Variable center frequency filter

[metal]

Hello

I saw this circuit some where on the net, the author claims that the center frequency can be varied using the potentiometer R1 & R2 which are on the same axis here, from 30Hz to 150Hz, this is a lowpass filter, second order one.

I have tested the potentiometer connections this morning, and measured that with an ohmmeter, the thing amazed me was that the overall resistance doesn't change while spinning the pot, from points A to E that are NOT connected to each other, which always measures to 22K. While the value measured at point S to either points A or E that are NOT connected to each other is variable from 0 to 22K.

Then, C2 sees a resistance of 4.7K + 22K, which doesn't change, and C1 sees a resistance of 4.7K + (0-22K), which can be changed from 4.7K to 26.7K.

The question is, does this badly affect the Q of the filter when trying to change the center frequency.....and, is this circuit worths the try....

Thanks

[12 Cents]

The resistance between E and A should change because the wiper is connected to point A and to point E, respectively...

In my uneducated opinion this thing should work as long as you set the two pots to an identical value, but notice that according to Linkwitz, the two C's should have the same value too (2nd order), unless you add another one of this stage (4th order).

[richie00boy]

If you study the schematic again you will see that both pots will vary 0-22k because one end is connected as an 'input' and the other end is connected to the wiper and works as an 'output'. So as you sweep the wiper along it shorts out part of the pot progressively more. Both pots work the same way -- it doesn't matter which way round you connect them.

The two capacitors should not be the same value as that will make the resonse too droopy IMO due to the Q being 0.5 rather than 0.7071 (Butterworth).

[12 Cents]

The desired Q depends on preference, application and the other filter branches, I would say.

[metal]

Hello guys

I am really glad to see that some guys here are interested, in deed, I have another circuit to talk about later , of course, when we finish this one !!

I agree, both pots are changing, ok, but I said that I measured the resistance between the pots pins which are not connected, so what ever the position of the wiper is, the value stays 22K.

Now, put the standard 2nd order filter circuit in your minds guys and remember that we can calculate Fc = sqrt (2 * pi * R1 * R2 * C1 * C2) , and look again to the circuit under surgery, if the wiper is turned fully CCW for example, then R1 is zero, R2 is 22K. Calculating Fc will show that it is 50Hz @ -3dB. Now, turn the wiper fully CW, then R1 becomes 22K, and R2 is zero, which yields the same Fc which is 50Hz @ -3dB. I can say that what ever the wiper position is, Fc will stay 50Hz. And this is not a bessel, nor a butterworth 2nd order response.


I don't know if I am right or not, but I spent the whole day trying to find where I got this filter circuit from to remove the vague, I found the original circuit now, but that did not introduce any new things to me, I hope that other guys may see some thing that explains the filter.

Thanks

[12 Cents]

Looking at the PCB, the still sane parts of my brain tell me "wait, the connection here in that stereo pot is somehow different than written in that schematic", so the wiper is connected to, say, E in both sections and not E and A respectively

[dimitri]

the schematics has a mistake, reverse A and E terminals of any pot

[metal]

Coooool notice 12 Cents, thanks alot man

If you calculate the frequency response again, you will see that its the 200nF cap with the 4.7K and 22K which determine the Fc, that is 29Hz to 169Hz. But my last question is, what is the 400nF supposed to do, and please explain the way its connected to the 22K pots, does it form some kind of integrator "Low pass filter". I f so, how can I calculate the resulting Fc afterall.

What about this one....which is used as a bass boost in a pioneer car power amplifier, it can be passed/bypassed by a switch, it only boosts bass, and doesn't affect other signals.

Can any one explain to me how this is working please, and better be mathematically.

BTW, dimitri, what did you use to plot this graph.

[12 Cents]

The 400nF is part of the filter and no, sorry I cannot explain exactly how it works with that feedback stuff, sorry, but let me point you to this site, I think you know it:
http://www.linkwitzlab.com/filters.htm

[metal]

Hey 12 Cents

I got it for the second circuit, this is a form of noth filter, which nothes the bass frequencies from the integrator feedback so that they get more amplification.

Thanks alot guys

I am really happy now