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  • 40W Stereo Active Speakers

  • Created: Mar 07, 2017

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Summary

This project is a 40W active speaker with dual channel suitable for stereo applications. Active speaker, also called powered speaker, is a loudspeaker system which uses an active crossover network to separate different frequency signals from an audio input before amplifying them individually to drive their respective speaker drivers. Commonly, this crossover and amplifier circuitries are built within the speaker enclosures but could also be externally connected to compatible built speakers.


The design is composed of two same circuits for both left and right channel. First, the signal goes through an input buffer circuit before being separated in the crossover network. In the crossover network, a turnover frequency of 1.8kHz is selected. Basically, a low pass filter is used to pass frequencies below the turnover frequency to the woofer driver and a high pass filter to pass frequencies above the turnover frequency to the tweeter driver. Before the low pass filter, a baffle step circuit is placed. This is to compensate for the “baffle” effect or the variation in the frequency response due to the diffraction effects of the enclosure. This phenomenon would likely increase the sound intensity at higher frequencies by 6dB and is common to woofers. To compensate this effect, a specific amount of attenuation must be applied to the frequencies where baffle occurs. A low-pass shelving circuit is used for the baffle step compensation circuit to compensate for the rise in amplitude with increasing frequency. After the baffle compensation, the signal goes through the low pass filter. Although the target corner frequency for the woofer would be 1.8kHz, this does not necessarily mean that the corner frequency response for the low pass filter would be also 1.8kHz. This is due to the baffle step compensation circuit affecting the woofer response. With this, the low pass filter is designed to have a corner frequency of 2.145kHz to produce an overall filtered woofer response close to the desired 1.8kHz. The low pass filter is a 4th-order Linkwitz-Riley filter constructed by cascading two 2nd-order Butterworth low pass filters. In the high pass filter section of the crossover network; a Linkwitz-Riley high pass filter with a corner frequency of 1.8kHz is used. After the high pass filter section, an all pass filter is added to add delay to the tweeter signal in order to compensate for the path length difference between the tweeter and woofer and also for the group delay in the circuitry. The path length difference depends upon the size and mounting orientation of the drivers as well as the desired listening position. In this design, a 3rd-order all pass filter is used providing 155us delay to the tweeter signal. After the tweeter signal has been corrected with the delay circuit it passes through an output level correction and buffering circuit. This circuit is used to attenuate the output level of the tweeter signal given that the tweeter driver has a higher sensitivity compared to the woofer. In the design, 8dB attenuation is introduced to the tweeter signal for the speaker system to produce a fairly flat response. Upon selecting the level of attenuation, aside from the sensitivity of the drivers, the baffle step compensation effect in the woofer signal is also considered. The drivers used to attain this design values are 87db woofer and 91dB tweeter.


After the crossover network, the tweeter and woofer signal goes to separate amplifiers. The amplifier used in this circuit is the TPA3106D1. It is a 40W mono class D audio power amplifier. Each channel has two amplifiers, each for the tweeter and woofer signals. A volume control is included in the circuit, placed in each output path of the crossover network for the user to balance the volume in each speaker to his preference.

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