There is a a bypass capacitor in the path of the audio signal on a board (from B&K) which feeds a microphone with a few mAs of current in CCLD (constant current line drive) mode.

I don't know its type and its capacitance value is strange: 2.2uj is written on it.

Capacitor And I have a capacitor that I want to replace is it ok and what is it's exact type, I know it's metalized film capacitor but what kind is it propylene?



2 Answers 2


It's not really possible to tell the dielectric of the SMT part without more information. It could be polyester, PET, PPS or PEN as well as possibly PP. Leakage is very low for any of those types, as is voltage coefficient, dielectric absorption (DA) is the main difference, as well as physical (temperature tolerance temperature coefficient) and, of course, availability issues in the required values and voltage ratings.

PET and PPN are more resistant to heat so more suitable for SMT capacitors, especially in lead-free processes.

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A through-hole PP (assuming it's actually polypropylene and not polyester marked as PP) capacitor should be an equal or superior replacement in any case.

The very best film capacitors from a DA perspective are polystyrene, however PS softens at below 100°C so it barely suitable for careful hand soldering and completely impractical for SMT processes (maybe you could solder a socket in). A 2.2uF PS cap would also be huge and quite expensive.

If that's a coupling capacitor then DA is not highly critical but I suppose if an audio golden ear was faced with such a situation they might report sensing higher harmonics of very low frequencies where the coupling is beginning to attenuate.

When we did a lot of work with dual slope converters, the DA was quite visible on the display as rollover error, so polycarbonate (now very difficult to find) or PP (as a poor second) were used for the integrating capacitor.

If you do a web search, you can find some DIY methods of measuring DA, but if it's actually important to you I imagine you'll have a distortion analyzer or equivalent test equipment to measure the effects.


This is an SMD film capacitor, most likely an MKP (metallized polypropylene) type. "2.2µJ" stands for 2.2µF with 5% tolerance (the "J" is a standard tolerance code for film caps). You sadly can't tell the voltage rating from these markings but given that it's audio stuff, it's probably not much.

Be careful with this type of capacitor, they are VERY sensitive to soldering heat and can easily be destroyed by soldering them for too long. They also basically don't break so there should be no need to desolder it in the first place.

  • \$\begingroup\$ Thanks, what is a good replacement for them? is propylene a good replacement? \$\endgroup\$ Jan 29, 2022 at 12:40
  • \$\begingroup\$ MKP is metallized propylene, so yes. \$\endgroup\$ Jan 29, 2022 at 12:44
  • \$\begingroup\$ Why they've metalized it? \$\endgroup\$ Jan 29, 2022 at 12:51
  • \$\begingroup\$ Why there is no match in internet? no shape of capacitor like it? \$\endgroup\$ Jan 29, 2022 at 12:59
  • 2
    \$\begingroup\$ Why? Because a capacitor is two conductive surfaces. For a little MKP cap is two layers of metallized polypropylene film laid over each other, a conductive wire lead is bonded to an edge of the foil. Then the two sheets are rolled or folded to the final size and sealed up in a package. Polypropylene alone is not conductive so it doesn't make a capacitor. A separate metal foil sheet is much, much thicker than physically deposited metal vapor onto a surface (PVD). Thinner is better because thinner means smaller. \$\endgroup\$ Jan 29, 2022 at 12:59

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