Intro: Audio Amplifier With Salvaged Parts
I enjoy salvaging electronic parts from discarded equipment. A lot of people will gladly donate unwanted radios, stereos, CD radios, and other devices to someone like me that wants to learn and is willing to come get them. A lot of modern electronic devices aren't good candidates for part recycling due to the size of the components. But in my area I have been successful obtaining recyclable equipment made in the 80's that have lots of components I can reuse. I got lucky and found three 1970's and 1980's vintage CB radios that weren't working. These radios have tons of useful parts like transistors, resistors, crystals, IC's, and ceramic capacitors. In older equipment you have to be careful with electrolytic capacitors and some power resistors due to changing values over time. But the rest have a good chance of being usable even after 40 years.
I am very new to electronics. My uncle took me to Radio Shack and bought me some soldering tools, parts, a digital multi-meter, and a Velleman LED blinker kit to practice soldering with. After he taught me how to solder and we completed the Velleman kit, he gave me a challenge to build an Audio Power Amplifier using only a schematic and a bunch of my salvaged parts.
This Instructable is the project I built with a schematic from the IC datasheet and some parts I removed from an old CB radio. Before I call the project completed, I need to add a heat-sink to the amplifier IC and some vent holes in my project box. But as it is right now the amplifier works and sounds great.
Step 1: Removing and Identifying Parts
I use the following tools to remove parts from salvaged equipment:
1. 25W Fine Tipped Soldering Iron (Radio Shack Catalog #6400206)
2. Desoldering Pump (Radio Shack Catalog #6400210)
3. Desoldering Braid (Radio Shack Catalog #6402090)
4. 60/40 Rosin Core Solder (Radio Shack Catalog #6400009)
My process for removing the parts I want is:
1. Identify the through-hole PCB pads that the part you want to salvage is soldered to. I mark the pads with a sharpie to help make sure I un-solder the right leads.
2. Melt the marked solder joints with a soldering iron and add fresh solder to the pad. I have found that adding fresh solder makes it much easier to cleanly remove the solder from the pad. The rosin from the fresh solder also helps to clean the solder joint and makes de-soldering faster.
3. Start by removing solder from any large ground pins or metal shields with the solder pump and then finish up with the solder braid. Use a small standard screwdriver to separate the pin from any small film of solder holding the pin to the board.
4. Un-solder component leads with the solder pump. Use a small standard screwdriver to separate the pin from any small film of solder holding the pin to the board.
5. Using gentle pressure, rock the component back and forth while watching the solder side of the PCB to verify that each pin moves freely.
6. Gently remove the part from the board.
I've had great luck removing most Resistor types, Disk Capacitors, Mylar Capacitors, Electrolytic Capacitors, Transistors, Diodes, LED's, most push-button and sliding Switch types, and some IC's.
I've had mixed success with 16-pin DIP IC's and iron-core Transformers.
I've had really bad luck with small ferrite core transformers in the metal can. Most times the pins end up pulling out from the bottom of the transformer rendering it mostly useless.
After I have removes the parts I want, I read the values and part numbers and keep a log of the inventory I have. I also download the data sheets for semiconductors and integrated circuits so I have a way to understand what the part does, how to connect it in a circuit, and what voltages and currents the part can be safely operated at.
Step 2: Selecting Parts for a Project
While salvaging parts from an old CD radio, I found a neat 10-pin SIP IC that was easily un-soldered from the circuit board. The part number was a Toshiba TA7222AP.
I searched for a data sheet using the part number and discovered that the IC was a 5.8W Audio Power Amplifier. That part had some possibilities for a fun project. And, the data sheet had some example circuits I could use to start with. I've discovered that many data sheets include circuits that can be used as starting points for some very interesting projects.
After reviewing the parameters of the part and the example circuits I decided to come up with a circuit I could use to amplify an audio signal and drive a small speaker.
My inventory list indicated that I had all of the parts I would need so it was time to draw up the circuit and start building.
When salvaging old equipment, it is possible that there might be bad parts pulled from the PCB. I try to test everything with my DVM after I salvage it but I don't have anything to test inductors and most IC's can't be tested without hooking them up to a test circuit.
Usually I just accept that as the risk of working with salvaged parts and if a part doesn't work, I try to find another that I can use and replace the broken one. Mostly my circuits are simple and if I can't complete a project because of a bad part, I just count it as good soldering experience and not much time lost. Then I move on to build something else.
If the circuit is more complicated, I will tack solder a minimum number of parts to the IC leads to see if it smokes, and to test if it appears to work as expected.
Step 3: Designing the Amplifier Circuit
My uncle taught me that every circuit should start with a schematic and then a component layout. If the schematic is drawn well, it can be used to do the parts layout which makes it easier to troubleshoot or modify the circuit.
Using the example circuits from the Toshiba data sheet we drew up the circuit I wanted to build:
1. Capacitor C1 is used to keep the amplifier from oscillating due to the resistance of the power supply leads, and to help remove noise from the power supply. I had a lot of 470uF capacitors so I used that for C1.
2. The IC is designed to provide a gain of over 400 with no external parts. We felt that was too much gain and the circuit would probably oscillate when assembled on a breadboard. We felt that a gain of 100 would be best but the data sheet did not recommend reducing the gain with external parts to 40db or below. So we decided to set the gain at 200 and reduce the gain further with input resistor R2. The data sheet indicates that the formula for IC gain is:
(40K + R3) / R3
The best value for R3 would have been 200 Ohms. Unfortunately I did not have a 200 Ohm resistor in my inventory so I used a 220 Ohm resistor and a 100uF bypass capacitor C3 to pin 6 as indicated in the data sheet.
