What do you think of this preamp design?

Ralph, don't you think you are asking a little too much? By doing this mod to a single ended amp, you likely will void the manufacturer's warranty and decrease the resale value of the amp. If someone modded one of your amps because another manufacturer says it will make your amp compatible with their preamp, wouldn't you void your warranty? I would not do this to my single ended amp or your amp for that matter. I would only let Atma-sphere touch my Atma-sphere amp.

Ngjockey, if the Audio-gd preamp sounds like any preamp made by PS Audio, I'll pass. I never liked any of PS Audio amps or preamps that I have heard over the years.

Apparently, the neat little thing about this preamp is you can swap different gain modules that were designed to sound like other classic solid state preamps by Mark Levinson, Accuphase, Music Fidelity, etc. As far as I know, Audio-gd does not have any gain modules that suppose to sound like PS Audio.

After a little bit of research, this I/V volume/gain control is based on a "Gilbert cell" and is available in CMOS in something like a AD835, although I have no idea of what is used in the Audio gd, PS Audio and Ayre VGT (KX-R) circuit. The advantage of this circuit is consistency through it's range, and allowing much higher impedance than the "standard" 10K to 250K ohms. Any claims of audible superiority I'll leave to the manufacturers and your own ears. Audio gd extends the concept with it's CAST-like I/O. Krell has a US patent but did not invent the technology. Seems like the most common use for this type of circuit is in phone systems.

We'll be seeing more of this type of control.

The Audio-gd C3 input impedance is 47 kOhms. I asked if it can be changed to 100 kOhms, and they said it was no problem. I wonder why they don't use higher input impedance.