High Power Audio Bass Amplifier by fjzhxb

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									High Power Audio Bass Amplifier
Proposal for Senior Design Laboratory

Steve Lapen, lapen@uiuc.edu Nick Sears, jsears@uiuc.edu Nithin Cherian, ncherian@uiuc.edu

Department of Electrical and Computer Engineering

Project Number 2 TA: Dave Crowe
February 6, 2002

I.

Introduction

1. Title: High Power Audio Bass Amplifier This project was chosen because all members of the team have an interest in audio amplifier design and because the energy efficiency of most common audio amplifiers leaves room for improvement. Specifically, linear amplifiers used at normal listening volume run at approximately 30 percent efficiency. By using a switching power processing scheme rather than standard linear amplification methods, we propose to build a more efficient amplifier with sound quality comparable to a linear amplifier of similar size. 2. Objectives The purpose of the project is to build a more efficient bass amplifier by implementing switching power processing techniques. Specific goals for the finished product are listed below. Product Features Signal output 30 Hz - 1,000 Hz Efficiency of 50% or better at normal listening volume Peak instantaneous output power of 1000 W into a 2 load Maximum time-average output power of 500 W into a 2 load

Product Benefits - Efficiency superior to that of a linear amplifier - Reduced heat production - Minimal distortion of audio signal

II. Design 1. Block Diagram: Switching Audio Bass Amplifier

DC voltage source +50 Vdc -50 Vdc

Low power analog audio signal

Capacitive filter

Adjustable low pass filter Shielding

Low power voltage converter for logic circuitry

Preamplifier stage with adjustable gain and protection circuitry

Signal processing circuitry (Converts analog audio to PWM digital logic signal)

Gate drive interface and isolation circuitry (converts digital signal to voltage and current levels necessary to drive the gates of power FETs)

Power FET bridge

Protection relay

Lowpass power filter

2 load

2. Block diagram explanation: Speaker, low power audio source, and DC voltage source: These components are external to the project, and will be purchased or borrowed in complete form. Rather than building an amplifier that is marketable directly to

consumers, this project is geared towards providing an alternative amplification solution to audio equipment manufacturers. Since the DC source is very similar to that required for a linear amplifier, an established audio equipment manufacturer could simply adapt an existing power supply to this amplifier with minimal complications. An integrated DC source is excluded from this project because DC power supplies are readily available and because building one would make the completion deadline unrealistic. Shielding: Switching of high currents and voltages in the power-handling subcircuit creates a significant amount of electromagnetic noise at the switching frequency and its harmonics. The noise is combated in several ways, and shielding is one of the most important elements that ensures interference with other equipment is negligible. Input and output low pass filters: Both of these are passive analog filters composed of capacitors and inductors. The purpose of the input filter is to remove the upper end of the audio band since the bass amplifier is not designed to accommodate this frequency range. The purpose of the output filter is to remove the switching waveform generated by the power processing subcircuit and to condition the signal before it is sent to the speakers. The output filter is a critical contributor to the overall audio quality of the amplifier. Low power voltage converter for logic circuitry: Depending on the total power and galvanic isolation requirements of the signal processing circuitry, this converter may be implemented with a commercially available single-chip regulator or a specially made flyback regulator that is powered by the DC input source. Preamplifier stage: This subcircuit is an interface between the audio source and the internal signal processing subcircuit. It regulates the signal level so that it falls within the range that can be handled by the signal processing circuitry. Adjustable preamplifier gain will provide one way of controlling the net amplifier gain. This circuit may also include a protection function which prevents unexpectedly strong or destructive signals from reaching the signal processing subcircuit. Signal processing circuitry: The “brains” of the amplifier, this subcircuit converts the analog audio signal to a high frequency logic level square wave. The frequency of the square wave must be much higher than the 1000 Hz upper limit of the amplification range for acceptable audio reproduction. The subcircuit implements a pulse-width modulation (PWM) signal conversion scheme, which means the square wave generated will have a “moving” duty cycle which is proportional to the amplitude of the audio signal at any given moment. This subcircuit is also the insertion point for any necessary feedback from the amplifier output.

Gate drive interface and isolation circuitry: This is the connection point between the signal processing subcircuit and the powerFET bridge. The logic signal from the signal processing subcircuit cannot be used directly to drive the gates of the power FETs because of mismatched voltage requirements. In addition to this, the high gate capacitance of the power FETs requires a specialized drive circuit to provide fast gate charging and discharging. Galvanic isolation between the logic circuit and the power circuit may be implemented with optocouplers. As a whole, matching the different needs of the logic circuit and the power circuit is the responsibility of the gate drive interface. Power FET bridge: The general topology of this subcircuit is relatively simple. Power FETs are used to alternately connect the amplifier output (through the final low pass filter) to positive DC voltage and negative DC voltage, producing a high powered square wave with duty ratio proportional to the audio signal. Though the general topology is straightforward, this is the heart of the power amplifier, and minor details here will have a profound impact on the overall performance of the circuit. This subcircuit is also the source of significant electromagnetic noise, which means it is a good place to implement preventative measures. “Soft switching” techniques can reduce the sharpness of the square wave, which will cut down the emitted noise. Power loss will also be most significant here, so snubber circuits or clamp circuits may be necessary to optimize efficiency. The output of this stage is a high-powered ac signal which will be conditioned to audio quality by the low pass output filter. Protection relay: This timer-controlled device keeps the speakers disconnected during turn-on and turnoff transients of the amplifier. These transients, caused by capacitances charging internally, can be destructive to the speakers. The relay may also be used as a protection measure which will disconnect the amplifier in the event of a short circuit load. 3. Performance Requirements: -Flat amplifier gain from 30 Hz - 1,000 Hz -Efficiency of 50% or better at normal listening volume -Peak instantaneous output power of 1000 W into a 2 load -Maximum time-average output power of 500 W into a 2 load -Minimal distortion of audio signal