3. The input impedance of the amplifier IC is approximately 33K, so we decided to add an additional 33K resistor to pin 4 to reduce the maximum input signal by half and the overall gain of the amplifier to 100 or 40db.
4. Capacitor C4 is recommended by the datasheet to help the amplifier output to swing between the power supply rails which improves the efficiency of the amplifier IC. I used 47uF for this capacitor as indicated by the data sheet.
5. Capacitor C6 allows the amplifier to be used with a single-ended power supply (+12V and GND). Normally the output of the IC is set at 1/2 of the power supply voltage which can't be directly connected to the speaker without burning it and/or the IC out. C6 blocks the DC output of the IC and passes the amplified audio to the speaker.
6. To adjust the input signal level we used a 50K ohm trimmer potentiometer which provides an input impedance high enough for the stereo line sources I wanted to use without being so high that hum would be picked up when the input was unconnected.
7. The datasheet recommended connecting a .1uF capacitor C5 across the output of the amplifier to prevent RF from being coupled into or out of the amplifier.
Once we had all the circuit values picked out, I verified that I had these values in inventory. After that I was ready to start building.
Step 4: Creating the Amplifier Layout
I bought a plastic project box and a perfboard to mount the parts on and created a layout diagram from the schematic. This helped me figure out the best place to put the parts so it would be easy to assemble the amplifier.
My uncle has Visio templates for different types and sizes of breadboards so we use that to draw the parts layout. But the parts could also be inserted on the board and adjusted to fit and then marked out with a Sharpie pen if you don't want to use Visio.
Below is the parts we used:
J1 - 1/8" Mono Phone Jack (Radio Shack Catalog #2740246)
C1 - 470uF 25V Electrolytic Capacitor
C2 - 4.7uF 25V Electrolytic Capacitor
C3 - 100uF 25V Electrolytic Capacitor
C4 - 47uF 25V Electrolytic Capacitor
C5 - .1uF 50V Mylar Capacitor
C6 - 1000uF 25V Electrolytic Capacitor
R1 - 50K Ohm Variable Resistor
R2 - 33K 5% 1/4W Fixed Resistor
R3 - 220 Ohm 5% 1/4W Fixed Resistor
U1 - Toshiba TA7222AP 5.8W Audio Amplifier IC
The breadboard is a Radio Shack Perfboard Combo Pack Catalog #2760328.
The project box is a Radio Shack Project Box Catalog #2701807.
Step 5: Building the Amplifier
I'm still working on my wiring skills but it was very easy to build the amplifier. There are probably a lot of ways to build the amplifier but here's what I did:
1. Mount the amplifier IC U1 on the breadboard.
2. Run a bare piece of wire around the edges of the breadboard and solder the battery GND wire to it as shown in the layout diagram. This is the common ground wire for all components connected to ground.
3. Solder a wire from U1 Pin 1 and solder it to the +12V battery wire from underneath the breadboard so it stays in place.
4. Solder the (+) side of capacitor C1 to the +12V battery wire.
5. Solder the (-) side of capacitor C1 to the common ground wire.
6. Install the 1/8" phone jack on the breadboard and solder the phone jack ground (ring) pin to the common ground wire using a short piece of hookup wire.
7. Insert the variable resistor R1 as shown on the layout diagram and solder the left lead to the tip pin of the phone jack.
8. Solder the right pin of variable resistor R1 to the common ground wire using a short piece of hookup wire.
9. Insert capacitor C2 as shown on the layout diagram and solder the (-) side of the capacitor to the center pin of variable resistor R1.
10. Solder one end of R2 to U1 Pin 4.
11. Solder the other end of R2 to the (+) side of capacitor C2.
11. Solder R3 to U1 Pin 5.
12. Insert capacitor C3 into the breadboard and solder the (+) end to resistor R3.
13. Solder the (-) side of capacitor C3 to the common ground wire using a short piece of hookup wire.
14. Solder U1 pins 7 and 8 together.
15. Solder a short piece of hookup wire from the common ground wire to U1 pin 7.
16. Insert capacitor C4 into the breadboard and solder the (+) side to U1 pin 10.
17. Solder a short piece of hookup wire from U1 Pin 9 to the (-) side of capacitor C4.
18. Insert capacitor C5 into the breadboard and solder one end to the (-) side of capacitor C4, and the other end to the common ground wire using a short piece of hookup wire.
19. Insert capacitor C6 into the breadboard and solder the (+) side to the (-) side of capacitor C4.
20. Solder the (+) side of the speaker to the (-) side of capacitor C6.
21. Solder the (-) side of the speaker to the common ground wire.
Step 6: Testing the Amplifier
I connected the amplifier to a 9V battery at first to avoid burning anything up too bad if I made a wiring mistake. Then I connected the input of the amplifier to my phone. The amplifier immediately powered up and was very quiet with no audio signal. There was no hiss or unexpected oscillation. I stared the music player on my phone and the audio output was easily heard on the lowest volume setting. So I turned down the variable resistor and then increased the volume on my phone. The audio output was strong and clear with a 9V battery. Very exciting, even if it was only mono audio.
Next I took the amplifier setup outside in the garage and connected it to a 12V battery. The power output of the amplifier was strong enough to overload the small speaker I had attached to it. After a few minutes the IC was becoming fairly warm so I switched off the amplifier and called the test a success!
My next step is to attach the heat sink I've ordered and test the amplifier with a bigger speaker. If I can find another TA7222AP IC in my pile of salvaged CB radios, I'll wire up a second one on the breadboard and have a neat home-made stereo amplifier.
This was a fun home-brew project. It was great to learn about IC datasheets and how they can be used to design a working circuit. I'm getting better at soldering and component layout. These kinds of projects help me get better without having to spend a lot of money.