Presto Electro

I apologize for the slow updates lately. This week was basically the perfect storm of exams and major projects. On the plus side, the exams all went great and I can add "Building a complete amplifier" (including pre-amp, audio filters, equalization, panning and power amplification) to my resume. And list of bragging rights.

As it turns out, amplifiers are a lot more complicated than I ever imagined. I tried to build one once when I was in high school, and it didn't turn out so great. For starters, the design centered around a 50 cent op-amp from radio-shack that really shouldn't ever be used anywhere near any sort of audio signal processing. I googled around and sorta half-figured-out that I could use an inverting amplifier configuration to amplify voltage signals.

So I did just that! I wired it up with a few resistors I took off of an old, broken VCR (of which there we about half a dozen to choose from in my room at the time) and the op-amp, and then I powered the whole mess with a wall-wart AC-to-DC adapter. I plugged my terrible guitar into this abomination of a circuit, and somehow nothing caught on fire. I plugged a pair of headphones into what I perceived to be the output, and out came the LOUDEST, most FIERCELY DISTORTED guitar sound I had ever heard. It was awesome.

Sure, I had completely failed in adequately amplifying the signal of my guitar, but instead I created a box that made my Squier sound like some kind of hideous beast out of hell. The trouble is, I couldn't really spend much time listening to it because it shredded my headphones pretty quickly. So where did I go wrong?

I've already outlined a handful of problems, but some of them were only implied. As it turns out, the source of the distortion (ignoring the distortion caused by my poor, rupturing headphones) was the AC adapter. Operational Amplifiers are supposed to have a bi-polar power supply: One positive voltage, and one negative voltage. My adapter was only giving it a +9 and a +0.

So the output couldn't give me any voltage at all when it was supposed to be putting out negative voltage. This brought out some weird clipping that REALLY got ugly if you played more than one note at the same time. Additionally, my headphones were linking Vout with ground, with only minimal resistance. In order for Vout to provide the appropriate amount of feedback current across Rf, the amp had to make Vout absurdly high. This elevated voltage made the incoming current cancel with the Vout/Rf current, but it also made Vout way louder than anyone could ever want.

I've still got that circuit bread-boarded in a box somewhere. I ought to get it out and see if I can fix it sometime. I've still got that old Squier… (because you can't even GIVE those things away…)