Change Center Frequency on Eq circuit

Hi,
The circuit showed in the photo it's from an EQ, thepot simpy controls the cut or boost in the center frequency, in this case 650hz.
From what I can understand by analyzing the circuit to change the center frequency is just a matter of changing the values of resistors R12 and R14. Higher Values will lower the Frequency.

Can you please help me on how could I calculate this values?
let's imagine I would like to have 250hz has the center frequency, what would be the values for R12 and R14?

Thank you

Whoops said:

The circuit showed in the photo it's from an EQ,

Click to expand...

it is from a bandwidth limited gain stage. Complete 3d night eq circuit is highly parts-values sensitive and relies on all frequency bands being present and exactly 2 octaves apart, so the sum of all bands would recombine for the total audio spectrum. Schematic parts values are only rough approximations because of parts tolerances.

thepot simpy controls the cut or boost in the center frequency,

Click to expand...

It doesn't. It controls the amount of boost (cut is only lesser boost). This is a non-inverting gain stage, so voltage gain within the pass band will not fall below unity gain 1. To get a partly similar to cut behaviour, there is a -22.2dB R25/R24 voltage divider in front of all bands. The rev.log.pot+R39 is one of the input resistors, setting the amount of gain in context with R30, the feedback resistor of the following summing stage.

in this case 650hz.

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626.594Hz with perfect shown parts values. If the neighbouring 160Hz band is 160Hz for real, you want this '650Hz' band being exactly 640Hz (not 650Hz or 626.594Hz), or the overall response will be a bumpy road with all level pots set for +/-0dB.

From what I can understand by analyzing the circuit to change the center frequency is just a matter of changing the values of resistors R12 and R14. Higher Values will lower the Frequency.
Can you please help me on how could I calculate this values?
let's imagine I would like to have 250hz has the center frequency, what would be the values for R12 and R14?

Click to expand...

1st.order filters with -6dB/oct. slope. -3dB cutoff frequency is 1/(2*PI()*R*C) with R in ohm and C in Farad.
For the HPF part R12=1/(2*PI()*250*2E-6)=318.31ohm and for the LPF part in the same -3dB spot R14=1/(2*PI()*250*200E-9)=3183.1ohm. The 2E-6 is the paralleled C5+C52 2uF cap value in Farad. Same goes for the paralleled 100nF caps C8+C17. The parts value relation of R12 and R14 also set the amount of gain for this specific '650Hz' band, so tweaking only one resistor in order to compensate for an off-value cap will as well change the amount of gain of this stage.
Maybe read the 3d night eq thread(s) in the Filters/Equalizers section ...

> change the center frequency is just a matter of changing the values of resistors

Almost always, the resistors values are set by DC conditions, the cap values may be changed freely to suit AC/Audio needs.

Change the caps.

In this case you *could* do either. You usually do have to re-pick resistors to cover a wide frequency change without difficult cap values. However it is good to start a habit of reaching for the capacitors first.

Bigger caps is lower frequency, "usually" in direct proportion.

Thank you so much for your help and clarification.

I can see now that the frequencies are dependent from each other.

I will just leave my Night Eq unit as it is.

Just one last question,
the 2,5K is a shelf filter, is that possible to change this between Shelf and peak without affecting the way the circuit works.
Basically the peak switch would make the 2,5Khz Filter work like the SUB, 40Hz, 160hz, 650hz frequencies.

Schematic of the Night Eq Attached

Whoops said:

Just one last question,
the 2,5K is a shelf filter, is that possible to change this between Shelf and peak without affecting the way the circuit works.
Basically the peak switch would make the 2,5Khz Filter work like the SUB, 40Hz, 160hz, 650hz frequencies.

Click to expand...

You may try putting a 180nF cap across R11. You may then find that changing the frequency of the variable shelf creates a hump (2.5k) or dips (10k and higher). The 5k position seems about right.

You may try putting a 180nF cap across R11. You may then find that changing the frequency of the variable shelf creates a hump (2.5k) or dips (10k and higher). The 5k position seems about right.
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thanks

Are you saying that if I put the 180 cap across R11 the variable shelf will change its behaviour?

Thanks