Audio Filters: Designing a two-way audio crossover – Part 6

In the previous tutorial, the fundamentals of audio filters were covered. As we learned, audio filters can be passive or active depending on the components used and whether or not it requires a power supply.

In terms of frequency response, filters can also be classified as high, low, band, and all-pass, as well as notch, T-notch, band-reject, and equalizer filters. Now, we’re ready to design an audio crossover. 

An audio crossover is an electronics circuit that splits the audio signal into two or more frequency bands. These frequency bands are then sent to the different audio drivers (such as the tweeter, woofer, or mid-range speakers). However, a single speaker is incapable of serving the full range of audible frequencies because of the limitations of its design. So, different drivers (speakers) are required to deliver different ranges of frequencies.

For example, tweeters are typically used for high-frequency audio signals, whereas woofers are used for lower-frequency signals. As the name suggests, mid-range drivers are ideal for mid-range signals. 

To split an audio signal into different frequency bands, separate audio filters are used in a crossover. They’re typically classified as either a two-way or three-way crossover. 

The two-way splits the audio signal into two frequency bands — a high-frequency band for a tweeter and a low-frequency band for a woofer — and it’s the most common crossover used in standard audio systems. 

The three-way splits the audio signal into three bands, which is less common but more efficient. It splits the audio signal into different frequencies to best match the tweeter, woofer, and mid-range speakers. 

In this tutorial, we’ll build a two-way crossover using active audio filters. The crossover will have a high-pass filter to deliver high-frequency signals to one speaker and a low-pass filter to deliver low-frequency signals to another speaker. Both circuits will use an operational amplifier (op-amp).

The audio will be input via a smartphone and output through two different speakers. The cut-off frequency for both filters will be 500 Hz.

To test the crossover, we’ll check the frequency response curve of the audio filters. This curve will be drawn by plotting the voltage levels of the audio signal with respect to the frequencies. A function generator will also be used as an input source to demonstrate the sinusoidal waves at different frequencies.  

We’ll be using some common terms associated with audio amplifiers or audio filters, such as gain, clipping effect, cut-off frequency, bandwidth, and the quality factor. We covered some of these in the previous tutorial: Understanding the filters.

Components required
Figure 1. List of components required for a two-way audio crossover

Block diagram
Figure 2. A block diagram of a two-way audio crossover

Circuit connections
In a crossover circuit, the audio signal is split between different frequency bands. Each band is amplified separately and the output is applied to the appropriate drive unit. Each frequency band has a separate knob to control the gain of the audio signal, as shown here: 

High-pass filter – An active, first-order, high-pass filter is connected in the circuit. For this filter, the audio input is passed via the op-amp’s non-inverting pin through an RC network (meaning it uses a resistor and a capacitor).

The audio signal passes through the capacitor. Its impedance is inversely proportional to the frequency and the capacitance — so, the lower the frequency, the higher the impedance and vice-versa. The high-frequency element of the audio signal will, therefore, have less impedance and easily pass through the capacitor and to the amplifier’s non-inverting input. The low-frequency element of the signal will contain a greater level of impedance. It’s bypassed through the resistor that’s connected to the ground.

The impedance of the capacitor can be determined using this equation: 

(Impedance), Xc= 1/ (2π*f*C)

The high-pass filter is designed using a capacitor (“C1” in the circuit diagram) of 100 nF and a resistor (“R2”) of 3.2 kilo-ohms. Using these values for the capacitor and resistor, the cut-off frequency of the filter can be calculated as follows:

FH= 1/ (2πR2C1)
FH= 1/ (2π*3.2k*100n)
FH= 500Hz (approx.)

The RC network forms a passive, high-pass filter. Through this network, the filtered audio signal — which now carries only high-frequency signals — is passed to the op-amp’s non-inverting pin. 

The IC has input and output overload protection and has zero latch-up when the common-mode range is exceeded. The IC can be provided a positive or negative supply voltage up to 22V and an input signal voltage (amplitude) up to 15V. Generally, it must be provided a positive or negative voltage of at least 10V. 

Gain = (R5/R3)

         = 22/2.2 kilo ohms

         = 10

As a result, the high-frequency element of the audio signal is amplified 10 times compared to the input audio signal. The op-amp’s output is drawn from the IC’s pin 6, which is connected to one of the speaker’s wires. 

Low-pass filter – An active first-order, low-pass filter is connected in the circuit. For this filter, the audio input is passed to the op-amp’s non-inverting pin via an RC network. The audio signal passes through the resistor, which has a frequency-independent response. The high-frequency elements of the audio signal are bypassed through a capacitor to the ground.

