What do you think of this preamp design?

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.

It looks overly complex... A very competent solid state line stage could easily be built with about 1/3 the components. The op-amp in the feedback loop looks to me like an outright design problem...

The opamp is obviously an integrator (DC servo) used to set the quiescent conditions of the "BPM-7110" module, which appears to use current feedback, and a very high output impedance (current drive). The buffer stages are used to interface with non-CAST components. Really not so complicated.

Atmasphere, there's certainly not enough information here to trash this design so thoroughly, and English is obviously not his native tongue. While this is also not the way I go about designing a linestage, it's about as thoroughly "Power Paradigm" as it gets. To me, his stuff looks interesting, unconventional, and definitely reasonably priced compared to many.

Kirkus, if that's a servo, as long as it is handled properly I would expect it to work fine. The schematic really did not offer timing data, and there should be some integration at the output of the opamp else the servo will not work correctly. We use servos in our preamps too as our outputs are direct-coupled. I agree his English could be better...

Dracule1, I was not proposing that the amp be modified by anyone in particular, and it is certainly work that the manufacturer could easily do. If you *did* decide that this is something you were interested, I would be happy to talk to whomever made your amp so they would know how to do the mod.

Kirkus, I completely agree with you regarding the price of this unit. I'm not an electronics expert so I can't attest to the soundness of the design, but there is no US or EU based company (even if they manufacture in China) that can make a preamp or amp of this build quality for the price they're asking for. If this was a US based company, I wouldn't be surprised if the preamp was priced at $6000+.