That all said, I went back to the drawing board about a month ago or so, with good initial results. Its better now. The current prototype has no EQ in the circuit yet. I wanted to get everything else performing perfectly before designing the EQ section. I'm writing this blog post today because I'm finally really satisfied with the amplifier. It sounds absolutely stunning. High resolution and pretty low noise with a decent amount of gain. The sound is what its all about and it sounds nothing short of amazing. The current topology is a single ended ultra-linear pentode amplifier driving a custom output transformer. THD is 0.185% with a 2kHz sine wave feeding the input with 80mV and the volume knob on the channel strip set to 100%. At that setting the noise floor is measured at -90dB A-weighted RMS and the gain delivered is 48dB. Turn the volume knob down to 75% and the noise floor drops to -100dB. Kick on the turbo and the gain jumps from 48dB to 57dB! With the turbo engaged, the THD is just over .3% with the volume knob cranked. With those settings, the A-weighted RMS noise floor is -80dB.
There's still a lot of work to do, but at least now things are looking better than ever and most importantly, it sounds absolutely wonderful!