Dr. Bob Odell's power amplifier squashed our best design efforts of that day.
It didn't just sound better, it literally stomped the prototype amp Rick and I had labored over. We welcomed Bob into the company and then and there committed to producing his amplifier as the next big PS Audio product.
Over dinner that evening I grilled Bob on the circuit details. I was surprised by his answers.
Let me give you a bit of background so this will hopefully make some sense.
A power amplifier is divided into two sections: a voltage gain stage and a current gain stage.
The voltage gain stage is at the input where you connect the output of your preamplifier. In fact, this voltage gain stage IS a preamplifier. Put one volt of music from your preamp into your power amp and, like a preamplifier without a volume control, you get a bigger signal out. Typically, 1 volt coming in produces 30 volts coming out.
OK. Got that?
Now we have a big signal that we want to use to power our loudspeakers. Problem is, while the voltage (big signal) is exactly what we want, we don't have any grunt power to drive those speakers.*
*Imagine a car's motor. A tap of the gas pedal gets the motor spinning at a high rpm. Connect that spinning shaft to the rear wheels of the car and, if you don't have much in the way of power, the spinning slows down and you don't move. To keep the rpms constant you need power. Horsepower, wattage, whatever you want to call it (FYI: one horsepower is the same as 750 Watts).
To add the grunt power we need to keep the voltage up, we have a current amplifier which does that work.
While the 2nd stage of an amplifier, the output grunt stage, is important (this is where we get the classes of amplifiers, A, A/B, D), it is this first voltage gain stage where the rubber truly meets the road when it comes to sound quality. (Despite all the marketing hoopla folks like us and everyone else in the industry who like to jump up and down about the output stage, it is this input stage that really matters for sound. Think of our Class D output stage on the Stellar M1200. It sounds amazing because of its vacuum tube input stage—or the M700 gains its magic from it's class A MOSFET input stage).
Bob's output current amplifier was pretty standard. Like ours. Like everyone else's when you got down to it.
The magic was in his input voltage gain stage.
