What do you think of this preamp design?

I've been on the hunt for a preamp and came across the Audio-gd C3 final version:


It comes with schematic of the signal section. Does it seem to be well executed? Any obvious or subtle design flaws? It has had some positive reviews by owners on the web.
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 ability to take a balanced or single-ended source is shared by all preamps with a differential input.
Ralph, I saw that opamp in the feedback loop was puzzled because they claim they don't use any chip based opamps in their amps.
BTW, I heard you're working on remote volume control for the MP3. When will it be ready? Did you see my thread on this forum about lack of a remote control being a deal breaker for preamps? More than I thought need remote.
We are trying to have it ready for RMAF... at any rate, like the MP-1, the remote for the MP-3 will be retrofit to any earlier MP-3.
Looks a bit like PS Audio's gain cells but more complex and less "empty" box.
Dracule1, you don't need RCA outputs on a preamp. What you need is XLR inputs on an amp. There is a way to do that such that both phases of the signal can be processed by the amp, without using a transformer and without changing the character of the amp.

If this sounds a little mysterious, its not- the technique is very simple. However, its not in common use in audio, which has always amazed me. I have a set of 45-based P-P amps at home that use a single-ended traditional driver circuit. Normally an amp like this would use only an RCA input, like a Dyna ST-70 or something like that. But I have been using this technique with this amp for several years, so it has an XLR input as well and the amp is processing both phases of the signal, even though the actual circuitry is the same as it was before.

This might be a matter for a different thread...
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.
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+.
Ralph, I appreciate the offer. I may offend the designer of my single ended amp if I ask him to change his amp in any way (He is pretty legendary and has been building audio equipment since the early 70's). But I'll ask.
What we're talking about is a textbook integrator circuit . . . the 1 Meg resistor and 100nF cap are even common values, giving a 0.1 second time constant, equivalent to 1.6Hz. The reason behind the extra RC on the input is unclear, as it doesn't create a second-order slope . . . but this is definitely more of a conceptual schematic than an accurate circuit reference. I'm giving him the benefit of the doubt.

I think that the Chinese high-end audio industry may be in the same state today as the Japanese industry was in the late 1960s . . . that is, with some interesting designs with their own unique cultural flavor are starting to be available for export. And amidst a sea of cheap manufactured goods and copies of Western legacy gear, I think this is the sort of thing that consumers of high-end audio should take notice of.