Class T Audio Amplifier Project Documentation
XXXXXXX June 17, 2007 Elex 146 – Applied Electronics Techniques Instructor: James Van Oort
Table of Contents Overview…………………………………………………………………………...……………………..2 Schematic ............................................................................................................................ 3 Libraries ............................................................................................................................... 4 Footprints ............................................................................................................................. 4 PCB ...................................................................................................................................... 5 Assembly .............................................................................................................................. 6 Testing……………………………………………………………………………………………………..9 Conclusion………………………………………………………………………………………………..9 Appendixes: Original Timeline Schematic Bill of Materials PCB Layout
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Overview Applied Electronics Techniques is a project based course that brings together all of the fundamentals that we learned in the previous courses. In this course, we had the choice of making a Power Supply, Audio amplifier, or a personal project. I chose the Audio Amplifier because I will use it a lot when it is finished. Also, the demo of a similar design from last year that I heard at the open house sounded fantastic.
Figure 1-TA2024C Chip I used the TA2024C Class T Amplifier chip form Tripath. It is relatively new technology and has both high power efficiency (81%) and high fidelity. It has 4Ω Impedance and 15W per channel. Figure 1 shows a typical performance of the chip:
Figure 2- Typical Performance of the TA2024C In order to implement this chip into a useable apparatus, I had to complete the entire fabrication process from schematic design to Assembly. The steps involved were as follows: Design Schematic Add Libraries and Footprints Complete PCB Layout including rules and ground planes Have board printed by Elexprep Populate the board with the parts Design/purchase case Wire the buttons, inputs and outputs. Test all the parts Troubleshoot if necessary Document the process 2
Schematic The schematic was the first stage of the project design. In our case there was a framework that we could to follow that was supplied by the instructor. Also, the data sheet for the chip had specifications for the pin outs as well. I would recommend using both because there was the following omission on the one form the class: From the datasheet from Tripath, pins 4 and 9 both need to be attached to pin 1 in order to get 5 volts. We used Protel (Altium Designer) to design the entire PCB from schematic to Gerber files. The design was fairly straight forward. It was mostly just finding the schematic symbols for our parts in Libraries (I will go over these later), and joining them with wires. There were a couple of optional circuits on the schematic that I decided to include. They were: Fault LED This was a connection from the main chip through the secondary LED chip (U2) to an LED. The LED will come on when there is a fault in the circuit. Overload LED The same as above however the LED will come on when the output power passes a certain threshold. It is important to note as well that the positive and negative leads on the audio outputs are reversed between the left and right speakers. Please not that the full-sized schematic that I made is attached in the Appendices.
Figure 3-Schematic Document
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Libraries Libraries are a collection of information about a number of specific parts. They include the schematic symbol, footprint and other technical specifications. Once again the Libraries were supplied by the instructor as they are specific to the project and the project had already been done last year. They were called: Audio Amp Project.PCBLIB Audio Amp Project.SCHLIB He supplied them thought the elexpub and the through his website at: vanoort.disted.camosun.bc.ca
Footprints The footprints are the design of the pads that the parts have to be placed on in order to be soldered properly. They were supplied by the instructor as the project had been completed by the previous year’s class. Some of them were pretty standard (surface-mount Resistors and Capacitors), while others had to be designed by the instructor (U1 and U2). It is very important that these are correct or the board will not be able to be populated properly.
Figure 4-Footprint of U2
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PCB The PCB Layout is probably the most challenging part of the whole design process. It involves logically arranging the parts onto a board in a way allows for all the necessary connections to be made while maintaining short trace lengths for maximum power use. I used Lawrence Cunningham’s, a student from last year, design as a guideline. Apparently his sounded the best because the traces were short which minimized interference. Below is a figure of my completed PCB layout as a reference: Dimensions of board: 3100 x 3600 mils Screw holes width: Inside hole width118.11mil Outside Diameter 236.22mil Screw size #4 Screw holes placement: 2780mils x 3300mils (centre) Clearance form edge of board: 50 mils
Figure 5- PCB Design As you can see from the figure, the inputs circuits are around the bottom and the outputs at the top. All parts, but two surface mount capacitors, are on the top layer of the board. The two capacitors were placed on the bottom layer because they did not fit on the top the way it was designed. The layout is designed the way it is to minimize the trace length to maximize power efficiency and minimize cross talk. The width of the traces from the cap and main dc power input were 70 to 100 mils. Most of the others were 30 to 50 mils, and the small input traces were 10 to 25 mils. The ground plane was done last as the rules had to be temporarily changed to accommodate a wider distance between it and the main power lines. This is to prevent any interference or loss of power at the high power level. The distance between the traces on the bottom layer and the ground plane was 20 mils. After completing this designing an making sure that there are no errors, the Gerber and Drill hole files were exported and sent to the technician at
[email protected].
