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December 15, 2005
For the Fall 2005 ECE 164: Electronic Design Project class, we chose to create an amplified audio distribution system for home audio entertainment. From early meetings in which we sketched out the various components necessary to create the device to our 'final week of lab sessions in which final connections were made and successfully demonstrated, we encountered and overcame a variety of engineering problems, adapting both our own skills and the specifications of the product in order to create a custom circuit that fulfilled the original design goals of the project. This report presents the design of the project in detail and chronicles the process of designing and building the final system.
The purpose of this system is to consolidate multiple output sources into one controlling unit and redirect the output of each device to any combination of “zones”. At each zone, a keypad controller will select one of four sources and use an integrated amplifier for playback to stereo speakers in the zone. By combining certain aspects for media output into one system with analog routing, one array of source components may be shared by an entire household instead of having multiple redundant source units scattered throughout the house. This results in significant cost savings to the consumer. Furthermore, this basic signal routing implementation provides all the necessary foundation functionality of many more complicated systems that would typically be many times more expensive. Aimed at the entry-level distributed audio market, this system provides substantial core functionality at minimal expense.
The system has three primary components:
- Main routing board with source inputs and zone audio outputs
- Keypad modules for each zone to control sources
- Amplifiers at each output zone for speaker-level playback
The main board provides the platform for signal routing. It receives an asynchronous discrete signal from the keypads in every zone that interfaces with an integrated "click-free" SSM2404 Quad Audio Switch (Appendix F) to pass the audio signals from the sources through to the appropriate zone. The model shown in the pictures and schematic (Appendix C) is only for two output zones and is limited to three possible input devices and an “off” position. Increasing the size of this to four output zones is a trivial matter of properly connecting two more SSM2404 chips, which could be supplied as a plug-in module. To adapt the main board for use with left and right channel stereo inputs, the setup as shown simply needs to be duplicated and attached to the switch controls for the complimentary chips.
The keypad is the controller for a specific output zone. It contains a single 4 position rotary switch which can select three devices or turn the output off, and a potentiometer to adjust the local amplifier gain. The input devices are assumed to be always on, so the user at any zone can not turn any device off, but can simply turn off the output to their specified zone. The source selection signal is passed along an 8-wire cable (such as CAT-5) to the main board, with connections for the logic voltage, ground, two for the outputs (left and right), and four for device selection. Multiple zones requesting the same source are allowed. The power supply provides the +5V routing signal voltage to the circuit.
Discrete amplifiers are used to drive the speakers in each output zone. Although only one amplifier was prototyped, a pair of identical amps would be used to deliver a stereo signal. At roughly 20 watts per channel, it provides adequate power for moderate sound levels in any envioronment.
A TLE2141CP (low noise, high voltage, high slew rate) op-amp is used as the main amplification component. It was chosen because it allows up to 44V of single or split power supply, easily accepting the 12V specification of our project, and has low- noise properties (less than 15nV/√Hz at 1 KHz). The design also includes a transistor stage consisting of two NPN transistors (BC182, TIP42A) and two PNP transistors (BC212B, TIP41A) that step up the output current to drive the speaker. Transistors Q3 and Q4 (see Appendix B) are connected to a metal heat sink to dissipate excess energy. Resistors and capacitors of various sizes protect circuit components and reduce the noise of IC components.
The amplifier circuit was tested using both a dummy 8 Ohm resistor and real 8 Ohm speaker. The output responds well with 100mV to 600mV peak-to-peak input sinusoids varying from 20Hz to 20 kHz in frequency. The output iss consistent and the system response is relatively flat with varying frequency. The output sinusoids are also stable and potentiometer adjustments have predictable effects on the output. With a 100mV input, no clipping is experienced even at the lowest potentiometer value; with inputs up to 600mV and with the potentiometer allowing maximum voltage, the output voltage is limited to 21.4 V peak-to-peak. The amplification multiple should not fall below 35 for normal inputs in our system. When connected to an 8 Ohm speaker, it consistently produces clear sounds of sine waves at different frequencies. The volume is easily adjustable with the potentiometer. When the amplifier circuit was tested in conjunction with the switching circuit and the independent power supply, it produced similarly satisfactory results.
A computer power supply capable of supplying +/- 12 V and +/- 5 V is used to power both the board logic and the output amplifiers in the prototype system. This unit provides all the voltage necessary for the design, as it is designed to deliver 200+ Watts to computer components. In a real-world system, each amplifier would require a local power supply, which could be easily incorporated into the amplifier design as either a linear regulator (simple) or switching power supply (efficient). The entire system is designed to be run from 120VAC @ 60 Hz main power to integrate easily with home wiring systems in the US. Slightly altered power supply design would allow production of a 220 VAC @ 50 Hz European model.
