Parafeed Capacitor Size
Different folks have there own favorite methods for
determining the value of the parafeed capacitor. I have not seen a procedure
that works well in all cases. I would guess that things vary a lot with
different iron and glass. There are several formulae to calculate the value for
the parafeed capacitor but they give different results by as much as an order of
magnitude so I
don't trust any of them. The only way I have found is to determine the value empirically. That is, measure different values.
I have had good experiences with a combination of
measuring the frequency response and listening. The good news is that there
seems to be some correlation between what sounds good and what looks reasonable
from a measurement standpoint.
If the capacitor is too small the bass will roll off and not be as deep as otherwise. If the capacitor is too large there will be a hump in the low frequency and the bass will not be as tight as otherwise.
Measuring is easy – just measure the frequency
response below 20Hz. You want extended bass with a smooth roll off. If the
capacitor is too small, the response will roll off early, like starting at 40 or
50 Hz. As the size of the capacitor is increased, the start of the roll off will
lower in frequency perhaps starting at 20Hz. As you further increase the size of
the capacitor a hump
will appear below 10 Hz. This hump will increase in amplitude as the size of the capacitor is increased. It may be at a very low frequency, like 3Hz so you have to measure all the way down. The correct value to use is just when the hump just starts to develop.
The test doesn't have to be expensive – use generic film and foil caps for your measurement and then when you determine the correct value buy an expensive cap of your choice. The brand of capacitor absolutely affects the sound but there is no "best," only what you like best. By the way, I like Kimber Kaps, which are relatively inexpensive, so I would suggest including them in your list of candidates. AuriCaps are a little better and a little more expensive. My current favorite is the Cardas, which are even more expensive.