Richard Knoppow wrote:
here?
Nope, very easy. Just don't use disk ceramics. Something
in a polyester,
or polypropylene would do nicely.


I am curious why you recommend against using disc
ceramics.
BB's are found in a lot of high-quality equipment. They
were supposed to be high-performance deluxe caps when they
were sold but very quickly got a well deserved bad
reputation.

Disk ceramics, particularly Z5U, or X7R, are a bad idea in any
signal path. The feature that gives them high capacitance for
their size also makes them highly piezoelectric: meaning they change
physical dimension with applied voltage. This large change in
dimension makes the capacitance non linear with voltage, and also
makes the capacitor's power dissipation change with applied signal.
That means distortion.

NPO's behave ok, but are of such low values that they would never
be able to substitute for a capacitor such as a BB.

I have used Z5U ceramic capacitors in high signal areas and actually
heard them "sing" along with the signal. Not a good thing!

Their capacitance is also extremely sensitive to temperature, which
makes them useless in most timing applications. [I did use one once
as a temperature transducer in a cheapy RF telemetry device...]

They work adequately in logic circuits as power supply bypass.

BB's are actually a pretty good capacitor, but they have a couple
of failure mechanisms that render them useless over time. They
are an oil filled paper capacitor with a black epoxy case. If you
look at the banded end of the capacitor, you will notice the lead
has a bulge where it leaves the case. That is actually a piece of
brass tubing that was used in filling the case with oil. After the
case was filled, the lead was stuck into the hole, swaged to keep
it from falling out, and then soldered shut. The problem comes when
the BB's banded lead is soldered into the circuit. If no heatsinking
is used, the solder seal melts, and the oil spoils the seal. From
that point forward, the oil will seep out of the capacitor, and moist
air will seep in... it's a slow process that takes many thermal cycles,
but we are talking about old stuff here. The other failure mechanism
is the plastic case shrinks over time and breaks the seal around the
leads, and sometimes even splits the case into pieces.

The BB's are all bad by now, and should be replaced on sight, but when
they were new, they were a nice high performance capacitor.

-Chuck