@soix in response to your initial post. You're being extreme. All DNA amplifiers are not doomed to failure. Do original amps have a much higher failure rate than normal, probably so. However, I'd be willing to bet that 50% or more of them will still be kicking with their original input boards 10 years from now. Being that this is the internet you're going to hear more about failed units than about those that are still working because people go out in search of help when they fail.
I've worked on a couple of DNA-1's recently. They had clearly different manufacturers of the input boards, but the actual layout was identical. One I worked on was a full upgrade on a late model, circa 1997 or 1998, DNA1 Deluxe for a local audiophile. The other was a dead early model that I picked up off of Ebay and have since fixed and fully upgraded including the "C" mod. There are some problems with the input boards and the construction of them. If you have one and take a look at the few IC's on the input board you'll notice that the solder didn't flow through to the top side of the board and up on the legs of those IC's on the board. This is a big issue on the pins that have through-hole connections because those pins can lose contact eventually.
I contacted SMc Audio a while back asking for schematics for the amps and, in addition to saying no to giving me a schematic, they pretty much confirmed my observations. Here is what I was told "The problem is with the through-hole plating between the top and bottom copper layers of these two sided driver boards and attempts to repair them is not recommended or supported."
All holes on the boards are not considered through-hole. Only about maybe 20% of them are. Through-hole is when a signal comes in on one side of the board and travels through the plated hole and then travels out the opposite side of the board to a different component on the board. Through-hole issues can be corrected and overcome. Its a lot of labor to do so, but the faults are not irreversible. Through hole connections need to have a solid solder connection on both the top and bottom of the hole. When these boards are upgraded care needs to be taken to offset the new capacitors about 1/4" off the board and apply solder to the leads on both sides of the board to make a solid and long lasting connection.
I think its not a stretch to say that the impetus for the "C" mod initially was to eliminate the AD712 opamps that were driving the DC servo in the DNA amps because of failures. The reason was that if a through hole failure occurred due to a poor solder joint on one of these 2 servo opamps then the servo could drive the output into a heavy DC situation and smoke the speakers. Pulling the servo op amp and installing a manually adjusted DC offset trimmer (aka "C" mod) into the same location basically solves this issue. So IMO the C-mod was a reliability upgrade and not necessarily a sonic improvement. The DC servo works great, when its working properly.
I would ask how many of the amps that SMc upgraded from the late 90's up until about 2014 have failed? In those years SMc was upgrading the stock input pcb's. Has anyone heard of one of those amps failing? Probably not, because the stock amps were already randomly failing back then so Steve implemented fixes for some of the issues he was seeing. However, SMc is now selling a new product to go into your old DNA chassis. Its not easy to sell that new product if the old parts are repairable and if you were happy with how the amp sounded before. How many people would pay $2k+ (I've heard of costs approaching $4k for some) when they might pay $500 - $600 to just get it repaired and not just functional but more reliable than it was and sounding just as good as it did prior?
