Work on both improvements to the “Concept Amps” and the SIT amplifiers proceeded in parallel through late 2010 and into 2011. In mid 2011 Nelson and the partners in Pass Labs heard the results of the work with the SIT devices in the form of a prototype of what would later be developed into the SIT-2 amplifier.
They found the sound of the SIT to have truly remarkable properties, and it was quickly decided “Whatever this is, we need to find a way to bottle it!”. Not only did they need to bottle it, but they needed to put it in a much bigger bottle.
In a subjective arena where engineering has limitations, it is extremely helpful if you can recognize what you want when you hear it. If you have an example of the sound you are looking for, there is great advantage.
Because this sonic quality was so striking, it became easier to discover what modifications to the circuit would make it go away, and by the process
of varying the design and listening to the result, Pass was able to objectively identify the qualities of the sound—information which remains proprietary. He began to alter the circuits of the prototype Concept Amp to bring out these qualities. In the end, a new output stage topology was chosen using power Mosfets with new values of single-ended and push-pull bias. A small amount of feedback was employed around the output stage.
The front end of the amplifier also was redesigned to a new topology which employed Cascode Local Feedback newly developed by Pass. The circuit bias was raised to a much higher Class A bias. It has a higher input impedance, a lower output impedance, wider bandwidth, and most important, it specifically can be adjusted to complement the sonic qualities of the output stage.
Eight months later, the listeners at Pass Labs judged the amplifier design ready, and the two versions were named the Xs 150 and Xs 300.