[sdiy] Noob Question: Passive Filter Design

[Byron G. Jacquot]

>So, I'm still slogging slowly through an EE degree, and we've gotten  
>up to the point where we're discussing passive filter design, and  
>we've learned how to construct passive highpass, lowpass, bandpass,  
>and band reject filters using various combinations of resistors, caps,  
>and inductors. Have seen a fair number of filters, I asked my  
>professor what the advantages/disadvantages of using caps vs.  
>inductors are, especially as the discussion relates to audio circuits,  
>but he was unable to answer that. Can anybody help me out? It would be  
>especially helpful if anybody has a suggestion for further reading.

Capacitors have the strong advantage that you can buy good ones, in a wide range of values, from a number of sources.  Good caps minimize some of the parasitic properties, as well...temperature stability, DCR, leakage, etc.

Inductors in values useful for audio work are less common.  Most of the inductor section of the Digikey and Mouser offerings are tiny ones for switch mode power supply work.  Inductors are pretty hard to get the parsitics out of, also.  By the time you've got any wire wrapped around something, you'll have some DCR...and from one wrap to the next, there's inter-winding capacitance.  They are also sensitive to hum pickup from magnetic fields...though with some luck, you can use shielding and humbucking techniques.

I've never met a coil that behaved anything like the "ideal inductor."

Byron Jacquot