powerampliﬁerdesignproject - wordpress.com...my general approach to designing this audio power...

Power Amplifier Design Project

Project report

Flagstaff, AZNovember 1st, 2018

Edited by

Michael FinleyNorthern Arizona University

CONTENTS Power Amplifier Design Project: Project Manual

Contents

1 Project Overview 3

2 Problem Description 3

3 Solution Plan 3

4 Implementation 54.1 Schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54.2 Simulations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54.3 Layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64.4 Populated Boards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

5 Resources 75.1 Software . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85.2 Textbooks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

6 Conclusion 8

of 9

Power Amplifier Design Project: Project Manual

1 Project Overview

This project manual contains a problem description section, a solution plan, and an implementation. Theproblem description describes the events that created the need for this project. The solution plan detailshow I planned to go about solving the problem. The implementation details how the problem solving waseventually put into place. And lastly the resources section details what resources were utilized in this projectto create a solution, including texts, and programs for simulations

2 Problem Description

A stereo receiver I owned and used in my personal entertainment system had a malfunction one day duringregular usage. Upon turning off and on the stereo receiver it would turn on but not allow any sound toreach the passive speakers. I noticed a protection relay was not clicking as per usual after a short amountof time like regular so I decided to bring the stereo into a research lab I worked in to do some trouble shooting.

My debug process involved probing various parts of the receiver with a digital multi-meter looking foroddities in node voltages that might be allowing the relay circuit to protect the passive speakers, as therelays are intended to do. Based on the owners and manual and schematics I was able to reference I noticedthat one of the power amplifiers had an irregularly high voltage at a specific node which lead me to believethat the irregularly high voltage was causing the relay circuit to stop the stereo from allowing any audiosignal to be sent to the passive speakers.

The next step was to determine what was causing the irregularly high voltage coming out of the poweramplifier, as any portion of the stereo receiver prior to the power amplifier. Upon completion of probingthe stereo as best as possible prior to the power amplifier, checking for blown capacitors, questionable solderjoints, and other possible issues, nothing was found to be problematic. So the power amplifier was thefirst proposed component to fix/replace. I was also able to compare node values on the power amplifier forSpeaker B, as only Speaker A’s power amplifier was seemingly not working. But the owners manual andschematic was used as the ’true value’ for comparison in case the power amplifier for Speaker B was alsoproblematic and there was another underlying issue going on.

The problematic component was an STK-0050 power amplifier which implements a darlingon pair of BJTtransistors for current amplification. Upon researching the part online, finding schematics, and data sheetsI was able to find out that apparently a lot of aftermarket "bad parts" have been made as replacements forthe STK-0050 so purchasing a replacement part would be just delaying a long-term solution. I decided itwould be fun to just design my own power amplifier mimicing the STK-0050 architecture based on the datasheets for the part and with using audio amplifier design textbooks I had available to myself.

3 Solution Plan

My general approach to designing this audio power amplifier was to reference heavily the data sheets andschematics provided to develop my design. First I wanted to create a schematic based on the equivalentcircuit in the STK-0050 data sheet, shown in Figure 1.

Unfortunately I was hoping for more information than what is found in the previous figure. But with someresearch I was able locate some more information for my design, see Figure 2 and Figure 3.

of 9


Figure 1: Equivalent circuit from owners manual, data sheet

Figure 2: Desired values for components, supplies

So at this point I had a general circuit to build off of, specific component values, and some information aboutthe supplies. I then developed a revision of this circuit using a textbook reference for designing audio ampli-fiers, Designing Audio Power Amplifiers by Bob Cordell. More specifically, the design seen in Figure 4 wasreferenced for the final design which would go through simulations before doing board layout considerationsand parts ordering.

of 9


Figure 3: Equivalent circuit from data sheet

Figure 4: Power amplifier design from Designing Audio Power Amplifiers by Bob Cordell

4 Implementation

This section details the implementation starting from schematic capture, simulation, to layout design, toboard mounting and population.

