Okay PB, I have been quite the busy bee...here is where we are at with the parafeed caps in this amp, which is otherwise complete
I took FR sweeps of five different capacitances: 3.3uF, 4.7uF, 6.8uF, 8.2uF, and 10uF. Not sure how to insert attachments on the BH forum, but I have included them in a GIF format attached below, easier to see the relative change of low end response this way, but let me know if you would like the individual plots.
As you can see the effect on low end response from 20-30Hz is pretty subtle, we are talking about a difference of ~0.3dB when tripling the capacitance from 3.3uF to 10uF.
*I am copy/pasting/reformatting from another forum, subjective impressions were done in two different sittings.
Part 1:Okay, so we've established there is a very small difference in frequency response from 20-30Hz. But how does the sound change subjectively? I first decided to compare at the extremes, 3.3uF and 10uF. The results were somewhat unexpected...
I much preferred the sound of the 3.3uF. Even if the low-frequency extension is marginally improved with the 10uF caps in, there is a very noticeable loss of soundstage, airiness, and detail. The treble sounds a bit more harsh to my ears as well with 10uF. The 3.3uF sound is much more spacious and cohesive. Up to this point I had mostly been listening with the higher capacitance caps (6.8uF and 10uF), but the sound is much improved with 3.3uF. In fact, the bass sounds better! Better definition even if the extension is objectively slightly worse.
So there are clearly two competing factors in play here that will need to be balanced when assessing the other three caps. I can say without a doubt I will not be using the 10uF, bigger does not necessarily appear to be better.
This is an interesting finding since using a lower value cap opens up many more possibilities in terms of the quality of caps that can be used. I had somewhat assumed I would need a value higher than 5.6uF, so my top choice was the Rike Audio S-Cap 2 (aluminum foil paper in oil) as these caps are the appropriate size, value, and are well-reviewed for their "neutral" sound. They are also quite affordable.
So, there is more listening to be done.
Part 2 (after the suggestion was made that differences heard between 3.3uF and 10uF may not have been all due to the capacitance - try the caps in parallel for lower ESR/ESL):Worked on getting the parafeed capacitance nailed down this morning. I started by comparing the 4.7uF and 3.3uF in parallel vs. the single 8.2uF cap. I can say without a doubt the two parallel caps sounded better. Better sounstage, air, and detail. Similar to what I said about the 3.3uF vs. 10uF, the singular higher capacitance cap causing something of a collapse of soundstage and more harsh treble.
Next, I compared the 4.7uF cap to the 4.7uF and 3.3uF in parallel to compare a singular lower capacitance cap with the higher capacitance combination. Again, I felt the two parallel caps gave better detail, soundstage separation, and this is the first time I felt the bass was significantly improved between two sets of caps. Fuller, tighter, more body.
Next, I compared two sets of parallel caps with different capacitances. With what I had on hand, I went for the two lowest capacitance combos for the sake of practicality, 4.7uF + 3.3uF and 6.8uF + 3.3uF. This is where differences started to get more difficult to detect...however, I did feel vocals started to get smeared a bit with the higher capacitance and focusing on other aspects of my test tracks revealed a loss of microdetail.
So, with the parallel combination of around 8uF, I was getting the best sound. But what about the relative values of the two capacitors? I then compared the 4.7uF + 3.3uF combination with an 8.2uF + 0.22uF combination. Not an exact capacitive match, but close enough with the 5% tolerances. I felt the two caps of similar size gave better nuance and vocal detail.
If two paralleled caps is good, is three better? I put the 0.22uF cap in parallel with the 4.7uF and 3.3uF for a total of 8.22uF. Adding this 0.22uF cap undeniably reduced the fullness and quality of the bass.
So my winning combination was the 4.7uF cap and 3.3uF cap in parallel. From the measurements, I know that diminishing returns in the bass response start at around 6.8uF, so I will be looking to find a parallel set of caps with similar values that add to ~6.8uF, perhaps 3.3uF + 3.3uF or 3.9uF + 3.9uF. Obviously this will make the size restriction more stringent, but I think the performance is worth it. It's pretty incredible the gains that can be made in this topology by tuning the parafeed caps.
I did briefly drop the 0.22uF cap in parallel with my 0.47uF coupling cap to see if similar gains could be had there. A few back-to-back comparisons was not enough to say definitively, and I was pretty fatigued at that point, so I'll revisit it.