III. Verification 1. Testing Procedures: Testing of the amplifier will be conducted using suitable resistors (separate trials with

2, 4, and 8 Ohms) in place of the speaker as a load, measuring the output voltage across this resistor with an oscilloscope. Since the resistors will necessarily be very large and possibly wirewound, they will have significant reactance. For meaningful testing, this reactance must be similar to that of a speaker load in the relevant frequency range (30 Hz -1000 Hz). Final testing and verification will be conducted with real speakers. Maximum Output Power A signal generator will be connected to the input of the amplifier, producing a sinusoid having frequency within the specified amplification band. The test will be done at the extremes of the band as well as in the middle. The gain of the amplifier is increased until “clipping” occurs or audio quality becomes unacceptable. The level is then reduced back to the highest acceptable level and the maximum amplitude of the sinusoid at the output can be measured. From this voltage, the maximum output power (peak and average) can be calculated. Efficiency A signal generator will be connected to the input of the amplifier, producing a sinusoid having frequency within the specified amplification band. The current drawn from the power supply will be measured, as well as the output power of the amplifier. The efficiency at a given output power is the ratio of the output power to the total power drawn from the power supply. The measurement will be taken and recorded for various output levels and various frequencies. Frequency Response Using a function generator, a sinusoid of constant amplitude will be applied to the input. Beginning at low frequencies (< 20 Hz) and sweeping upward, the amplitude of the output will be recorded for varying input frequencies, with the results plotted on a logarithmic scale of amplitude vs. frequency. Dynamic Range With the input terminal shorted to ground, the peak to peak output noise can be measured and expressed in decibel form as the ratio of the maximum unclipped signal from part A to the noise signal measured here. Distortion The amplifier will be connected to a single frequency source and the output will be recorded using HP Vee. A discrete Fourier transform will then be conducted on the data to identify the frequency components of the output signal. Any output frequencies not present in the input signal are considered distortion. This procedure will also be done using two simultaneous input sinusoids, and intermodulation distortion will be measured. EMI Electromagnetic noise emissions due to switching within the amplifier will be measured with a spectrum analyzer.

2. Tolerance Analysis: Facets of the project which require particular attention to detail are listed and explained below: Component layout: When switching large amounts of current, wire inductance and undesired electromagnetic coupling have very significant impacts on circuit performance. Because of this, the component layout must be as tightly packed as possible, and wires carrying signals that might interfere should be placed at right angles to one another. Rds of the power FETs: Most of the power lost in the circuit is dissipated in the switching FETs. Minimizing Rdson reduces the on-state losses and improves overall efficiency. Thermal stability of all components: Heat will be generated in the amplifier, so the circuit components must behave predictably under cool and hot conditions. Low ESR in capacitors filtering the DC voltage source: Since power is drawn from the input source in high-current pulses, it is critical that high quality capacitors be used to maintain a steady rail voltage. Capacitors with low ESR will minimize deleterious voltage ripple normally caused by the pulsed loading of the source.

IV. Cost and Schedule 1. Cost Analysis Labor Cost: (3 people)*($30 / hour)*(2.5)*(120 hours) = $27,000

Parts List and Cost:

Part Description Power MOSFETS Misc. Inductors Misc. Capacitors Protection Relay Op Amp Casing(EM shielding) Processing circuitry ¼" jacks Heat Sinks 250Kohm Potentiometer

Price Per Unit ($) 20.00 varies varies 5.00 3.00 10.00 varies 1.00 8.00 3.00

Quantity 4 x x 1 1 1 x 2 4 1

Line Total ($) 80.00 20.00 20.00 5.00 3.00 10.00 20.00 2.00 32.00 3.00

Total Parts Cost

195.00

Total Cost = Labor Cost + Total Parts Cost = $27,195.00

2. Schedule

Week

Prospective Goal

2/4/02 2/11/02 2/18/02 2/25/02 3/4/02 3/11/02 3/18/02 3/25/02 4/1/02 4/8/02 4/15/02 4/22/02 4/29/02

Complete and submit proposal Create overall circuit schematic and order needed parts Prepare schematics, parts list, part order status, and simulation results for design review Test DSP with preamp to find parameters of the input signal and build all other subcircuits Debug subcircuits and combine them Debug system as a whole, design shielding, and address thermal concerns Spring break, individual mock demo preparation Mock-up demos Troubleshoot remaining problems and optimize circuit Outline presentation, start final report Complete and rehearse formal presentation Final demo, presentation Turn in final report


								
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