building a passive crossover

I understand this is basic, just trying to build something that will work. I would like to build a low pass crossover for an 8"  4 ohm sub with a cutoff of 80 hz.  This sub is only going to be powered by the radio.  21W RMS. The crossover would attach to the speaker wire.  First of all is this possible? If it is, I am not sure whether to use a LC or RC ciruit,  I have found diagrams showing you could use either the resistor or inductor to make one. It seems easier to find the resistors than inductors at the local ratshack.  But my question then becomes,

do I use this schematic with 80 Hz cutoff. L2= 11.25 mHy  C2= 351 uF.

or this one.

where R= 20 ohm  C= 100 uF

I that wasn' t enough- how do I know which resistors or inductors to buy, they may fit into the right ohms or millihenries but how do tell which watt and q ratings, respectively.

Thanks from a newbie with little knowledge and little research.

But it does not tell me which is the right coil to buy.

I can find similiar mHy ratings but not sure about the Q min. current ratings.and dcr and other properties

You do not want to use the RC topology.  A simple 1st order crossover A 80Hz for a 4-ohm woofer is to simply place a 7.96 mH inductor in series with the driver.  Use the lowest DCR rating you can obtain (or should I say, afford.)  You are not likely to find audio-quality crossover inductors at Radio Shack.  The best bet is to order parts from www.partsexpress.com or www.madisound.com

You probably don't want to try and build a 2nd order passive at 80Hz as the component values (especially the capacitor) become quite huge at that frequency. (the 321 uf value in your initial post, for example.)

Thank you for the information, especially where to find these parts.

One more thing.

With the setup above, how would I figure out which amp rating to use for the inductors.

8 mH 100 mA ?    8 mH 1A?      8 mh 6A?

Not sure if that makes a difference. But would like to understand for future knowledge.

I was told by a Radioshack guy that they don't even sell inductors.

I just ordered 2 of these http://www.partsexpress.com/pe/pshowdetl.cfm?&Partnumber=255-026&Cfid=454888&CFTOKEN=53922125 (6db octave, 3500Hz @4ohm) for the 4x6 woofers in my front doors. They will ultimately be bandpassed as they are highpassed from the HU. -The 4x6 woofers are running off HU rear channels, 18wrms. Highpassed @180Hz.

Q: I didn't see any kind of amperage ratings for the inductors. They claim 200wrms, will this be adequate?

I actually am (eventually...) planning on building this: http://www.partsexpress.com/projectshowcase/tritrix/index.cfm I just got a new house and currently have no audio system for the living room. I figure I can build a pair of these for $100, get a 200wrms x 2 @ 4ohm home reciever, add a 10" powered sub, and party on! -This's prolly plenty for me in a quiet setting like my house.

stevdart, thanks. So then the 20g's i'm using do handle 200wrms like they claim? Any hysteresis distortion with these? What about the amp rating mentioned before, do the air core inductors have this? Only iron core?

Oh sure, they'll handle 200 watts.  Figure amp rating, if you want to, using Ohm's Law.  Generally, it's watts rating that is used.  What's hysterectomy distortion?  Oh yes, that's with iron core inductors, not air core.

Those speakers look like the top half of the main speakers I built for my system.  The bottom houses a 6.5 Vifa woofer that takes care of the range of 80 Hz to around 450 Hz.  I've never attempted a TL design.  Good luck with your speaker build!