Thanks. That was actually quite helpful.

I'd rather not mess with loss values and calculations of the resistance
needed to compensate. Sounds like my best bet is to replace the
polystyrene capacitors with some higher quality films caps of the same
value, and replace the electrolytics with the same as what's already
there. Agree?

Steve


In article ,
"All Ears" wrote:

Here is a snip from B&Ws FAQ section, it may help....
=20
KE
=20
"We usually find that customers who alter crossover components are not fu=
lly
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 othe=
r
very low-loss type substitutes an electrolytic capacitor. We all know tha=
t
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 w=
hen
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 capacit=
or
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 =3D d / 2=F0fC
where R =3D resistance in ohms, d =3D loss factor, f =3D frequency in Hz and C =
=3D
capacitance in farads.
Loss factor is usually expressed as a percentage at 1kHz. For a "low-loss=
"
electrolytic such as the values between 1=B5F and 20=B5F 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%. Typical=
ly
therefore a good electrolytic capacitor of 5=B5F 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."
=20
"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 (polystyren=
e
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 i=
n
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 littl=
e
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 fe=
d
through a set of capacitors with a combined value of 75 mfd. That's muc=
h
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.

--=20
Steve Cohn


--=20
Steve Cohn