Name:
         
         Ken Gilbert
  E-Mail:
         ride5000@ride.ri.net
   Date:
         1/13/100 6:00 PM
 Subject:
         Re: simple low pass fiter







Ok Ricky, here's a few hints for you.

As the order of a filter goes up, the "exact-ness" of it's
characteristics goes up as well... ie you get a steeper slope of
rolloff. OTOH, the complexity of the circuit goes up, as does it's
tendency to resonate at some given frequency. A first order filter
does not self-resonate. Anything above that will.

I suggest either a first or second order filter.

A first order filter is simply--in your case--an inductor in series
with the woofer. The equation for the value of inductance is:

L = Rl / 6.28 * f

where Rl is the load impedance, and f is your desired crossover
frequency. In your case, we've got the following equation:

L = 4 / 6.28 * 100 = 4 / 628 = .00637 H, or 6.37 mH of
inductance. 

These inductors should be available from MCM Electronics or
Parts Express, or any number of mail order places. Again,
AIR-CORE inductors are MUCH better than any cored types for
high level (speaker) use.

Now, for a second order filter, you need to add a cap. There are
also different RESPONSES of 2nd order filters, namely
Linkwitz-Riley, Bessel, or Chebychev. They all have slightly
different phase responses and propensities to ring. The best, in
this application (and in most) is the Linkwitz-Riley.

The equation for the inductor is:

L = 0.3183 * Rl / f

Substituting:

L = 0.3183 * 4 / 100 = .01273 H, or 12.7 mH.

for the cap:

C = 0.0796 / Rl * f

Substituting:

C = 0.0796 / 4 * 100 = .000199 F, or about 200 microfarads.

In the second order filter, the cap goes in parallel with the
woofer, and the inductor in series between the + lead of the
woofer and the + lead of the amp.

The cap will need to withstand the peak voltage impressed
across the woofer's voice coil, which can be solved with:

P = V^2 / R

solving for V:

V = sqrt (P*R)

remember that an amps PEAK power is double the RMS rating.
subbing:

V = sqrt (800) = 28.3 volts.

So you should use a 30V rated cap, at least. Also remember that
the cap must be NONPOLAR, since the waveform is completely
AC with no DC offset.

Hope this helps.

ken