4.1 Schematic

The final schematic can be seen in Figure 5 below:

4.2 Simulations

I designed the circuit in dip trace for schematic capture as well as layout as they were easily available andopen source. Simulations were done in LTSpice, again, because of their availability and being open source.The main reason for simulation was to confirm the working behavior of the circuit and quantify gain andexpected total harmonic distortion as useful performance metrics. Total harmonic distortion (THD) is acommon performance metric used to market receivers and or amplifiers and so simulating this was beneficialand worthwhile. I also wanted to be sure that I wasn’t significally lowering the performance capabilities of

of 9

4.3 Layout Power Amplifier Design Project: Project Manual

Figure 5: Final power amplifier circuit design (after simulations)

my stereo with my replacement power amplifier circuit. Worst case expected THD of an original STK-0050(based on the data sheet) is .1% THD. The expected, or simulated, gain of the power amplifier replacementboard was 23.5dB. A table of the THD expectations of the power amplfifier replacement board from LTSpicecan be seen below:

THD (%) Frequency (kHz) Output Resistance (Ohm) Current Bias (mA)

0.0002 1 4 310.0197 20 4 310.0001 1 8 310.0116 20 8 31

4.3 Layout

The layout was fairly simple as there was a generous amount of room to place the part and with moderncomponents (SMD packages) available I knew I would be able to fit everything in an efficient manner whilestill fitting onto the form factor of my desired heat sink. Auto routing in dip trace yielded good results but Iended up hand-routing my traces so as to have a board that was visually pleasing to my preferences as therewas plenty of real estate left on the PCB dimensions I specified and intended to use. In reality, the cheapestway for me to order PCBs required a minimum amount and a minimum size so I used that minimum size asmy maximum board space for cost efficiency.

4.4 Populated Boards

Figure 6 showcases a populated board, sans heat sink and silicon pads to insulate the heat waste tab ofthe BJTs. The first iteration of the board allowed me to learn how to properly use the TO-220 packageswhich have an intended use in power semiconductors and are typical in through-hole electronic design withlarge power consumption. I did not properly ground these tabs which resulted in me shorting my board andfrying my first prototype board as my heat sink was not properly grounded. Upon populating componentsonto my second board I was sure to properly mount my BJTs to my heat sink with silicon pads and thermalcompound to properly utilize my components, but did not know to purchase an insulating washer and soI ended up improperly mounting my BJTs again and shorting my circuit to ground inadvertently causing

of 9


component failure. Figure 5 showcases the issue, for clarity, and I now have a much better understandingof integrating heat sinks into my power amplifier designs moving forward after learning my lesson the hardway, twice.

22 AWG was used for mating the board to the stereo receiver and functioned as a nice convenient fuse tomy power amplifier. I have not run into any issues yet with this mating method.

Figure 6: Components mounted, no heat sink attached yet

Figure 7: Proper mounting of non-electrically isolated tab TO-220 packages

Figure 8 showcases a populated board mounted onto a heat sink, properly, with the daughter-board properlymounted in place of the original part.

5 Resources

This section details the various resources I used for this project to bring it to a successful closing/conclusion.The audio amplifier textbook proved to be most useful and educational with regards to designing the poweramplifier and understanding how it worked but the data sheets and user manuals were a necessary first stepto find a better solution to implement in the end.

of 9

5.1 Software Power Amplifier Design Project: Project Manual

Figure 8: Populated board, mounted to heat sink, soldered to receiver

5.1 Software

• DipTrace: an open source Schematic and PCB Design Softwarehttps://diptrace.com/Used to create schematic and layout for PCB design

• LTSpice: an open source Spice simulation softwarehttps://www.analog.com/en/design-center/design-tools-and-calculators/ltspice-simulator.htmlUsed to simulate circuit and characterize expected behavior/performance and confirm working behavior

5.2 Textbooks

B. Cordell, Designing audio power amplifiers. New York: McGraw-Hill, 2011.

This textbook was borrowed from heavily for guidance in designing this power amplifier circuit.

6 Conclusion

I learned a lot during this project. I was able to debug a system with bench-top tools. I read through datasheets and service manuals to understand the problem and understand how I might implement a solution.I got to practice amplifier design for audio signal paths, I learned hands-first about the various groundingissues and heat transfer nuances of high voltage components in audio systems and how troubleshooting themcan be important prior to board population, I practice schematic capture and layout, as well as spice-basedsimulation. Furthermore I practiced some through-hole PCB component population.

What I am most proud of in this project was my addition of a resistor trim pot to cancel out any off-set in the system which improved performance. I was also impressed with how similarly my power ampli-fier matches the performance of the original STK-0050 power amplifier through comparing Speaker A and

of 9


Speaker B outputs when listening to audio. I also developed a greater appreciation for how serviceable oldconsumer electronics used to be, something no longer currently available in consumer products in most cases.

Moving forward I would like to consider designing this with exclusively SMD packages to see what kindof minimal form factor is achievable for this circuit. I would also like to improve my soldering skills a bit soas to clean up the final product a bit.

of 9

powerampliﬁerdesignproject - wordpress.com...my general approach to designing this audio power...

Documents