Introduction

Problem

Typically, audio amplifiers operate exclusively on 120VAC (home applications) or 12VDC (automobile applications). This design excludes the use of home amplifiers in automobiles and vice versa. For the user who does not want to purchase several amplifiers, it would be advantageous to have an amplifier that is capable of operating on either power source. Currently, amplifiers that support both power sources do not exist or are very rare and, as a result, this dilemma provided an opportunity for our design project.

Solution

To address the power source issue, we proposed to design an audio amplifier with 120VAC or 12VDC power capabilities. This AC/DC Audio Amplifier was designed to support two independent audio channels (stereo) fully tune-able via 6-band graphical equalizer circuits. Naturally, standard connectors would be made available for each power source and the appropriate power source would be selectable via a toggle switch on the amplifier unit. For either power source, the power supply circuitry of the amplifier unit transforms that power source into +/- 30VDC @ 5A for the audio amplifier circuits. More specifically, both power sources would be converted into an intermediate 160VDC and subsequently passed to common circuitry that would convert this voltage level into the desired +/- 30VDC. Using this design, the amplifier circuitry can remain unaltered for either power source and consists of the standard design: buffer (pre-amp), graphic EQ, volume/balance, peak limiter, and power amplifier circuits.

Block Diagrams

Power Supply Block Diagram

Audio Amplifier Block Diagram

Block Diagram Description

AC Rectification Circuitry
The AC rectification circuitry converts the 120 VAC signal into the 160 VDC signal that is required by the inverter circuitry.

DC Boost Circuitry
The DC boost circuitry converts the 12 VDC signal into the 160 VDC signal that is required by the inverter circuitry.

Half Bridge Inverter
The inverter circuit transforms a standard input of 160 VDC into a +/- 80 V square wave. This is achieved through making use of an oscillator IC that controls two MOSFETs in a push-pull setup.

Control Circuit
The control circuit samples the output voltage levels and adjusts the inverter circuit to provide more or less power as required.

Buffer Circuit
The buffer circuit presents a high input impedance to the attached device while maintaining sensitivity for the incoming signal.

Graphic Equalizer, Volume and Balance
The equalizer, volume and balance circuits all provide the user with control of the desired output of the audio amplifier

Peak Limiter
The peak limiter circuit protects the power amplifier circuitry by suppressing any extreme voltage spikes on the signal.

Power Amplifier
The power amplifier provides the final current gain required to drive the speakers.

Tentative Project Timeline

The design of this project was initially laid with the following timeline. This timeline was designed for the best case scenario project development including the design and fabrication of the PCB and chassis for the amplifier. Pratical progress did not follow this optimistic schedule.

Task

Persons Responsible

Start Date

Completion Date

Comments

Amplifier cct Design Shawn

14-May-01

28-May-01

part list to Dr. Bhat May 28
DC- DC cct design and testing Mike, Chris

23-May-01

8-Jun-01

simulations as required
Amplifier Simulations Shawn

28-May-01

4-Jun-01

 
Amplifier cct testing Shawn

11-Jun-01

28-Jun-01

dependent on part availability
AC - DC cct design and testing Mike, Chris

9-Jun-01

28-Jun-01

simulations as required
Full cct testing Mike, Chris, Shawn

4-Jul-01

16-Jul-01

 
PCB Design Mike, Shawn

9-Jul-01

16-Jul-01

PCB sent for manufacturing July 17
Chassis Design/ Fabrication Chris

4-Jul-01

16-Jul-01

Add of Mark Ricci (Mech student)
Poster Presentation Mike, Chris, Shawn

16-Jul-01

27-Jul-01

 
Full cct and PCB testing Mike, Chris, Shawn     Dependent on arrival of PCB
Web Page Development Mike, Chris, Shawn

16-Jul-01

3-Aug-01

 
Report Documentation Mike, Chris, Shawn

16-Jul-01

3-Aug-01

 

Actual Project Timeline

The following timeline summarizes the actual progress made on the project given a 4 month development period. A more complete discussion of issues discovered during the development process can be observed in the Power Supply and Amplifier sections.

Task

Persons Responsible

Start Date

Completion Date

Comments

Amplifier cct Design Shawn

14-May-01

28-May-01

part list to Dr. Bhat May 28
DC- DC cct design and testing Mike, Chris,Shawn

23-May-01

  ongoing
Amplifier Simulations Shawn

28-May-01

4-Jun-01

 
Amplifier cct testing Shawn

11-Jun-01

  Final testing still required
AC – DC cct design and testing Mike, Chris

9-Jun-01

  ongoing
Full cct testing Mike, Chris, Shawn     Time did not permit
PCB Design Mike, Shawn     Time did not permit
Chassis Design/ Fabrication Chris     Time did not permit
Poster Presentation Mike, Chris, Shawn

16-Jul-01

27-Jul-01

 
Full cct and PCB testing Mike, Chris, Shawn     Time did not permit
Web Page Development Mike, Chris, Shawn

16-Jul-01

3-Aug-01

 
Report Documentation Mike, Chris, Shawn

16-Jul-01

3-Aug-01

 

Future Considerations

Given that a 4 month development process proved to be an insufficient length of time to complete this project, the following items remain to be resolved. Furthermore, possible enhancements to the original design are including in the discussion.

Power Supply

A large portion of the outstanding work remaining is focused in the power supply section.

  1. The series resonant inverter requires additional troubleshooting to overcome the failure experienced when the input voltage exceeds 60VDC.
  2. The boost circuit requires assembly and testing. It is likely that two cascaded boost circuits will be required to achieve a duty ratio of 0.925.
  3. The control circuit, using negative feedback, requires design and implementation in order to achieve regulated outputs. The SG3524 is equipped with a comparator for such a purpose.
  4. A selector switch is required to connect the AC and DC supplies. The switch must enable one supply, while disabling the other.
  5. The power supply requires testing of voltage ripple. If the voltage ripple is too high modifications must be made.
  6. Fusing and protection circuitry should be implemented in both the AC and DC supplies.
  7. Printed circuit boards need to be designed and manufactured for the power supply.

Amplifier

The work remaining to be done on the amplifier mainly consists of final testing.

  1. The Equalizer circuit needs to be tested to determine the range of gain that is available. Also the Q of the filters should be measured to determine if the 6 bands of equalization is optimal.
  2. The buffer coupled with the peak limiter need to be tested to determine the proper buffer gain. Given a standard 1 Vrms input, the peak limiter should just start acting when the volume is at a maximum. Once the gain of the buffer is set, the gain of the power amp can be adjusted so that no clipping occurs.
  3. Improvements that could be made to the amplifier involve adding a microprocessor to the circuit. This would allow for the use of active switches to disable the balance control, disable the equalizer section, and utilize the mute and standby function of the LM1876. Also, a remote control sensor could then be added to control the different functions of the amplifier.
  4. If higher power applications are desired, all that needs to be change on the amplifier are the power ratings of the two current gain transistors. The LM1876 may also need to be replaced with either an op-amp that can handle large voltage rails or a couple of transistors used to form a differential input.

Final Integration

Once the power supply and amplifier sections are complete, integration of the two stages needs to take place. The amplifier must be tested while running from both the AC and DC supplies. The power supply should also be tested under heavy load requirements set by the amplifier. A chassis design incorporating proper heat sinking should also be designed and implemented. Many obstacles stand in the way before the stages can be integrated. At the time of integration, a more rigorous final product testing procedure should be developed.