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In Reply to: RE: Metallized caps where space is tight? posted by Michael Samra on March 25, 2017 at 00:45:58
I need a 0.1uF 600V capacitor, and the PCB holes are 20mm apart. It's between the input gain stage and the LTP PI/driver of a Dyna MkIII-based amp. The PCB is crowded, so there's not much room to work.
perpendicular to the plane of the PCB? If so, you could mount a variety of film and foil caps in that orientation (i.e. vertically with the "bottom" lead buried into the PCB). The distal lead then just bends around 180 degrees to enter the PCB 20mm downstream. I've done this many times to circumvent space problems.
I could mount axial caps vertically. I work on Farfisa Combo Organs, and they did that all the time. Of course, the fact that Farfisa did it doesn't exactly inspire me to adopt the practice. It does cause the end caps of some brands of capacitor to pop loose over time.
In the end, I think I've figured out a way to mount a film & foil capacitor on the underside of the PCB that won't cause any problems.
I guess my takeaway from this thread is that no commercial metallized film capacitors are acceptable. But then, how is it that capacitors like Auricaps and SonicCaps are OK? Winding capacitors is not something you can do in your garage. It has to be done by a capacitor manufacturer that's contracted to make them. Do they use a thicker metallization layer?
When circumstances dictate, I have used Auricap XO and Dynamicaps, which are both metallized polypropylene film capacitors, and both types have been acceptable to me for sonics. I'd use them in a heart beat, ahead of using an electrolytic, for example. Some like them better than some film and foil types. This is all a matter of opinion in the end. I have no experience with Sonicaps, but Mike Samra has more experience with a wider variety of capacitors than anyone I know.
I don't know what a "Farfisa Combo Organ" is, but I have had zero issues mounting film and foil capacitors in the vertical orientation on a horizontal PCB, when space is at a premium. I've done this extensively during the restoration and tweaking of a pair of Beveridge direct-drive amplifiers that develop mega-high voltages in the output stage. Any capacitor that is so cheaply constructed as to have the end cap "pop loose" must be regarded as a POS to begin with. Perhaps an organ is subject to severe vibration so as to stress the leads; this is pure conjecture on my part. Even then,...
How much of Auricap, Sonicap ... is "boutique" panache? Metalized polypropylene is not terrible, but it is not as good as film and foil. It definitely is more volumetrically efficient and that is what the OP needs.
A link to a mechanically suitable part (20 mM. lead spacing) follows below. The data sheet suggests (happy surprise) copper leads.
My post was in response to the OP who wrote at the end of his previous post, "I guess my takeaway from this thread is that no commercial metallized film capacitors are acceptable."
I was offering up two brands of metallized polypropylene that I have found to be "acceptable" when using film and foil types was not an option. Of course, "acceptable" is in the ear of the listener. I can only name two brands, because I found those two to be superior to a few others, which I did not name. (For sure, I've tried Solens, REL, and Sprague brands.) But I certainly have not tried every single other option, and I certainly would not say that Auricaps or Dynamicaps are superior to even film and foil types, always my first option.
In answer to your question, I have no idea how much of Auricap's or Sonicap's reputation is BS. I just listen for myself. I wouldn't recommend Sonicap, because I have never experimented with that brand. For that matter, Dynamicaps are in some cases as physically large as a good F&F, so not much of a help in tight spaces.
Just like Teflon and Polystyrene, as well as using High Voltage rated caps in Low Voltage applications, break-in time can require dozens or many dozens of hours playing real music, before sounding "relaxed."
Even the smallest, thinnest metallized propylenes can sound "usable" after they settle in.
I do believe in film capacitor break-in, and for more critical applications, I will put them across the AC mains for a couple of days.
A materials engineer working with polypropylene told me that he found it plausible that changes would occur at the boundary between foil and dielectric, especially with applied AC voltages.
Do you use anything to limit current, when running caps. in?
Another factor that may be relevant is dielectric absorbtion.
No, I don't limit current. I thought that was the point of breaking a capacitor in via this method. The rapid cycles heat the capacitor slightly. Why should being placed across 120V AC mains be harmful to a 600V capacitor -- unless it's very large?
Leslie amps use 0.1uF 600V capacitors for arc suppression in the 120VAC motor switching, and orange drops last for decades in that application.
I believe one has to approach cap ratings with caution when the cap is going across the AC line. Depending on mfgr, a cap rated for 600/630VDC may have a very much lower AC line rating. If the cap is designed for across the line use it will almost always carry a specific AC rating and an "X" designation. As I understand it, the problem with AC line ratings is that while the nominal voltage may be 120VAC, there can be multi-KV spikes due to switching of inductive loads and other events. X/Y caps are designed to deal with these events and also have a defined (safe) failure mode.
I'm not aware of any audio grade cap carrying an X or Y rating and so would be inclined to use some form of current limiting during an across-the-line break-in process such as a properly rated fuse or even an incandescent bulb matching line voltage.
I believe one has to approach cap ratings with caution when the cap is going across the AC line. Depending on mfgr, a cap rated for 600/630VDC may have a very much lower AC line rating.
I have actually seen those caps with a 1.2kv rating,mainly because of AC spikes.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
One does have to approach capacitor ratings with caution, but I am a professional amp tech with years of experience at this. Yes, capacitors do have different DC and AC voltage ratings, ratings that are usually noted on the datasheet. For example, most 1600 to 2000V DC rated capacitors are only rated 500 to 700VAC, but it's not a linear relationship. As the DC voltage ratings go up, the AC ratings plateau.
You frequently find 600V polyester film capacitors installed in guitar amps as "death caps." They could easily end up across the AC line, depending on how the ground switch was set or how the amp was plugged in. And some amps have capacitors across the output tubes plates in parallel with the output transformer. Those can see very high AC voltage swings.
For reference, both the 400VDC and 600VDC 716P types have a 200V RMS frequency-dependent AC rating. The rating starts to decrease around 12kHz, but at lower frequencies like 60Hz, the full rating applies.
...as a pro, you already know of cap rating idiosyncrasies and what might happen if not heeded. And that if used, a "death cap" should carry a type "Y" rating.
I/T do you have any experience with exposing MKP caps. to AC, as a run in method?
I call for MKP bypassed by film and foil, when large valued O/P couplers are needed.
i like the blue phillips pp for cheap compact cap. good enough for nagra tubes. I haven't used them since the ky,s came but they are nice for power supply. I thought about trying them again recently for signal they are nice soft blurry mid and real clean sweet treble. much better then the awful wima red. I know this is bad but years ago I used to put one across a electro for ps bi pas in preamps and they used to sound really good.
Can you gain room by mounting the part on the "abnormal" side of the PCB?
I'm among the very last people to suggest ceramic dielectric in a coupling cap. position, but your mechanical constraints ARE a problem. Unlike most ceramic caps., NP0/C0G dielectric is tolerable as a coupler. A link to an AVX brand part is provided below. Not surprisingly, the part is big, but lead forming could make it a "winner". Also, some ambient heat is OK.
They are awesome.
Polypropylene Film & Foil Capacitor, Copper Leads, 31.8mmL x 13.7mmT x 23.6mmH
you will have to bend the leads to fit 20 mm spacing, but if you have 6mm on each end space, these are definitely a good way to go.
"Polypropylene Film & Foil Capacitor, Copper Leads, 31.8mmL x 13.7mmT x 23.6mmH"
I'm not sure which capacitors you mean.
Oh, must be 716P. I have a drawer full of those. The overall dimensions still make it pretty cramped and put it on top of other hot parts.
These caps are very tolerant of heat...
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