Question on replacing output caps

[A Jedi]

The Crack manual states that the banded terminals on the factory caps should go to a specific terminal. Is there also a correct cap "direction" for after market caps? For reference, I bought Solen PA10000 caps (https://solen.ca/products/capacitors/fast-capacitors-250v-pa-series-metallized-polypropylene/solen-pa10000/) which have no directional markings. TIA.

[Tom-s]

The first sentence of the data sheets says theyre biaxial. These are bipolar. Can be used both ways.

[A Jedi]

The first sentence of the data sheets says theyre biaxial. These are bipolar.

Yep that's what was confusing me.

[Deke609]

That first sentence of the data sheet is a bit funky:

"All of our metallized film capacitors use the maximum lowest possible resistivity metallized layer for a given film thickness which is vacuum deposited on a biaxially oriented polypropylene film."

"Biaxially oriented polypropylene film"? Never heard of that so I googled it, and am still confused about how it relates to the cap being non-polarized (which it clearly is, or I'd have had a number of solen caps explode by now). A bit of googling suggests that "biaxially oriented" polypro (or "BOPP" as it is apparently known) is mostly used in clear plastic packaging - but also has applications for capacitors. See, e.g., this link to a short abstract about BOPP from the Society of Plastic Engineers: http://onlinelibrary.wiley.com/doi/abs/10.1002/pen.760281307

All of which is to say, I find Solen's reference to "biaxially oriented polypropylene film" pretty confusing too, and even after reading a bit about it, I still don't understand if or how it relates to the cap being non-polarized.   

Much better would be a sentence to the effect of the following "All of our metallized film capacitors are non-polarized".

cheers, Derek

[Doc B.]

Polypropylene dielectric are the words that matter regarding what makes it bipolar. The biaxial orientation of the molecules just means they stretch the PP film in two directions by inflating a tube of the stuff to make it stronger. It's all on google.

[A Jedi]

Polypropylene dielectric are the words that matter regarding what makes it bipolar. The biaxial orientation of the molecules just means they stretch the PP film in two directions by inflating a tube of the stuff to make it stronger. It's all on google.

Ok, so long story short, I don't need to worry about directionality correct?

[Deke609]

Correct. It's a film cap, so it has no polarity. It will block DC and pass AC in any direction.

Edit: To expand a bit on the above, my understanding is that only electrolytic caps (including tantalum) are polarized and are marked as such. Film and ceramic caps are not. But searching the question "Are film caps directional?" on audio sites can get confusing b/c some film caps have an "outer foil" marking on one end, and some people claim that film caps sound better if installed in a certain direction (for example, VCap recommends different outer foil directionality depending on application). But this is totally different from the polarity of an electrolytic cap, which will overheat and explode if installed backwards - I know this from experience. 

[[Edit 2: I see that Tom-s posted about outer foil markings as I was typing this]]

cheers, Derek

[A Jedi]

Thank you sir. Cheers gang!

[Tom-s]

Only thing i could add is that if the outer foil of a cap is marked, connect this side to ground (or lowest impedance point (previous stage of amp).

[A Jedi]

Actually, now that I'm looking at the cap closer, there is a small "+" on the right side of writing. Sooo....?

[Deke609]

I just checked a bunch of solen film caps I have on hand. Some have the + sign and some don't - even from the same line. For example, I have a bunch of Solen PPE 630V caps and only some have the + sign.

I wonder whether Solen is starting to mark the outer foil? Dunno. So maybe install them with positive connected to signal input. But either way, they're still polypropylene film caps, so you're not at risk of destroying the cap or your amp if you install them in one direction of the other. 

cheers, Derek

[A Jedi]

Thank you sir.

[2wo]

Witch way you connect the outer foil probably won't make much difference on an output cap, maybe worth the effort on low level stuff.  Go with the + sign if it's there.

There is a way to tell, if you have a scope but except for a few critical applications, if I can't tell, I run the music through in the same direction as the  writing on the cap and sleep well at night...John

[A Jedi]

So, got the new caps in and it sounds amazing - I can't believe the difference it made.

Now the question...got the caps hooked up, powered on the Crack and had one channel out. Went back to see what the problem was (with it unplugged from the wall) and got a pretty huge shock when my hand came into contact with something in the area of the transformer. Was that the caps discharging into me? Is there a safe way to discharge them in the future if I need to work on it? TIA.

[Deke609]

Went back to see what the problem was (with it unplugged from the wall) and got a pretty huge shock when my hand came into contact with something in the area of the transformer. Was that the caps discharging into me? Is there a safe way to discharge them in the future if I need to work on it? TIA.

Sounds like it. How long after unplugging the amp did you wait before poking around? The Crack has a 270K Ohm bleeder resistor (on page 40 of my version of the manual) that will drain the stored charge of the power supply capacitors when you power down. If you look at the Crack schematic, it's the first component connecting "B+" and "PSU Ground". But it will take some time to do its job of draining the charge. PB or PJ can probably tell you exactly how long it takes to get the voltage down below "shock level". 

You can check the charge of a power supply cap in an unplugged amp by measuring the voltage between its two leads (or lugs to which they are attached) -- just be careful not to short them: you may see a spark. I suggest using alligator or grabber hook clips leads. Then you can watch as the caps drain.

A faster way to discharge the caps is to make your own discharging thingy: get a 5K or 10K Ohm, 5+ Watt resistor and solder a wire with alligator clip to each lead. You can then connect one clip to the lug to which the negative of a power supply cap is attached, and the other clip to the positive lug. This will drain all power supply caps - i.e., you don't need to do each cap.  And to be sure that the power supply is discharged, you can measure the voltage as above.  I suggest measuring the voltage across the power supply cap farthest from the one to which you attach the discharging thingy - b/c the farthest cap is separated by approx. 500 Ohms of resistance and will take a little bit longer to drain.

cheers, Derek