The impedance of the capacitor is inversely proportional to the frequency and its capacitance — so the lower the frequency, the higher the impedance and vice-versa. The high-frequency elements of the audio signal, therefore, experience less impedance and are easily bypassed through the capacitor to the ground. The low-frequency elements of the audio signal experience more impedance and cannot bypass through the capacitor.

The impedance of a capacitor can be provided with this equation: 

(Impedance), Xc= 1/ (2π*f*C)

The low-pass filter is designed using a capacitor (“C2” in the circuit diagram) of 100 nF and a resistor (“R1”) of 3.2 kilo-ohms. Given these values of the capacitor and resistor, the cut-off frequency of the filter can be calculated as follows:

FH= 1/ (2πR2C1)
FH= 1/ (2π*3.2k*100n)
FH= 500 Hz (approx.)

The RC network forms a passive, low-pass filter. Through the network, the filtered audio signal — which now carries only low-frequency elements — is passed to the op-amp’s non-inverting pin. 

Gain = (R6/R4)

         = 22/2.2 Kilo ohms

         = 10

The low-frequency element of the audio signal is amplified 10 times compared to the input audio signal. The op-amp’s output is drawn from the IC’s pin 6, which is connected to one of the speaker’s wires. 

Speakers – Two speakers are used in the circuit, with a 25 mW power rating and 8 ohms of impedance. One wire from each speaker is connected to the op-amp’s output pins and the other is connected to the common ground.

The speakers regenerate sound from the the audio signal. Ideally, the high-frequency signals should be sent to a tweeter and he low-frequency ones to a woofer. For this tutorial, however, basic speakers are used in the circuit. 

Safety first
The following precautions must be taken when assembling this circuit:

1. Only use speakers that are equivalent to the amplifier’s output — or of a high-power rating.
2. Avoid clipping the output signal as it may damage the speakers.
3. Always place the components as close as possible to reduce the noise in the circuit.
4. The bread board produces a lot of noise and loose components, so it’s recommended to make this circuit on PCB for clear, distortion-free noise.

How the circuit works
A single channel of audio is fed as input to the circuit, and the high and low-pass filter circuits receive this audio signal. The high-pass filter extracts the high frequency audio signals (frequencies above 500 Hz) and sends them to the op-amp, which amplifies the signal by 10.

Similarly, the low-pass filter extracts the low-frequency signals (frequencies below 500 Hz) and sends them to the op-amp, which amplifies the signal by 10. The output from the high and low-pass filters is directed to different speakers. Since the low and high frequency elements of the audio signal are separated and sent to the ideal speakers, the amplified sound is crisp and clear. 

Testing the circuit
To test the filter circuit, a function generator is used as an input source to generate a sine wave of constant amplitude and variable frequency. Since an audio signal is, essentially, a sine wave, a function generator can be used instead of using a microphone or other type of audio source.

You’ll note, headphones are not used during testing as their speakers are resistive and inductive. At different frequencies, its inductance changes, which in turn changes the impedance (R and L combination) of the speaker.

So, the use of a speaker as the load for deriving its specifications at the op-amp’s output could lead to false or non-standard results. Instead, a dummy load that’s purely resistive is used. As resistance does not change with frequency, it can be considered a reliable load independent of the frequency of the audio signal’s input.

The peak-to-peak amplitude of the signal from the function generator is set to 23 mV and a resistive load of 100 ohms is connected to the output (instead of the speakers). The cut-off frequency for both the high and low-pass filters must be 500 Hz and the op-amp’s voltage gain should be 10.

In this case, a voltage gain of 11 was observed. The frequency response of the high and low-pass filters was as follows: 

This table is then used to plot a frequency curve for the high and low-pass filters.

The frequency curve of the low-pass filter: 

The frequency curve for the high and low-pass filters can be more precisely plotted by taking a reading of the voltage level for a greater number of frequencies. 

In summary, we designed a two-way crossover in this tutorial, which has a maximum output power of 22 mW and a signal gain of 20 dB. Its cut-off frequency is 500 Hz. The crossover for other cut-off frequencies can also be designed by selecting suitable values of the resistor and capacitor for the high and low-pass filter circuits.  

This audio crossover can be used for driving different types of speakers such as tweeters, woofers, and sub-woofers. It can also be used in Hi-Fi audio systems for separating the frequency band from the audio signal.

This was a simple circuit design using few components, but it can be converted to a three-way (or more) crossover by adding more filters in the circuit. 

In the next tutorial, we’ll design an audio equalizer. 

Project Video

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Filed Under: Audio Electronics, Tutorials

 

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