Two-way Loudspeaker with High and Low Frequency

The circuit was designed to create a two-way loudspeaker with the use of a tweeter for producing high frequency or pitch signals and a woofer for producing low frequency or bass signals.

• TL071/TL072 – a low noise JFET input operational amplifier with features such as common-mode input voltage range, high slew rate, operation without latch up, compensated internal frequency, high input impedance at the JFET input stage, low noise, low total harmonic distortion, protected from output short circuit, low input bias and offset currents, wide common-mode and differential voltage ranges, and low power consumption
• NE5532 – an internally compensated low noise dual operational amplifier with features such as full power bandwidth up to 140 KHz, input noise voltage of 8 nV, common mode rejection ratio, 9 V/us slew rate, high DC voltage gain, 32 V peak to peak voltage swing, wide supply voltage range from 3 V to 12 V, unity gain bandwidth at 10 MHz, and internal frequency compensation
• STK4042 – An AF power amplifier with features such as reducing pop noise when the power supply is turned on or off by adopting constant current circuits, facilitates thermal design of slim stereo sets by distributing the heat dissipating ICs in the set, supporting the design of supplementary electronic circuits, availability of 20 W to 200 W outputs as pin-compatible amplifier, and allows audio sets to be made slimmer due to its miniature package
• CMOS 4093 – a quad 2-input NAND with Schmitt trigger inputs integrated circuit, generally characterized by small fluctuation in voltage supply, very high impedance, outputs that can sink and source, one output can drive up to 50 inputs, high speed gate propagation time, high frequency, and low power consumption
• 7812/7815 – 3-terminal 1A positive voltage regulator with short circuit protection, thermal overload protection, output transistor safe operating area protection, output current up to 1A, and output voltages from 5V to 24V
• 7915 – a 3-terminal negative fixed voltage regulator with features such as surface mount package availability, contact factory for other voltage options, availability of voltages -5V, -12V, -15V, fold back current limiting, internal thermal overload protection, compensated output transistor safe area, good load and line regulation, current output to 1.5A, no external components required, internal short circuit, and 1.5% internal output voltage
• G2R2 Relay – a power relay used for a variety of purposes with various models with features such as availability of high capacity and high sensitivity types, availability of double winding latching type, highly functional socket, and with clearance and creepage distance of 8 mm/8 m
• Diode Bridge – also known as bridge rectifier which has four diodes arranged in a bridge configuration where the output voltage has the same polarity with either polarity of the input voltage
• Tweeter – a small loudspeaker designed to produce high pitch and high acoustic frequency sound in a high fidelity audio system
• Woofer – a large loudspeaker designed primarily to produce low audio and bass frequency signals in a 2-way or more complex speaker system

Different speaker sizes are being reproduced by different sound waves. The very fast movement of the speaker can produce high pitched sounds like cymbals while a larger speaker that moves a greater volume of air can produce a low frequency that will sound like a bass drum. A two-way loudspeaker can separately produce a specific range of sounds into two speakers even though one speaker can do both. This implies that each speaker can work better within its frequency range when reproducing the sounds. To make this possible, the speaker utilizes a crossover circuit that receives the signal from the amplifier and divides it into two frequency ranges. The signals are sent to the speaker that is designed only for handling those frequencies. An audio crossover is normally found in full-range speakers whose portions are decoupled at increasingly higher frequencies and can be constructed mechanically. Approximately 2 KHz to 20 KHz is the crossover frequency of the tweeter while the crossover frequency for the woofer can be is typically around 40 Hz to 1 KHz.

The performance benefit of active loudspeaker is significant over passive loudspeaker especially in the separation of frequencies. An active loudspeaker system contains its own amplifier, making its construction bigger and costly but proportional in this circuit. Since it does not inductors, the distortion is very low thus, producing a sound with more quality. Large capacitors are built in the path of the signal which have a very small value and is made of good quality. This does not mean that a passive loudspeaker will not provide good benefits, some passive designs are better than an active loudspeaker. In this circuit, one active loudspeaker would be enough for the design.

An active loudspeaker has a built in active crossover filter which should be precise in the design. The drivers of an active loudspeaker are connected directly to the power amplifiers. This would result to a much simpler and well known load of the power amplifier. The power amplifier is placed after the crossover since each power amplifier has a limited frequency range to amplify. Electronic amplifier components comprise an active crossover which are operated at signals levels appropriate for power amplifier inputs. This is different in passive crossovers which operate at the high levels of the power amplifier output, which handle high voltages and high currents. The combined effect of the power amplifier responses, driver responses, and crossover filter responses produces the flat frequency response of a high quality loudspeaker. The use of active method enables the optimization and adjustment of frequency response for the full loudspeaker system without expensive external equalizers. The outcome would be an efficient, more reliable, consistent, precise and simpler active loudspeaker system.

Figure 1 illustrates a 2-way active loudspeaker while figure 2 illustrates a 2-way crossover with cross frequency equivalent to 3.1 KHz. This frequency was set for the reason that it is close to the frequency of commercial loudspeakers in the market. The frequency may be modified depending on the requirements of the design. IC1 TL071 is located on the input stage. IC2 TL072 creates a high pass filter of frequencies while IC3 NE5532 creates a low pass filter for frequencies below 3.1 KHz. NE5532 are high-performance operational amplifiers with the combined AC and DC characteristics. They possess the trait of very low noise, low distortion, high slew rate, high output drive capability, input protection diodes, maximum output swing bandwidths, and high unity gain. They have specified maximum limits for equivalent input noise voltage and internally compensated for unity gain operation. The inputs of the NE5532 are different from the inputs of the TL072 in that they do not have any input current. As a result, the NE5532 is 3 times quieter due to the very low impedance.

The presence of the trimmer TR1 in the connection of high frequencies can be adjusted for the level of the two speakers. The level of the tweeter is normally adjusted 10% below the level of the woofer. The filter consists of capacitors and resistors which are reserved for future use. Resistors R6 and R10 are not used in the cross frequency. The two power amplifiers are driven by the two outputs of the filter with IC4 for high frequencies while IC5 for low frequencies. Both ICs are of the same Sanyo STK4042 with good sound characteristics and output power of 80W/8ohms. The filters RLF1-2 in the output of amplifiers are made of resistors R27 and R38 and an inductor wound in three layers. The inductor is made with 25 to 30 coils in a 1 mm diameter.

A relay RL1 is located in the output amplifier that performs the DC protection and delay system. This is operated by the IC6 4093 and it works when the circuit is driven and a 5 second delay is applied in the connection of the speakers in the amplifier outputs. When the power supply is stopped, the contact of RL1 is open, disconnecting the speakers from amplifiers. The output of power amplifiers provides circuit protection on the speaker by opening the contacts of RL1. The visible LED D20 illustrates the delay/protection of the circuit. The power supply consists of a rectifier, main PCB, heatsink, diode bridge BR1, and a toroidal transformer. They are all enclosed in an aluminum casing that is fit to the dimensions of the speaker box. The tweeter and the woofer dictate the total performance of the loudspeaker.

Source:users.otenet.gr/~athsam/2_way_active_loudspeaker_eng.htm