Fig. 2: Block Diagram of 2-Way Audio Crossover
2-Way Audio Crossover Block Diagram
Fig. 3: Typical Image of 2-Way Audio Crossover
Fig. 4: Circuit Diagram of Dual Power Supply for Active High Pass and Low Pass Filters
Audio Source – The audio input is provided from a smart phone. For receiving audio from the smart phone, an audio jack of 3.5 mm is plugged into the phone. The 3.5 mm audio jack has three wires – one for ground and two wires for left and right channel. The two wires for left and right channels are used for stereo systems. In such system, the audio signal from two channels is transmitted with a phase difference of 180 degrees. At the audio system, these phase shifted audio signals are combined to produce noise free audio signal. Such an audio system is called balanced audio system. In this circuit, only one of the channels is used for audio source while the ground wire of the jack will be connected to the common ground of the circuit. So, this audio system will be unbalanced audio system and the audio source will be connected as single channel or mono source channel.
High Pass Filter – An active first order high pass filter is connected in the circuit. In this filter the audio input is passed to the non-inverting pin of the OPAM through a RC network. The audio signal passes through the capacitor which has impedance inversely proportional to the frequency and the capacitance. So lower is the frequency, higher is impedance and vice-versa. Therefore, the high frequency component of the audio signal experiences less impedance and is easily passed through the capacitor to the non-inverting input of the amplifier. But, the low frequency component of the audio signal experiences more impedance and is bypassed through the resistor which is connected to the ground. The impedance of a capacitor is given by the following equation –
Fig. 6: Circuit Diagram of 741 OPAM based Active High Pass Filter
For OPAM, LM741 IC is used. The LM741 is a general purpose OPAM (operational amplifier) with a low input impedance (mega ohm) as compared to FET OPAM. The FET OPAM has a high input impedance in Giga ohms. The output impedance of 741 should be ideally zero but it is usually about 75 ohms. The maximum supply current of 741 IC is about 2.8 mA with a supply voltage up to +/- 18V.
Fig. 7: Typical Image of LM741 IC
Fig. 10: Internal Circuit Diagram of LM741 IC
The LM741 can be configured as both open loop as well as closed loop amplifier. It can be used as either inverting amplifier or non-inverting amplifier. In this circuit, the LM741 IC has been used as non-inverting amplifier as the input signal from the passive high pass filter is applied at the non-inverting input pin (pin 3) of the IC. A resistor (Shown as R5 in the circuit diagram) of 22 Kilo Ohms connected between pin 6 and 2 of the IC provide negative feedback. The inverting pin (pin 2) is grounded through a resistor (Shown as R3 in the circuit diagram) of 2.2 Kilo ohms. The gain of the amplifier is set by these resistors and can be calculated as follow –
Fig. 11: Circuit Diagram of 741 OPAM based Active Low Pass Filter
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 at the output instead of speakers. The cut off frequency for both high pass and low pass filters must be 500 Hz and the voltage gain from the OPAM should be 10. Practically, a voltage gain of 11 was observed. The following frequency response of the high pass and low pass filters was observed –