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Genius!2: Pat Quilter and power amps #1

From classic nerd to transforming power amps with QSC: the first in our excerpts from Genius!2, published this month

I grew up as a pretty classic nerd, a skinny kid who was mostly interested in mechanical contraptions, writes Pat Quilter. It was not however until college that I got interested in audio experimentation. I discovered you could play around with these nifty little parts and end up with something that could play music, without even getting your hands dirty. So in 1965, I set myself to the task of re-inventing basic audio technology, occasionally “looking in the back of the book” for more hints when I would get stuck. Within several years I had built a 5W transistor audio amp and speaker that could run off a 12V battery and play back music from a tiny portable tape recorder I had. Essentially, I made a crude but effective “boom box” well before these were commercially available.

In 1967, the bass player in my brother’s band needed a high-power amp, but he had no money. I figured I could scale up my 5W amp to 100W. “How hard could it be?” I did everything over twice to get results, which doubled my parts cost, but I still made about three cents an hour on the project. So in 1968 I started Quilter Sound Company and decided to make a few more amps, this time only once-through, and thereby make a reasonable profit.

But the high-power germanium transistors used on amp ‘Old #1’ became unavailable, so I took the opportunity to upgrade to silicon devices. Once again: “How hard could it be?” As it happened, it took years to develop a somewhat reliable design, during which time I gained the partners who would form the key management at QSC Audio Products. By the mid-Seventies, it was obvious we had missed the opportunity to be a guitar-amp leader, and so we re-focused on pro audio, a decision which proved to be far more rewarding over the succeeding decades. We also made the conscious decision to focus on power amps, since this had been the most difficult part to get right, and we wanted to capitalize on this hard-won knowledge.

Power amplifiers embody every difficult challenge in audio electronics. Ultimately, they must combine a great deal of refinement and precision, along with brute-force power delivery and protection from getting hot and power spikes – and they must be price competitive if one expects them to be commercially successful.

In terms of design, this obsession with ‘minimum active stages’ got me to a certain point, but a critical driver stage still had too much voltage across it at all times, which would not be reliable. Also, the final high-power output transistors needed to be insulated from the heat sink, which is a fussy production detail and results in additional temperature rise.

So, in one of my few genuine “lightbulb moments” it occurred to me to float the power supply, ground the output devices, and let the power supply voltage swing instead. We could then mount the power transistors straight to the metal heat sinks using simple screws, and this also solved the excess voltage on the driver stage. The only drawback was the need for a separate power supply section for each channel, but one of our salesmen turned this into another key benefit by noting that if one channel failed, you could still use the other.

This was a major selling point in the days when amp failures were still common.

So: our small shop of hand-assembled products was now competing with the ‘big boys’, and establishing a reputation for sincere, reliable products that just kept working. And it also resulted in our first truly well-protected amplifier, which would withstand all the usual hazards, yet continue playing well for as long as possible. Many ‘QSC Series One’ amplifiers we produced in the 1980s are still in daily service in cinemas and clubs nationwide.

Although we eventually outgrew this exact design, which was somewhat limited in the power it could reach, it gave QSC enough volume to invest in more automated assembly, and I continued to exploit key production elements such as metal-to-metal power device mounting for decades to come, even as we developed more sophisticated, higher power amp designs.

Pictures: Top: Pat Quilter. Second: Quilter in 1979. Third: Testing kit in 1983. Last: Quilter at his workbench in 2008.

Launched last year, Genius!2 is the second edition celebrating those clever people whose inventions have transformed the world of professional audio. The 30-page supplement is also available to read in a handy digital-edition form.