Designing by ear

In yesterday's postwe talked about how different design paths can give the same measured results without sounding the same. How does someone train themselves to add their ears to the design tool set they use to build equipment and systems? The answer's the same as the age old joke about "how do I get to Carnegie Hall?" asked by a lost violinist in New York City. The answer was "practice, pratice, practice." When we started learning this skill set in 1973 we had no roadmap to go by. In fact, we were simply clueless designers trying to figure out a mystery - of which there were many. My post about designing a preamp among them. One of the simpler mysteries we solved, at the time, concerned our first power amplifier. Power amplifiers are relatively simple devices to design if you're not trying to get fancy (we weren't). Our first design, which later became the Model One, was about as simple a design as they come. Class AB, good specs, good parts, nothing out of the ordinary. It sounded good but not yet great. We played with all the tools we knew about, how much class A, speed of the devices, etc. All mattered, none clicked in the sound. Then, a friend asked us if we had paid any attention to the power supply. We hadn't. An amp's power supply doesn't get any simpler: power transformer, AC to DC and big capacitors - not much more. "Have you heard about bypassing?" Bypassing was the new thing: adding small capacitors in parallel with the big capacitors. We tried it and the amp went from ho-hum to high-end in an afternoon. Our listening tools were sharpened, we added another piece of the puzzle to our arsenal and created a high-end product. The amp measured identically to the same design without the bypass caps. The two didn't remotely sound the same. Only our ears could tell the difference.