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Here is a throwback for me. This was the first audio amplifier I had ever built. This was back in 2003, and I was 13 at the time. I didn't really know what I was doing, but I put this circuit together, and by chance, it worked! I mean, it's called the EMITTER, right? That must be what EMITS the signal :)
I had used 2 2N3055 transistors for the output, and 2 2N3906's for the preamp. They were robbed from a dead stereo amp.
I found that 12 ohms seemed to be about right for the load for this amp. So for an 8 ohm speaker, I would put a big 4 ohm resistor in series with it. Forget coupling the speaker through a capacitor. That was beyond me at the time!
This amplifier was a beast to tame, but if you got everything dialed in right, it actually sounded REALLY good, and I remember that the frequency response was ridiculous!
This amp had a lot of quirks. For one, you can see from the design that it always put out a DC voltage through the speaker. The voltage powering the PNP transistors were critical! At the time, I knew nothing of voltage regulators, so I used batteries. I used a 3 AA battery holder, and would mix and match used batteries until the amp worked the best. I think that in practice, 3.6V was about ideal. Not a good way to do it, but it was functional!
At the time, I called it "Current Differentiation," a term that I more or less made up. Looking back at it, that is actually a rather fitting name. It does consist of two stages of current amplifiers, and it does take a differential input!
Maybe I wasn't as dumb as I thought I was back then!
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