The audio distribution system is ideal for any environment where distributed audio is required, although the simplicity of this implementation make it more suitable for the home market that will not require advanced configuration ability. This system allows individual audio source components such as CD/DVD players, Radios, or portable MP3 players to be accessed from any room in the house at the touch of a button, reducing not only the cost of equipment but also consumer electronic waste. We anticipate great market enthusiasm because of the high demand for audio entertainment systems in the household and the desire for a centralized control mechanism over all different pieces of equipment in the systems. We are pricing our system in line with market expectations, which can be measured by marketing surveys, but will likely be in the $150-$200 range. With a unit build cost estimated to be $145 (see Appendix A), we could potentially profit at $5-65/unit. The product will generate savings for a house of 3 or more rooms that require media access as compared to purchasing equipment separately for individual rooms. At the same time, we will employ incremental pricing techniques where plug-in accessories can be sold separately from the main router to expand the capabilities of the system.
- Prototype: $31.42
- Market-ready Product: $145
Total man hoursEdit
- Planning: 50 hours
- Contruction & Testing: 120 hours
While the final product does bear some resemblance to our early design concepts, it is the result of many redesigns and revisions in order to make it practical and realizable within the resource constraints of the course. The following section will discuss the experience of developing each component of the system, describing the challenges that were faced and how they were overcome.
The main switching board is the central piece of the audio distribution system. The main board interfaces with both the source devices and the keypad controllers to route the appropriate signal to each zone. The main board accomplishes one primary goal: source selection. The source playback control option that was originally considered is discussed at the end of this section.
Taking into consideration the need for an audio switch, we researched possible devices. We came across the Quad Audio Switch, which was designed for "click-free" audio pass-through applications. We created a new design for our main board using this chip. At this point we also changed our design for source selection. Originally we had envisioned a button selection method, with a button for each source on each keypad. For ease of implementation and to ensure no two sources could be switched “on” at the same time we decided to use a simple rotary switch.
The SSM2404 analog switch uses CMOS logic to control the switching of up to four input audio signals. The function of the chip is fairly straightforward and we integrated it into a design that would realize our objectives (Appendix C, fig 1&2). Initial tests of our circuit yielded more bad results than good. When originally attaching multiple sources and floating the switch inputs for circuits on the chip not in use, we saw some interesting effects (Appendix D). Sometimes the switch behaved as expected, passing through a signal when the switch was closed and passing nothing when the switch was open. At other times, however, we measured unexpected voltages on the output and when checking the input voltage we found that it had dropped below the specified level as well. Additionally, when hooking up multiple sources we saw distortion at both the input and output end of the signals. After observing the signal “jump” inexplicably, we tried testing the voltages at all the switch inputs using a multimeter. For each node we read the multimeter gave the expected reading, and interestingly enough if the output was in an unexpected state this seemed to rectify the problem. The determination was that there had been some low-level noise that had affected some of the CMOS switches when they were in the low or open position that caused them to jump high or close. In order to rectify this problem, we attached each switch across a high value resistor (~47k) to a –VL voltage source. After re-wiring the circuit in this manner, the design met our goals of complete source selection as demonstrated. Different scenarios for the switch state of two zones and their outputs are included in Appendix E.
The keypad was developed alongside the main board throughout the project, as the two components were closely linked. The assumption that all input devices are always on simplified the design for multiple users listening to the same output (i.e. the on/off state of the device was not a design factor for user source selection). The first plan to implement source selection was the use of TTL logic. We designed a digital logic circuit using flip-flops and elementary gates so that only one device could be accessed by a particular zone at a time. (Add more about early considerations and faults) Although the logic was implemented, and simulated and physical tests showed it met design goals, there were two major problems with the initial design. First, it was too bulky. It took an entire bread-board to implement one channel (i.e. one device to one controller). In order to design a full system using this set-up, we would have needed approximately 24 ICs for what we would later implement with two. Additionally, this design did not take into account the switching of analog audio signals. The outputs of the logic design would still need to be fed into some digitally controlled analog switch to fully implement our desired system.
If we wanted to scale our board so that it was capable of having more input devices we have two options to address the need for more unique signalling. The first and trivial solution is to simply add more wires. A cleverer implementation would be to use an encoder on the controller end and a decoder on the board end to scale down the number of wires need for the device selection by log2(N).
Originally, our plan was to use a low voltage amplifier based on the LM386N op-amp to power each zone output. This circuit was favorable because of its simplicity and impressive gain factors, as well as its low voltage input requirements. The suggested input voltage range that the amplifier could handle was + 400 mV. Although this value was adequate enough to accommodate the line-level audio signals from the main board, the amplifier output wattage had a nominal rating of 325 mW, a small amount when considering that most audio amplifiers range from 10W to 1500W. Nevertheless, for our initial expectations of the output zones, we felt that this amplifier would suffice.
