audio in this experiment is demonstrated in the video using a normal headset, since the headset speakers are designed for the good quality reproduction of both the high frequency sound and low frequency bass.
Video: