a very enthusiastic review by Herb Reichert. He mentions that he likes it better than the HPA-1 - XA-25 amplifier combination. Smoother sounding or some
thing like that.... I wonder what topology is implemented in the front end to make that happen.
I think the first link was Int-25 and second link was xa25.....the output stage seems quite similar. I was interesting in knowing the output devices…...and realized after posting that it was not the exact same amp.....but in close family I guess.
The INT-25 has a slightly different layout with all the power supply components on one board now and that has carried over to the XA-25. This way the front panel display board or just a switch on the XA-25 will turn it on.
Amatherm makes the thermistor for inrush, they have some big ones.
"There are several unique things about the amplifier that make it a departure from the rest of the Pass Labs amplifiers. It's a fundamentally very simple topology made special by new parts and a unique approach to operating them in Class A without degeneration in the circuit (simplified circuit shown below). Years ago, we discovered that degeneration—that is to say placing resistors in series with FET source pins (or emitter pins for bipolar transistors)—has an impact on the sonic performance. My late business partner Joe Sammut was adamant about the qualitative difference, and could spot it in blind testing, so I took it seriously. The difficulty comes the fact that there is a reason why people use degeneration (the "other form of feedback") in gain stages—it stabilizes the characteristics of the parts so that you don't have to do precise matching and compensation to keep circuits stable. At the same time, it acts like the feedback it is. Routinely, your "no feedback" solid-state amplifiers depend on degeneration in the gain stages to control the performance, and so it does not achieve all the goals that make SET tube amps attractive."