Nice work (that's the understated 'nice' that comes with a subtle nod of approval and really means superb). Do I spot about 10 of the Vishay 'naked' foil resistors on your PCBs?
Yep. Anand likes those.
You don't need the fancy resistors to get the full performance. They do look cool and make for great marketing, though. And, more importantly, the Z-foil resistors don't degrade performance.
To my knowledge they do represent the real world resistors that come closest to behaving as a theoretical ideal resistor does in the textbooks.
They're nice resistors for sure. Whether they're closer to ideal is debatable. The resistor tolerance and temperature coefficient are extremely low for the Z-foil resistors, so in that sense the resistors are closer to ideal. They also offer good aging properties (basically they don't change throughout life) and low excess noise. They are, however, rather large and the resistive element follows a serpentine pattern, so I would think they'd be a bit more inductive and offer higher end-to-end capacitance than a typical leaded resistor, so in that sense the Z-foils are a bit further from an ideal resistor. That said, those parasitics have minimal impact in the Parallel-86. In fact, they're likely swamped by the layout parasitics (trace inductance, in particular).
There are cases where you want low temperature coefficient and good matching between resistors. The feedback network of an audio amp would be such a place. There's definitely a technical reason for using a low temperature coefficient resistor there, which is why I specify such a resistor on the bill-of-materials.
Note that expensive and "audiophile" don't equate to high quality, however. In many cases - in particular when it comes to capacitors - the "audiophile" components actually offer higher parasitics (i.e. are further from the textbook ideal components) than commonly available parts. And the commonly available parts are 20-60 dB lower cost.
Apparently 'resistor spotting' is this most geeked out audiophile equivalent of 'train spotting' among railroad enthusiasts.
Not a bad way to spend an afternoon. Resistors are worth paying attention to for the reasons I've pointed to above. There are some SMD resistors that are nearly as good as the Vishay resistors. I use them in my Modulus-186, -286, and -686. Unfortunately, they look just like any other SMD resistor.
This aligns with my experience. SMD (Surface Mount Devices) are harder for casual DIYers to work with but the more compact layouts they allow yield tighter current loops in the circuit which yields lower noise and real world performance gains in other areas like wider bandwidths.
Yep. The more exciting opamps are now SMD-only as well. It's unfortunate from a DIY perspective, but good from a performance perspective.
From a DIY perspective, the SMD components can also be rather fickle to work with. I've had a couple of interesting support cases on the Modulus-286 Rev. 1.1, where ceramic capacitors had cracked during soldering, resulting in weird issues like audible distortion. Those were rather challenging to debug by email, but everything worked out in the end. This type of support case is one of the main reasons I no longer offer SMD builds for DIYers. By offering the boards fully assembled, I can give everybody lower prices and better performance. The downside is that it's less DIY.
In a professional SMD assembly setup, the parts are baked for 24-48 hours to get all moisture out of them before the assembly. This is to prevent cracked parts. The local assembly house I use go a step further. Their reflow oven has 13 (thirteen) temperature zones, again to prevent cracked parts. Many of the overseas outfits use 3-zone ovens. Think about that next time you read DIYers advice on toaster oven or frying pan reflow.
For SMD hand-assembly, I much prefer Anand's approach and solder each part individually. Just be careful with the heat.