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Assembly PCB: During the Assembly of the PCB, start with the Chip first as it is the hardest to do. Run a bead of solder paste along the bottom that will be melted later for bonding with the ground plane. I would solder the four corners first to be sure it is in place. Then I would carefully dab a little paste on each pin and solder them down one by one. This way you will minimize bridging between pins. After this, start from the smallest components and move up so that room to move is less of an issue. I recommend doing the chip pins and the small components under the microscope to minimize mistakes. After each component is placed, check the continuity to be sure that there is not a short there. Also make sure that there are no shorts to ground from the main power inputs as this could be very damaging to some components.
Figure 6- Sam's PCB CASE: For the Case, I chose the ZN-45 from Polycase. (http://www.polycase.com/item/zn-45.html). It is good size for the parts that need to fit in it and it is made of a soft plastic that is easily drilled.
Figure 7- ZN-45 Case
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The components that need to be fitted to the case were: The Power button The Volume pot The LEDs for Overload and Fault (I included another spot for one in case I wanted to add something later) The audio inputs both RCA L/R and 3.5 mm stereo input The 4 speaker outputs The DC power input
Figure 8- Case Front
Figure 9-Case Back
The choice of audio inputs was optional but I decided to go with it. With that, we needed to tie the two connections to each other in parallel. We also had a choice of a battery pack or a DC plug that would go to an AC wall adapter. I choose to go with the DC power although you could have them both there and have it switch between them when necessary. When drilling the holes for the various inputs and out puts, I used a drill press with a ¼ inch drill bit at first and then used a hand file to do the precision fitting. This was because the parts were all roughly the same size, apart from the power button which I did in sections with a bigger drill bit. It was easy because the plastic is quite soft. Table 1- Buttons and Connectors
Figure 103.5mm Audio Input
Figure 11RCA Input
Figure 12Pin-outs for Pot
Figure 13Power Button
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For the wiring, it is better to use different kinds of wire for the different inputs and outputs. I used: Table 2-Wire Types Audio Inputs Stranded Wire
Audio Outputs 16 Gauge Speaker
Power Inputs Heavy Stranded
Led Connector Solid Core
Table 3- Audio Amp Top Interior
Check the data sheet for the Potentiometer to get the correct pin outs for the audio inputs. I have included a small picture of them in Table 1.
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Testing When testing it is important to remember a few things: 1. Clean off the board thoroughly with alcohol and/or compressed air. 2. Look under the microscope for any solder balls that can be joining traces or pins. 3. Apply a small DC voltage at first (1V) with the current limited to almost nothing. Ramp it up to the desired 12V. 4. When that is functional, apply a sine wave from the Function generator to the Audio input (500mVpp, 1 kHz). 5. Use the Oscilloscope to measure the output. Vary input if necessary. Be sure to be aware of any smells or excessive heat coming from the board. This way you might catch a problem before it damages any components. If you have to use the DMM to check voltages on the board, be conscious of what you are doing. Even with the power off, there is a lot of power that can come from the large capacitor.
Conclusion This is a fantastic project for someone just finishing the Electronics Technician Program. I brings together all of the things that we have learned over the last year, and puts them into a product that can be very useful. IN retrospect, the most important things to remember would be to plan ahead, be patient, be careful and manage your time. Any under taking such as this takes time and care and should not be rushed.
