In my previous post on the topic, I reviewed the original schematic of Krell KSA-5 and concluded that is has a nicely linear front end that cannot help the output stage stay linear because of low feedback.
My plan was to improve the linearity of the output stage and wrap it in a feedback loop with as much loop gain as possible. An additional self imposed limitation was to keep the PCB and the general schematic intact as much as possible and use the parts that I had readily available, so as to complete the upgrade over one weekend.
I decreased the emitter resistors of the output stage by an order of magnitude, converted the output drivers to Class A, decreased the amount of local feedback (degeneration), dramatically increased global feedback loop gain - the revised KSA-5 has at least 50dB of loop gain over all audio range - and adjusted the compensation to keep the amplifier stable.
It sounds immaculate and is a massive upgrade over the original version. I have heard it so far driving Sennheiser HD595 and Grado GS1000 headphones and B&W 602.5 floor standing speakers, with great results on a variety of music material. A brief head-to-head against Musical Fidelity X-CANv8 revealed that the amplifiers are somewhat different but without a clear preference one way or the other.
Below are some measurements. I believe that at this point, the distortion performance is largely limited by the original topology, such as the JFET buffers outside of the global feedback loop (note that First Watt B1, which is a JFET buffer, has similar distortion at similar input levels), and by the PCB layout.
1kHz THD and 19+20kHz 1:1 IMD @ 4Vpeak (1W) into 8 ohm:
The amplifier clips nicely:
and delivers a good looking square wave:
The phase margin is more than adequate:
