I had been re-reading some old posts on crossover mods on the SDA’s and it seemed to fit the bill for the next task I wanted to undertake on my SDA-1C’s. I did a lot of research on capacitors, and who sells what, and who likes what, and why this one is better than that one. I noticed many of the forum members like the Solen caps for their mods. So I purchased all the Solen capacitors necessary for the mod.
On some other forums I encountered in my subsequent research I saw a few posts from people who had the opinion that Solens can sometimes be a bit bright. Now of course everyone has an opinion that’s true, and I have not had the experience of listening to different caps in systems, but now I got this bright word gnawing at my brain. Through further reading in and out of the Polk forums I see a lot of good things on the Sonicaps including a reasonable source for them. The Sonicap Gen I has many good reviews at a reasonable price point. Hmm… http://www.soniccraft.com/sonicaps.htm
They are a little on the pricy side for sure, but not the most expensive by any stretch. They look like a great price vs performance capacitor. Looking at the SDA-1C schematics I see only the 4.3uf and 12uf are directly in the signal path, and in the tweeter circuits, which is where brightness would occur. I decide to order Sonicaps for these circuits and use them there, but stay with the Solens at 20 and 40uf (39uf used) in the mid and mid SDA circuit to keep costs down. Brightness should not be an issue in the mid crossover circuits.
Now the pesky 12uf has another issue. Polk has done a bypass with the 750pf silver mica cap. My best guess is the engineers used it as a bypass to improve transient frequency response at that point. There are several articles/opinions out there on bypassing capacitors to improve tonal quality or transient response. Some are for and some against. The ones that are against it make a valid electrical argument.
Bypassing was done on a lot of older gear because very high-end capacitors were quite expensive. The Mfr would often go with low or mid grade capacitors, and place a smaller value (cheaper than the big ones) higher-end capacitor in parallel (bypassing). The higher end cap has much better response than the low/mid grade main capacitor. It reacts much faster to subtle and fast changes in the music allowing the capture of quick details, or high frequency harmonics in the music. Thus the main capacitor passes the primary high frequency music, and quick high frequency information that it may have missed due to a slower response is passed by the smaller value, higher quality capacitor. There is much discussion across the net on bypassing to improve high frequency response. There is a fair amount of controversy on the subject. Imagine that…audiophiles disagreeing. ;)
It sounds good in theory, but the problem that can occur is the faster response capacitor has a faster time constant. This can cause portions of the high frequency signal to arrive on the other side of the paralleled capacitors slightly ahead of other portions, in effect widening or smearing the higher frequency information on the other side as the entire signal does not arrive at exactly the same time. We are only talking milliseconds perhaps in difference here. There is of course more discussion on how audible this waveform smearing is. There are even high-end cap mfrs who now recommend against the bypass practice in the signal path. For example http://www.audience-av.com/auricap_a...tion_notes.htm
This would explain why the 750pf cap Polk used has such a wide tolerance. At .00075uf bypassing 12uf, its contribution to the overall capacitance of the circuit is negligible. It appears to exist purely to pass some of the faster high frequency response the slower cap may have missed. As such its absolute value is not critical. After much research and much contemplation I have decided to try completely removing the 750pf silver mica bypass capacitor from the tweeter circuit. I will trust the higher end 12uf Sonicap capacitor to pass the high frequencies and transients that the lower quality 12uf stock capacitor needed bypass help with. In the event there is a serious degradation of the sound (which I doubt) I can always reinstall the silver mica.
I decided to leave the resistors stock as I do not want to risk too much change to the high frequency sound at this time. I believe the caps will have more bang for the buck anyway. I’ve also decided on a mid grade upgrade the binding posts.