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Bill of Materials (for PCB) Description
Designator
Footprint
LibRef
Quantity
Diode (D1)
Diode (D1)
1
Capacitor
C5
CC2012-0805
Cap
1
Capacitor
C6
CC1608-0603
Cap
1
Capacitor
C7
CC1608-0603
Cap
1
Capacitor
C8
CC1608-0603
Cap
1
Capacitor
C9
CC1608-0603
Cap
1
Capacitor
C10
CC1608-0603
Cap
1
Capacitor
C11
CC1608-0603
Cap
1
Polarized Capacitor (Radial)
C12
CC2012-0805
Cap Pol1
1
Polarized Capacitor (Radial)
C13
CC2012-0805
Cap Pol1
1
Polarized Capacitor (Radial)
C14
CC2012-0805
Cap Pol1
1
Polarized Capacitor (Radial)
C15
CC2012-0805
Cap Pol1
1
Capacitor Capacitor (Semiconductor SIM Model)
C16
CC1608-0603
Cap
1
C17
CC1608-0603
Cap Semi
1
Polarized Capacitor (Radial)
C18
Bulk Capacitor
Cap Pol1
1
Polarized Capacitor (Radial)
C19
Bulk Capacitor
Cap Pol1
1
Polarized Capacitor (Radial)
C22
CC2012-0805
Cap Pol1
1
Capacitor
C23
CR1608-0603
Cap
1
Capacitor
C24
CC1608-0603
Cap
1
Polarized Capacitor (Radial)
C25
CC2012-0805
Cap Pol1
1
Capacitor
C26
CC1608-0603
Cap
1
Polarized Capacitor (Radial)
C27
CC2012-0805
Cap Pol1
1
Polarized Capacitor (Radial)
C28
CC2012-0805
Cap Pol1
1
Capacitor
C29
CC1608-0603
Cap
1
Capacitor
C30
CC1608-0603
Cap
1
Capacitor
C31
CC1608-0603
Cap
1
Polarized Capacitor (Radial)
C32
PS Capacitor
Cap Pol1
1
Capacitor
C33
Cap
1
Zener Diode
D2
D Zener
1
Schottky Rectifier
D5
Diode 10TQ035
1
Schottky Rectifier
D6
Diode 10TQ035
1
Schottky Rectifier
D7
Diode 10TQ035
1
Schottky Rectifier
D8
CC1608-0603 Diode (Shottky and TVS) Diode (Shottky and TVS) Diode (Shottky and TVS) Diode (Shottky and TVS) Diode (Shottky and TVS)
Diode 10TQ035
1
Fuse
F1
Fuse Mount
Fuse 2
1
Inductor
L6
Inductor
Inductor
1
Inductor
L7
Inductor
Inductor
1
Inductor
L8
Inductor
Inductor
1
Inductor
L9
Inductor
Inductor
1
P1
Connector (input) Connector (pwr + spkr) Connector (pwr + spkr)
Input Connector
1
Power Connector
1
Spkr Connector (L)
1
P2 P3
10
P4
Connector (pwr + spkr)
Spkr Connector (R)
1
P5
Connector (LED)
LED Connector
1
P6
Header (2-pin)
Header (2-pin)
1
P7
Header (2-pin)
Header (2-pin)
1
Resistor
R2
CR1608-0603
Res1
1
Resistor
R4
CR1608-0603
Res1
1
Resistor
R5
CR1608-0603
Res1
1
Resistor
R6
CR1608-0603
Res1
1
Resistor
R7
CR1608-0603
Res1
1
Resistor
R9
CR3216-1206
Res1
1
Resistor
R10
CR3216-1206
Res1
1
Resistor
R16
CR1608-0603
Res1
1
Resistor
R17
CR1608-0603
Res1
1
Resistor
R18
CR1608-0603
Res1
1
Resistor
R19
CR1608-0603
Res1
1
Resistor
R20
CR1608-0603
Res1
1
Resistor
R21
CR1608-0603
Res1
1
Resistor
R22
CR1608-0603
Res1
1
Resistor
R23
CR1608-0603
Res1
1
Resistor
R24
CR1608-0603
Res1
1
Tripath Class T audio amplifier
U1
TA2024C
TA2024C
1
U2
Buffer (SOT23-8L)
Buffer
1
VR1
Regulator
5V Regulator
1
For a complete list of the parts, go to: http://vanoort.disted.camosun.bc.ca/Audio%20Amp%20Project/Protel/Audio%20Amp%20 BOM.pdf
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Figure 14 - Schematic
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Figure 15- PCB Layout (this doesn’t include Ground plane to improve visibility)
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