My daughter came home one day carrying a big honking Crown amplifier.  It turns out it belongs to a friend of hers (who’s in a band), and it needs fixing.  So I said “Okay, I’ll look at it.”

I’ve never really done  amplifier repair. Much less repair a pro-audio power amplifier.  This will be *fun*!

That last power amplifier I built was way back in the mid 80s.  There was this excellent teacher, Mr. Alexis, very knowledgeable in electronics and someone I look up to.  Me and a few other high school classmates sometimes will just hang out with him and just talk “shop.”  One day, he was at his desk and me and the other guys hanging around his office. He was drawing something, and it turned out to be a power amplifier schematic. I’m talking about discrete NPN/PNP power transistors (2N3055, MJ2955 TO-3 packages), and just a “complicated” looking of a schematic. It was not a one-chip power amplifier using an STK chip or something like that.

First of all, who draws a complete power amplifier schematic from memory? With part values and all that?  It’s freaking amazing and I know he drew it from memory because I watched him from start to finish! That left a very great impression in my young mind.

So okay, he handed me the piece of paper and said it was a schematic for a 50W (or something) power amplifier and said to me “go build it.”   So imagine this, it’s like the heavens parted, angelic and choir music played as he handed me this paper, and like the booming voice of God commanding Noah, “go build the ark” — yeah, the effect was something similar to that.

When you get handed over this command from a teacher you look up to, you take it very seriously! So I started designing a PCB for it.  (Back in the days, we don’t have Eagle CAD software or anything like that… you design a PCB using pencil, pen, eraser, and tracing paper.) Then I stencilled the bare copper PCB with permanent marker pen, and nail polish, and etched it myself using Ferric Chloride, drill the holes at our school shop (they have drill presses), and assembled, troubleshooted and built that amplifier board  in a matter of weeks. And IT WORKED!

So what’s the memory trip back to the 80s have got to do with this?

Well, I googled and  found the schematic for this Crown XLS 202 amplifier (it’s great Crown is offering schematic downloads!) and looking at it’s schematic, I saw the similarities… the class AB, push-pull NPN/PNP complementary output stage design, and all that.   So I thought to myself, yeah I can do this… and started opening up the amplifier’s case.

…. to be continued