After constructing the simple circuit, we encountered a number of problems. First and foremost, no clean output signal was obtained from the circuit as one expected to receive an output sinusoid greater than or at least equal to 400mV peak to peak. In fact, the output was largely filled with static with little or no tone when connected to the speaker. Moreover, the oscilloscope did not display any readable data for the output signal to the amplifier. This could have taken place for a number of reasons. The primary reason for this was likely because the amplifier did not have sufficient output power capability needed to significantly amplify the output zone, and closer inspection of the op-amp specs revealed a 325 mW nominal amplification, well below our needs. To meet the power supply and output wattage requirements, a more sophisticated multi-stage amplifier system was built that provided the output current necessary to drive an 8 Ohm load.
For the foundation of our high-current amplifier, the op-amp (TLE2141CP) was requested from Texas Instruments as a free engineering sample. Four NPN and PNP transistors were ordered online from Mouser Electronics. A TLE2141CD chip was also ordered from Mouser, but when it arrived, we realized that the pins did not fit the bread board, and it had to be re-ordered. Pin layout (often denoted by the last letter of the part number) is an important consideration in prototype development, and it is imperative that one consult the datasheets before ordering parts. Some specialty capacitors and a 1N4148 high voltage diode were purchased from Radioshack to complete the final amplifier circuit.
The implementation of the amplifier stage on the circuit board proved to be time-intensive as errors repeatedly occurred on the wiring. Because these parts did not come with datasheets, we had to rely on online sources for the pin layouts of the transistor chips. Two transistors were improperly connected at first, resulting in very large current drain at even small input voltages. As a consequence, one of the capacitors C2 was burned out; luckily, replacements were readily available from the lab. Another serious mistake while wiring was a reversed diode. These problems were detected with Dr. George’s help. An isolation technique was used in debugging the circuit, narrowing the scope of the problem until it was pinpointed, which proved effective for such a complex circuit.
The power supply proved to be only a minor challenge for the prototype circuit, largely because it was decided early on that development energy should be directed toward more complex parts of the system. We therefore chose to use a switching power supply taken from a junked computer case in the lab, which would provide the +/- 12V and +/- 5V supply voltages necessary for the amplifier and switching circuits respectively. This proved to be an adequate power supply, though a pair of simpler custom linear regulator power supplies would likely be necessary for a production unit due to cost considerations. One +/- 5V supply would be connected to the main board, while a +/- 12V (or greater) supply would power the amplifiers in each zone, along with a +5V tap for the keypad signal.
Original design plans called for a source control function in which each keypad would have playback controls (Play/Pause, Stop, Rewind, Fast Forward) relayed back to the main board, which would then interface with the source equipment through a universal remote control. While the concept is sound, the physical implementation proved to be prohibitively difficult. After buying a suitable remote control, we attempted to solder onto the PCB contacts that corresponded to specific button presses. These solder points would then be connected to a transistor switch that would short circuit the connection when given an “on” signal from the keypad, sending the appropriate control signal through the IR transmitter. The soldering was the breaking point, as the laminate used on the remote’s PCB melted when heated, coating the contacts and making a strong connection nearly impossible. The playback control option was scrapped as we realized that the physical and design difficulties of incorporating it into the system would have put us well behind schedule.
With minimal lab resources and zero working budget, the final hardware implementation proved to be the most difficult phase of the project, as we needed to source a number of logic components as well as analog amplifier components. While we were able to use various wires and basic circuit components from the bins available in the lab, the critical transistors, op-amps, and switches had to be ordered from electronics suppliers. Both the main board switching and the amplifier clearly presented major challenges in the build stage, requiring a large amount of time to get each piece to operate as desired. While the design concepts were well founded, real-world limitations required flexibility and adaptation to overcome and the final result is the culmination of these efforts.
The audio distribution system we developed has great potential to be a commercially successful product. Although our prototype unit can only be considered an alpha-level product, with careful, standardized construction, it has great potential succeed.
Appendix A: Cost ListEdit
- $2.40 - 4 transistors
- $1.52 - TLE2141CD op-amp
- $9.00 - Misc capacitors and 1N4148 high voltage diode
- $5.00 - shipping
- $9.50 - rotary swtiches+shipping ($2.55 per unit)
- $4.00 - Universal Remote Control
- $Free - Quad Audio Switches
- $Free - TLE2141CP
Full Product (estimate)Edit
- $80 - amplifiers: $10/amp * 1 amp/channel * 2 channel/zone * 4 zone
- $16 - main board: 4 * $4 Quad Audio Switch
- $10 - keypads: $2.50/switch * 1 switch/zone * 4 zone
- $20 - cable, wiring
- $19 - cabinet/packaging
Appendix B: Amplifier SchematicEdit
- source of the amplifier circuit: http://www.redcircuits.com/Page1.htm. cite it as a reference.
Appendix C: Main Board + Keypad SchematicEdit
DONE - Included in Jake's submission
Appendix D: PSpice Simulation of AmplifierEdit
DONE - Printouts with Mike
Appendix E: Other Simulations?Edit
Appendix F: Data sheetsEdit
- The TLE2141 datasheet is 70 pages. Mike, you probably want to print just the first few pages.***