Please help with some filter maths

[Nordic]

I'm trying to make a little subwoofer controler, based on a parametric equaliser design by Rod Elliot... the problem is I can't seem to replicate his math, maybe I'm makeing a mess of decimals or something... but help would be much appreciated...

The circuit is based on two simulated adjustable inductors...

Which is then used as part of a filter... as far as I understand...



The example he gives...

Looking at the first section (without the switch, though), there is a 47nF capacitor, 470 Ohm resistor, and a resistance that can be varied from 47k to 1.047M Ohms (the 47k fixed resistor plus 1M Ohm). The approximate formula for the inductance is:

L = R1 x R2 x C
So
L = 470 x 50k x 47nF = 1.1 Henrys
... and with the pot at maximum ...
L = 470 x 1.05M x 47nF = 23 Henrys
With a series capacitance of 1uF, and since the resonant frequency is determined by ...
Fo = 1 / 2 x pi x sqrt( L x C )
We obtain
Fo = 1 / 2 x 3.141 x sqrt( 1.1 x 1uF ) = 151 Hz with the pot at minimum, and ....
Fo = 1 / 2 x 3.141 x sqrt( 23 x 1uF ) = 33.2 Hz when the pot is set to maximum.
These figures are more than accurate enough for our purposes. With this information, you can modify the frequency ranges to suit any application.

Would someone please be so kind as to type the sum out with proper decimals so that I can figure this stuff out more productivey... I even made a PCB, which I am now stuffing, but I don't know which value caps to use yet....

I want the first control to run from about 20 to 100Hz and the second from about 80 to 200Hz...

Please, I'm begging you for help...

[BWRX]

Hi Nordic.

It is hard working through equations that don't use parentheses where they should!

Here's the first two in scientific notation:

L = (470)(50e3)(47e-9) = 1.1 H
L = (470)(1.05e6)(47e-9) = 23 H

Here's the second two with parentheses:

Fo = 1 / (2*3.141*sqrt(1.1*1e-6)) = 151 Hz
Fo = 1 / (2*3.141*sqrt(23*1e-6)) = 33.2 Hz

[Nordic]

as soon as I figure out how to type that into excel or windows calculator, I will continue...

school was very long ago.... lol

[Nordic]

OK think I got somewhere... not the innitial goal, but somewhere nonetheless...

1uf and 150nf with 470R should give 18.5Hz to 87.5Hz

and for control two, 470n and 22nf should give 70.5Hz to 333Hz.

[AKN]

Hi Nordic,

Originally posted by Nordic
as soon as I figure out how to type that into excel or windows calculator

You can also use Google....
Like the first expression here
Or combine the two expressions into one, like this.

[Nordic]

Nice google trick there...
I typed it into the windows calculator in scientific view, hen copied and pasted to excel, and made those cells scientific format...