Variable Frequency Low Pass
« on: May 08, 2008, 01:56:10 PM »
I am looking for ideas for an easily freqeuncy variable lowpass filter, ie adjustable with a single pot.  Want to maintain a constant 6 db/oct rollof as the frequncy is adjusted.

One way that could work I think is a simple RL voltage divider where the L is a gyrator circuit.  Effectively this is identical to using the standard graphic EQ circuit, remove the C of the "RLC string", and set for full cut.  Although I guess this is actually a very deep shelving EQ, but would work for my purposes.

A state variable filter takes a dual gang pot to vary frequency (IIRC?) but that is a possibility.

Any other simple ways to do this?  Switching capacitor size in an integrator circuit is another option, but that is not continuously variable.


JohnRoberts

Variable Frequency Low Pass
« Reply #1 on: May 08, 2008, 02:00:53 PM »
??

A simple C and variable R (pot) followed by an opamp buffer.

JR
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.

Variable Frequency Low Pass
« Reply #2 on: May 08, 2008, 03:56:47 PM »
Yes indeed i knew there had to be an easy way I was overlooking...

And the frequency range I need it for does not create any opamp drive or capacitor related issues, so I think we have a winner!

Thanks JR

Variable Frequency Low Pass
« Reply #3 on: May 08, 2008, 05:00:43 PM »
Idea:
Steal from some analog synthesizer. ;)

gyraf

Variable Frequency Low Pass
« Reply #4 on: May 09, 2008, 02:14:34 AM »
naa, synths are usually much more than 6dB/oct..
..note to self: don't let Harman run your company..

Variable Frequency Low Pass
« Reply #5 on: May 09, 2008, 02:51:29 AM »
Quote from: "gyraf"
naa, synths are usually much more than 6dB/oct..


They have filters with variable slopes. However, he did not specify the range of tuning, so may be a pot and a cap will be enough...

JohnRoberts

Variable Frequency Low Pass
« Reply #6 on: May 10, 2008, 03:25:09 PM »
You can make whatever you want, but in general if you use an opamp buffer you can make a 2 pole or even 3 pole filter without much more cost/complexity.

A one pole rolloff is gradual and has significant interaction in band before it rolls off. For a given -3dB half power tuning frequency, it will also be -1 dB at 2f and  -.1 db at 10f.

Using an active 2 pole or 2 pole + 1 passive, you can under damp the active section to give you steep attenuation in the stop band with less loss of in band gain just before roll off.

If one pole is all you want or need, go for it, but don't expect to find much written on the subject. Back when I was selling kits I sold a little stereo 2 pole HPF and LPF set.

JR
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.


 

Related Topics

  Subject / Started by Replies Last post
12 Replies
4255 Views
Last post December 31, 2006, 11:08:47 AM
by Kit
4 Replies
2403 Views
Last post July 23, 2005, 03:48:41 PM
by Ufdah
5 Replies
3508 Views
Last post June 12, 2010, 02:15:40 PM
by Gold
2 Replies
400 Views
Last post January 01, 2019, 04:24:12 PM
by Tubetec