Microphonics

[voice of the theater]

I thought this post by westend9 deserved it's own thread. I don't recall this concept being discussed here much before.

It looks from the pictures that the soldering has gone well for you. Congrats!
Since you've chosen to solder to the leads, it appears that the Solen caps are now suspended above the circuit board. It might behoove you to use some Blue-tac or even silicone to attach the caps onto the boards. There is a thing called microphonics, also, that attachment to the board will alleviate.

The original caps in the Model 14 crossovers were touching the board, but not glued securely to it. I'm sure they vibrated quite a bit when I played the speakers at a high SPL. As you pointed out, now the caps in my 14's aren't touching the board at all. I did a little research and from what I can find, these metalized polypropylene capacitors aren't very prone to microphonics. I hope that's true because if it isn't, I'm sure they would have suffered the effects even before the upgrade as the caps just "grazed" the boards but were not securely attached to them by any means. In fact, they were probably practically "bouncing"/"buzzing" against the boards when I played them at a high SPL. I think for stability/durablity it makes sense to "glue" the caps to the boards, so I'll probably do that soon--but I'm not sure if that will have any impact on microphonics (especially if these caps aren't very susceptible to that). Here are a couple of things I've read online regarding this:

"Axon: Private brand of capacitors by Orca Deisgns, manufactured by SCR (who also produces Solens caps for them)"
"AXON True Caps are manufactured on high precision Swiss machines with high winding tension to suppress microphonics. They use a multi-wrap encapsulation (white epoxy ends) for protection and oven annealing to improve internal damping."
"MICROPHONICS: Not generally an issue in the kind of capacitors you?d select for loudspeakers"

All above quotes from Crossovers: Passive Parts Selection Guide - AudioKarma.org Home Audio Stereo Discussion Forums

Other quotes I've found online regarding this type of capacitor:
"Consistent, even, winding tension assures freedom from microphonics"

"I tested a few capacitors for microphonics. I took the capacitor and charged it up to 10V, hooked a fairly sensitive scope across it, then banged the capacitor with a pair of pliers. Not a very sophisticated experiment, but almost certainly valid in giving a hint as to microphonic susceptibility. Banging the cap hard and directly with a pair of steel pliers probably creates an acoustical vibration in the cap that is many tens of dB higher than any acoustical vibration, even for a cap in a crossover in a speaker box. Out of five caps, only one showed any evidence of a microphonic response - an old GE mylar cap, which showed a 2 mV p-p response (0.02% of the initial charge of 10V). All others, which included a Dayton metalized polypropylene, showed no discernable response. I was quite impressed by how quiet they were. One could do an even more sensitive test. Do the same thing, but instead of hooking a scope across the cap, connect the cap wires to the microphone input of a cassette recorder (though an appropriate dc block), then later listen to it. Although one will always be able to find poor or outright defective capacitors that display microphonics, this preliminary test suggests that microphonics are unlikely to be much of a problem. Note that the amplitude of microphonics from a capacitor will tend to be larger for larger voltages across the capacitors. Thus, a d.c. blocking cap in a tube amp with 250V across it will likely be a bit more prone to microphonics. "

Above quote from Pjay's capacitor test page

I found many more results that indicated that microphonics wasn't an issue with this type of capacitor, but the bottom line is that I've cranked up the 14's to a very high SPL with no audible evidence of a problem. I'm not sure what I'd be listening for, but I heard nothing but crystal clear sound even at extremely high volume levels. No distortion, unwanted harmonics, etc. BTW, if I had ceramic caps (which are known to be very susceptible to microphonics), what would I be hearing if it were a problem? Distortion, unwanted harmonics, feedback, extraneous sounds (clicking noises, etc.)?

[bfish]

Microphonics is a vacuum tube phenomena/concern. Only pertains to caps, resistors, etc in speculative or uninformed web discussions. XO caps are subject to vibration. Potting, staking with glue etc, or even zip ties prohibits lead-in fatigue and eventual failure caused by minute but repetitive movement.

[aditya]

Yes, for the audio BW, microphonics is limited to the tubes mostly, and any other worry is over-kill.

But for BWs below and above that it is an issue. I still remember, while designing a FM receiver as a final yr college project, we had similar issues. One tap on one of the ceramic disks was enough to throw the radio station out of tunning ! We had to change the design concept & bypass the cap out of the loop. Smilarly with a VLF amp design, we had found the whole building structure to have become microphonic ! So the solution learnt was that for HF related issues one gains by joining everything together, and for the LF one by separating everything from each other.

Aditya