Feedback In Amplifier

Entry # MT2292

Digital Subwoofer Amplifier With Electromotive Feedback

Nowadays, a home cinema system is present is almost every household. Their price range is very wide – from $100 to tens of thousands of dollars. Usually, the subwoofer of a low or middle end system is small so it needs to be with ported design to achieve reasonable response in low frequencies. For movies this seems to be allright, because the subwoofer is used for effects. For playing music, though, the ported design is a problem – it has greater total harmonic distortions (THD) and group time delay than the closed box. To have good bass response in a relatively small closed box subwoofer, speaker driver is usually pushed to its limits where it has high THD. In this project one very interesting approach is used for reduce the distortions of the speaker and also to extend its bass response. A dual coil speaker driver is used but only one of the coils is connected to the power amplifier – the other is used for electromotive feedback. This method is not new, but is difficult to implement in pure analogue world because of the many parameters that have to be tuned. For easier experimenting and tuning, a dsPIC30F2020 device is used. It not only implements a ProportionalIntegral-Derivative controller for the feedback, but with its powerful SMPS controller and ADC drives a ClassD amplifier for the subwoofer! This type of amplifier, called also “digital” amplifier uses pulse-width modulation (PWM) to drive power transistors in switching mode. The main benefit is the losses are very small compared to the ClassAB power amplifier. The heat dissipated and more power can be transferred to the speaker driver with only small heatsinks used. Two of the bonus parts for the contest are used – the dual MOSFET driver TC4427A (here used for fast level shifter) and the dual lownoise operational amplifier MCP6022 (used for input and feedback signal amplification and filtering). Also, for easing the design process, the 28-pin Starter Board together with the ICD2 programmer was used.

The block diagram of the device is presented on Figure 1. It is seen that the dsPIC resources are the core to the system.

dsPIC30F2020

SMPS PWM

PWMH, PWML

MOSFET Drivers

H-Bridge

Active coil Digital Low-pass filter

Digital PID Contoller

Closed box subwoofer

Dual-coil Speaker

ADC

Input Amplifier

ADC

Velocity Feedback

Audio Input

Figure 1 The schematic block diagram

Band-pass filter

Passive coil

Power Supllies

There are a lot of formulaes assossiated with this theme, but since they are widely available they are not presented here. Some important points need to be said, though: 1. The closed box subwoofer sound pressure level (SPL) decreases by 12dB/oct at low frequencies. 2. The higher the volume of the closed box is the closer the resonant frequency of the speaker-box system is to the open-baffle resonant frequency of the speaker. 3. The second voice coil voltage should be processed by 2nd order filter to be proportional to the cone velocity. By having these in mind, the theory of operation can be easily understood: The input audio signal is amplified and then digitalized by the dsPIC’s ADC. Since this is a subwoofer, the signal should not contain higher than some frequency (here 150Hz chosen). This is provided by a Digital Low-Pass FIR Filter. The output of this filter is the signal which the subwoofer shall output as a sound. That’s why this signal is the setpoint of the digital PID controller, also realized inside the dsPIC. The feedback for the PID controller is the digitalized voltage from the passive coil via an analog bandpass filter. This voltage is proportional to the velocity of the speaker’s cone. Output of the PID controller drives the PWM – a part of SMPS’s controller inside the dsPIC30F2020. The very high operating frequencies of the PWM and ADC modules allow noise-free ClassD amplifier and fast and accurate control loop. The power section of the amplifier is built with MOSFETs which are driven by two-stage drivers.