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highpass passive line level crossover pllxo design

I am looking for information on the design of a very simple passive line level crossover for bi-amping an old set of maggie tympani iii. I have found two different first-order RC filter designs, and wonder if they work equally well. They appear at:

http://www.silcom.com/~aludwig/Sysdes/RCfilter_circuit.gif
and
http://www.t-linespeakers.org/tech/filters/passiveHLxo.html

The difference is that the resistor is in series before the capacitor in the latter and in parallel with the amp following the capacitor in the former. Can anyone explain the different effects of these two simple circuits?
thanks
memeboy
karls
332 posts
 
Fundamentally, all you need is a capacitor in series. Most preamps or amps will have a specified input impedance, and if designed properly, it will be resistive in nature. The standard RC network equation f=1/(2piRC) can then use this input impedance as R, to find the correct value for C. Adding a resistor in series (as in your first link) will add to the overall R by simple arithmetic addition. Adding a resistor in parallel (as in your second link) will decrease the overall R according to the given equation, which gives the net resistance of two paralleled resistors. Note that the series resistor will decrease the input sensitivity of your preamp or amp, while the parallel resistor will not change it. Neither resistor is strictly necessary, but neither will cause any harm either if they are good quality and don't cause a major deviation in the input impedance. This all assumes that the input impedance is resistive to begin with. If it isn't, then a passive filter is not the way to go. Hope this helps.
memeboy OP
46 posts
 
Karls,
That is indeed very helpful! Just what I needed to know.
Thank you very much!!
jeff
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