Jeeze... everyone gets so critical. Where's the Fun and Learning?? Can't we all get along? Can't we accept something for what it is?
:rofl:
Ok, enough o' that... lemme put my electronics pro hat on...
Why? ESD. So you dont blow things out when you plug it and you're charged from the carpet or whatever. And, any spikes on the power lines would be clamped down. Theres probably one or two TVS's (or Zeners, they work too, not as well but cheaper) in your DAC input, it's common design practice. More would be better, and wont hurt.
Ehhh.. yeah a fairly sloppy soldering job, maybe not too bad. That's one of the major reasons to clean the flux, to be able to see the joints better. Theres a spot where it looks like a solder bridge, might be flux. Most of the time flux wont affect the circuit... depending. Costs $$ in cleani g labor, tho... (at the hand soldering level that is) I'd do a lot better, out of personal pride.
Hand soldering... always worse looking than machine reflow. But so what, if its functionally correct? Machine soldering... makes sense if your making many 1000's or millions. Very cheap. Building a couple hundred or less, not so. Even more so including the cable attaching and box assy. You gotta think, there might be only 100 in the world, as a gamble that they sell enough to warrant "actually going into production" with a larger batch. Bigger investment, more $$ to lay down first.
Circuit... seems OK. If the flux was off, and good pics of the traces, easy to reverse engineer. Dont know all those SMT values, of course... an advantage of thru hole parts, even caps and resistors were marked. Overall, as some have noted, seems like a reasonable filter for USB. (If the parts are sane)
BUT... we are talking >$10 parts in a $10 box, and even with a x5 markup to try to support a biz... this thing has no right costing >$100. Mass produced, could possibly do it for $10-20, but do you wanna drop $100k to have 10k made to try and make more than your money back? Hey, there's ASR out there!
Key issues. Does it do something? Have to come up with a filtering test to determine how much of what frequencies it filters. Do you need it? As stated, probably not unless you have an unusually sensitive DAC or unusually noisy power supply or RF neighborhood.