Group 2: High power audio bass amplifier

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Group 2 High power audio bass amplifier

• Nithin Cherian
• Steve Lapen
• James Sears
• ECE 345
• Spring 2002

2
Objectives

• Use switching power processing techniques to
  achieve
• Product Features
• Frequency response 30Hz - 1,000Hz
• Efficiency 50 or better (at normal listening
  volume)
• Peak instantaneous output power 1000W (into 2?
  load)
• Maximum time-average output power 500W (into 2?
  load)
• Product Benefits
• Efficiency superior to that of a linear amplifier
• Reduced heat production
• Minimal distortion of audio signal

3
Preamplifier basics

• Amplify signal to usable level
• Add Controls for User Interface
• Filter signal frequencies that are too high for
  proper amplification
• Limit signal to an acceptable level for PWM
  generation

4
Preamp Components

• Voltage Buffer
• Inverting Gain Stage
• Overall LPF to limit output frequencies to 20
  1000HZ
• 3 Band EQ
• Inverting Summer

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EQ

• 3 Bands
• LO 0 250Hz
• MID 250 750Hz
• HI 750 1250Hz

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EQ Filter Implementation

• Utilized LP, BP, HP Butterworth Filter Model
• 2nd Order Filters using Sallen-Key Topology

8
Preamp Hi Band

• HPF response

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Preamp Lo Band

• LPF response

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Preamp Mid Band

• BPF response

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Preamp Design Problems

• Ground Noise
• Filter Resonance
• Precise Components for prototyping
• Output Voltage amplitude
• Integration with PWM circuit

12
PWM Generation

• Triangle wave generation
• Compare triangle with incoming signal
• Duty cycle varies with input signal

13
PWM Generation (component selection)

• Function Generation
• MAXIM MAX038
• Wide range of stable oscillation (0.1Hz 20MHz)
• Output symmetrical about ground
• Comparator
• MAXIM MAX913
• Stable with slow moving inputs
• Low power consumption (6mA _at_ 5V)
• Ultra fast propagation delay (10ns)
• Dual Supply Operation

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PWM Generation results

• Noise coupling through supplies
• Supply decoupling
• Attention to layout
• Short wires
• Short component leads
• Stable operation

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PWM Generation results (contd)
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Dead-time
Prevents both FETs from being on
simultaneously Two general approaches Inverter
propagation delay Asymmetric RC delay
17
Power Bridge

• Half bridge topology consists of two MOSFETs
  operated as switches
• The purpose of the half bridge is to alternately
  connect the load to positive and negative DC
  voltage such that the output is AC.

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Choosing Power Components

• FETs
• - Blocking voltage
• - Current capacity
• - On-state drain-source resistance
• - total gate charge

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Choosing Power Components

• Diodes
• -Ultrafast reverse recovery necessary to avoid
  FET damage
• Gate resistors
• -Proper resistance is required to avoid
  deleterious ringing.

20
Output Filter

• MATLAB design for inductance and capacitance
  values
• Saturation constraints
• Good agreement between idealized design and
  reality

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Problems Addressed

• Noise coupling between high-power and low-power
  ends of circuit
• Steps taken
• Numerous layout revisions
• Metal shielding
• Increased dead time
• RC filters on optocouplers
• Reverse parallel FETs
• Gate resistance adjustments

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Describe project build and functional tests

• Protoboard test
• Illustrated the importance of layout
• Permanent construction
• Test functionality
• Lowered voltage / smaller load

23
Testing

• Efficiency
• Output Power
• Frequency Response
• Gain linearity

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Efficiency

• Power out / Power in

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Max Output Power
Average power 140 W Peak instantaneous power
280 W Load current 11.3 A amplitude
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Frequency Response

• Measures output level vs. frequency
• System is flat (within /- 3dB) over the range
  20Hz 5kHz

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Gain linearity

• Measures the increase in output level for a
  corresponding increase in input level

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Future Improvements

• Add proportional integral controller to eliminate
  DC offset.
• Pinpoint cause of unsymmetrical gate drive glitch
• Consolidate entire circuit and enclose in housing