Variable frequency low pass filter

[DigitalJunkie]

I've used this circuit before with TL072/82's and it works nicely.
http://www.users.qwest.net/~ptaylor/Audio/xover-2.jpg

[jackinnj]

There are application notes on both the Analog Devices www.analog.com and Microchip Technologies www.microchip.com websites which specifically address tuneable filters using digital potentiometers -- if you don't want to program the digital pots you can use the Dallas Semiconductor devices which don't require programming.

Here's the link to the Microchip AN737:
http://ww1.microchip.com/downloads/e...tes/00737c.pdf

if you don't want to use a digital potentiometer you can use an inexpensive 8-bit ADC and a bunch of relays to switch values of resistors, proportioned to the transfer function.

One other thing about filters -- don't scrimp on the operational amplifiers -- they need high bandwidth to function properly.

With respect to the digital filters from Linear Tech and Maxim -- there are some with very low noise, low THD. You have to carefully check the specs since some aren't designed for the audio bands.