Originally Posted by Tjj226 Angel
Three last questions guys.
1: As I look at the schematic for the 12au7 mod, I see that there are two 0.22uf caps in parallel with the 470uf caps. The 0.22uf caps are optional, and with such a little change in capacitance, I am wondering if it would be smarter to buy higher quality 470uf caps, or simply add in the 0.22uf capacitors and leave it alone.
2: Resistors R2 and R8 on the schematic are listed as 390K ohm resistors. However, on the BOM page, they are listed as 220K ohm resistors. Which one should I get?
3: Resistors R15 and R14 look like they just essentially lower the volume a bit. Is this true, or do they serve a greater purpose?
The last two posts kind of give you an indication that the opinions for capacitor bypassing run the gamut. It depends on an awful lot of things - including the amp circuit design and the capacitors involved. You guessed correctly that they're not there for the extra capacitance. They're there to "bypass" the electrolytic capacitor.
Capacitor bypassing is based on a few things that are empirical except #3:
1. Film capacitors sound better than electrolytic capacitors
2. Large film capacitors - enough to match an electrolytic in ratings - are so large and so expensive that they're impractical.
3. This is the based-in-engineering part: any audio output capacitor (used for AC-coupling) is going to form an RC circuit with the load (headphones). This means that depending on the farads and impedances, certain frequencies will pass through better than others.
Because of the RC "filtering" effect, it can be argued that the bulk of the very detailed frequencies - highs and midrange - will by and large pass through the small film cap, while the mid-bass and deep-bass will go through the electrolytic. One might say it's an ingenious way of applying a high-quality film cap for the "most" sound, while the electrolytic ensures that all the bass frequencies still get through. Some people say there's actually a cross-over of sorts and claim they can hear a "smear." Others may notice that the highs are clearer, less-distorted and that the bypass arrangement sounds better than the electrolytic alone.
One could also argue that the capacitor pair actually forms a brand-new, hybrid-style of capacitor and that the actual frequency bandwidth is the same for both, but the combination itself sounds/performs better.
I don't know which is true, but I can tell you that there are definitely cases where the pairing sounds better than an electrolytic along. There's also very little predictability in whether one pairing sounds better than another. Generally speaking, Wima film caps (typically MKP10's at 0.1uf, 0.22uf, 0.33uf) perform fairly consistently with many electrolytics. Even then, though, some combinations work, some don't. Pairings of other type of capacitors are a crapshoot. Luckily, there are several sources of documented results with bypass capacitor combinations - Dsavitsk's "Some Notes on Output Coupling Capacitors" at ECP Audio, Humble Homemade HiFi capacitor reviews, etc.
Finally, Pete Millett uses Wima's as capacitor bypasses in many of his designs. We thought it was a nice touch to put on his Starving Student, too.
Edited by tomb - 8/7/13 at 8:21pm