Here is a snip from B&Ws FAQ section, it may help....

KE

"We usually find that customers who alter crossover components are not fully
satisfied with the results. They find that some aspects are improved, but
others made worse. A classic case of this is when a polypropylene or other
very low-loss type substitutes an electrolytic capacitor. We all know that
polypropylene capacitors can sound inherently better, but the change in
internal losses changes the response of the filter, which is designed
assuming the losses of the electrolytic component. What usually happens when
the low loss component is fitted is that the corners of the roll-off are
sharpened, giving a peak in the combined response that can make the sound
unpleasant in various ways depending on the crossover frequency. One way of
getting round this is to wire a small resistor in series with the capacitor
to approximate the original losses. I say approximate because the loss
factor is a frequency dependent resistance. The actual value you need
depends on the original capacitor loss factor and its capacitance value. The
larger the value, the lower the resistance for a given loss factor. The
formula for the equivalent resistance is:
R = d / 2ðfC
where R = resistance in ohms, d = loss factor, f = frequency in Hz and C =
capacitance in farads.
Loss factor is usually expressed as a percentage at 1kHz. For a "low-loss"
electrolytic such as the values between 1µF and 20µF found in tweeter
circuits, d is of the order of 0.025 (loss factor of 2.5%). For values in
the hundreds of microfarads it may be of the order of 0.07 or 7%. Typically
therefore a good electrolytic capacitor of 5µF would have an equivalent
series resistance of 0.8?. If the capacitor has a much larger resistor in
series with it anyway, it's probably not worth altering."

"Steve Cohn" wrote in message
...
I want to recap the crossovers in an old pair of high-end speakers I
own. I've opened up the cabinet and diagramed the circuit, but have a
few questions about what I found. Perhaps somebody can help.

The first thing I noticed is that many of the "better" caps (polystyrene
in this case) are run in parallel with electrolytics to reach the
desired value. I spoke with a tech who said that this was done to
achieve the filter affect of the combined capacitance while taking
advantage of the better signal path through the one quality capacitor in
the circuit. Does this make sense?

I certainly understand that there is a cost advantage of doing it this
way because the circuit uses some very high capacitance values.
Replacing these caps with modern, high-end versions would be
cost-prohibitive, if not impossible. I just want to compromise as little
quality as possible.

That leads me to the next question: how do I achieve the desired values
and maintain the quality without going broke? The midrange signal is fed
through a set of capacitors with a combined value of 75 mfd. That's much
higher than I've seen for sale from any high-end capacitor
manufacturers, and anything close is prohibitively expensive.

The next issue is with matching. I don't own a capacitance meter, so
matching the pairs myself is not an option. Is it worth buying matched
pairs of replacement caps? And what do I do if I need to add
electrolytics into the circuit to achieve the desired value? Can I buy
those matched as well?

Lastly, what are the preferred brands of capacitors to use? Are there
any good web sites that do comparisons, including listening tests?

Thanks very much in advance.

--
Steve Cohn