Variable frequency low pass filter

[pk386]

hello all,

I'm working my senior project a monobolck caraudio subwoofer amp.

I have just now ran in to a brick wall, the low pass filter.

The problem I have is this; I cannot find a good knowledge sources on active, variable frequency, filter designs. I would like to vary the cut off frequency and the gain. Roughly 50-300Hz for the frequency range, and about 5-10 for the voltage gain.

I'm currently useing the Burrbrown OPA 2132 to acheive this design. (because I've worked with them before and I had a few extras to work with.)
I cannot come up with Ideas on how to achive the variable frequecy with Pots. You are to avoid the use of variable capisitors in audio circuits correct?

I've found a few books and sources on-line but these only seem to deal with set or fixed cutoff frequency designs.

Finally What other characteristics should I be consitering.

I would be grateful if you all could point me in a good direction.
Thanks For your time.

 

[ingrast]

If you can make do with second order filters, a ganged dual pot will work.

Otherwise I guess a switched capacitor filter is the answer but I don't know if there are audio quality units available.

Rodolfo

 

[theChris]

DSP is a choice, as it is by definition adjustable.

for ASP, you can do things with changing gains on opamp stages. look into "control systems" and "mason's rule" (i think), which will explain how things will affect the pole-zero plots.

 

[HFGuy]

Run a google "Sallen Key" . Its the most common type of active filter. Its used because its very flexible and simple. Its easily made such that its Fc, Q and DC gain are adjustible

 

[sauuuuuce]

here's an idea

This has a hp filter in it two but just pretend that isn't there. Replace the two 5.35k resistances with a dual gang pot. Figure our what gain setting resistors you need in place of the two 1k resistors and you are set. Adjustable lowpass. The pot value you will have to figure. Right now it's set for f-3dB at 3kHz if i remember correctly.

 

[HFGuy]

Thats a Sallen Key

 

[sauuuuuce]

yep

easiest active filter to design ever!

 

[pk386]

Thanks for the replies especially you sauuuuuce. This definitly will give me a good starting block.

I'll Keep you all up dated on this project.

pk386

 

[matjans]

you might also check planet10's site for more in-depth tech info on active/passive filters.

about halfway down the page there's something about the stuff you want.
mind you, there used to be a calculation error somewhere on the page (don't know if he fixed it yet).

/matti

 

[planet10]

about halfway down the page there's something about the stuff you want.
mind you, there used to be a calculation error somewhere on the page (don't know if he fixed it yet).

I think i fixed it... (i'm sooo behind with updates to my site)

there is also a set of active XO links...

http://www.t-linespeakers.org/linx/xolinks.html

dave

 

[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/en/AppNotes/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.