RC filters question?

Question

how do I hook up the following:
V_in -- [series] -->
hi-pass filter -- [series] -->
low-pass filter -- [parallel] -->
(1) hi-pass / tweeter
(2) lo-pass / woofer

How would this look if I were to draw the circuit? Neither the textbook nor any web sites help

Answer 1

See http://www.st-andrews.ac.uk/~www_pa/Scots_Guide/experiment/lowpass/lpf.html.

These filters depend on the properties of the capacitor. A capacitor will not transmit a DC current - the ultimate in low-frequency signals - but will transmit a high-frequency one. Therefore, the low-pass filter runs the signal through a resistor, with a capacitor to ground; the high-pass filter funs the signal through a capacitor, with a resistor to ground. The Web site I cited has circuit diagrams as well as Java applets where you can play with R and C and see what it does to the signal.

Answer 2

Be careful when you try to access the ref. site in answer 1; there's a final period that shouldn't be there.
If you're interested in making a passive crossover network for a woofer and tweeter, check out the ref. web page below. Note RC or LC filters will waste amplifier power while L-only or C-only do not. (However a tweeter shunt R is often used; most program material has less tweeter power anyway.) Also you shouldn't connect either L or C directly across the amplifier output as this may overload the amp. (In other words use a series connection only as shown in the ref.) You have more freedom in design, and can avoid making/buying expensive inductors, if you use separate amplifiers for woof & tweet preceded by RC filters. One useful tip: A simple RC filter has a time constant of R*C seconds, and this results in a "rolloff" frequency of 1/(2*pi*R*C) Hz. At the rolloff frequency, attenuation of the signal is 3 dB. (This is sometimes called the "knee" in the response curve.)
Example: You apply signal to a 1K series resistor leading to a 0.1 uF shunt capacitor to ground. This is a high-pass filter with a 1592 Hz knee. Below this frequency, attenuation approaches 0; above it attenuation continuously increases at a rate of ~3 dB/octave or 10 dB/decade. Exactly the reverse happens if you interchange the positions of the R and C. Now you have a low-pass filter, knee is still at 1592 Hz, and attenuation approaches 0 as frequency increases, and contnously increases as frequency decreases.