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A Push Pull Amplifier

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7-23-04

I decided one day, for lack of funds at the time, to build my own speaker boxes. I had recently received a home brewed software program for DOS that basically calculated the air volume necessary for a specific speaker. I also had been told that cheap speakers did not sound good, and I was out to prove that theory wrong. The secret was in the box. The box had to have the correct volume for the speaker and also have two chambers for the air to flow at the correct pressure to create a good sound. Too much air volume and the speaker would rattle, too little volume and the speaker would choke down. The program did not have the particular speakers that I purchased for $8 each, but had a speaker that was very close. I calculated the volume for my speakers and set out to build the boxes. I chose ” particle wood to build the shell and carefully measured and cut the parts. I pre-drilled all of the screw holes and used 2 ” coarse dry wall screws. I placed a screw every 2” so as to ensure that I get a good seal. I then used a general purpose silicone sealant for the sealing material. I waited for the sealant to set up and affirmed that I was now ready to test the speakers. Now I needed a driver for the speakers.

Again, not having the funds to go down and buy the best amplifier, I decided to use whatever I had in my lab. I found a complementary pair of transistors and a 714 operational amplifier. I proceeded to go through the specifications search and determine the operational parameters that I wanted for my circuit. I started building the prototype circuit and found that my initial calculations were very close. After bench testing the circuit and tweaking the resistors and components a bit, I was ready. I duplicated the circuit for the second stereo channel. I plugged the input of the 2 amplifier circuits into each channel output of a small FM radio that had stereo output for headphones. I hooked up the outputs of my amplifier to my 2 speakers and hooked up a 100 Kohm pot as the feedback/gain resistor of th 714 so that I could control the volume of my amplifier. I moved the output radio volume to the mid-range and powered up the amplifier. Keep in mind that my ‘lab’ was my garage, in a middle class neighborhood (even though I had the doors closed, so I was a little subdued as to what was an ‘acceptable’ volume for a stereo system). At the time, my teenage son (late teens) had an older Ford pickup with a six cylinder engine (300 straight six) and he had rigged the exhaust to be a little louder than factory. Well, to get on with my initial test, the stereo was playing and I cranked the gain pot until I couldn’t hear myself scream for about 5 seconds, and then quickly rotated the pot in the opposite direction to bring down the volume. Well, about that time, I could hear my son’s truck at the turn to our street, about a quarter mile away, so I quickly turned the volume back up to near distortion to see if he would hear it. I could barely hear my son’s truck pull into the driveway and come to a stop. When I heard his car door slam, I quickly rotated the pot back down to a conservative level. The next thing that I heard was the front door open and close suddenly and rapid (running) footsteps to my garage as he slammed the door open with a sudden swoosh and said very excitedly that he could hear my system when he turned the corner to our street and what kind of system did I buy? (He had one of his friends with him). I quickly explained what I had been up to and showed him the circuit, etc. Also, I showed him my homemade speaker boxes with $8 dollar speakers. He and his friend were still in disbelief, so I demonstrated the system one more time for the final convincing. The look on their faces was priceless!

I was convinced that the design had potential, so I laid out a rudimentary PC board for my circuit and took the design to work the next day to use the PC board etching materials there. The PC board came out acceptable enough and I took it home and promptly soldered my components in and bench tested again. The results were the same, and I new that I had a winner!

I still have that original board to this day!

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