TAC Scorpion II - EQ Circuit Mods

[critterkllr]

I was wondering if somebody could help me understand this EQ circuit from the TAC Scorpion II. I'm interested in modifying the selected frequency ranges on the high shelf, low shelf and high pass filter. The high shelf has a switch to select between 6kHz and 12kHz. The low shelf is switchable between 60Hz and 120Hz. The high pass filter is set to 120 Hz at 12db/octave.

My idea is to customize a few channels to give me frequencies that I commonly use on specific instruments. Since these will all be slightly different, I need to understand how to calculate what value cap will be needed to result in the desired frequency.

For the HF Shelf, I understand that R22 is boost. With the 6kHz switch selected, C11 is bypassed leaving 330pF from C12. With the 12kHz selected, C11 is in series with C12, making 165pF.

For the LF Shelf, I understand that R36 is boost. With the 60Hz switch selected, C20 is bypassed leaving .47uF from C21. With the 120Hz switch selected, C20 is in series with C21, making .235uF.

For the HPF, the switch puts two HFPs in the circuit. One before and one after MF 1. Is this a second order HPF since there are two together? Looking at the first half, it would seem as though C36 and R27 combine to create the filter. But does R42 do anything to the filter? If I use C36 and R27 alone, that would mean that the filter is set at 45hZ. But I don't believe that is where the filter is stated to be set.

Thank you in advance for anybody that can help me figure out how to calculate this!

 

[PRR]

Lot of stuff there.

If you just want to change frequency, leave the resistors alone. They were picked for system impedances or available pot values. Change the caps in a section, smaller for higher.

 

[abbey road d enfer]

For the HPF, the switch puts two HFPs in the circuit. One before and one after MF 1. Is this a second order HPF since there are two together?

It is indeed two cascaded 1st-order filters resulting in a 2nd-order filter. Somewhat jurassic as a concept, but it works.

 

[critterkllr]

Lot of stuff there.

If you just want to change frequency, leave the resistors alone. They were picked for system impedances or available pot values. Change the caps in a section, smaller for higher.

That's what I'm looking for some advice on. How to determine the cap value that I need to use to result in the desired shelf frequency boost.

I've found some examples of shelf filters that resemble the filters that TAC used, but not exactly. The formulas that accompany the filters that I've found are pretty intense (for me). This makes it difficult to figure out what to plug into the formulas given.

 

[moamps]

How to determine the cap value that I need to use to result in the desired shelf frequency boost.

fxC=const.
HF:
6000x330=12000x165=1980000
so Cx=1980000/fx (pF)
LF:
60x0.47=120x0,235=28.2
so Cx=28.2/fx (microF)

For the HPF you should use R42 and R24 also in the calculation. But if you know stated HPF frequency of the module,  you can easily find constant of the filter so you can find desired capacitor for another frequency using the formula above.

 

[critterkllr]

How to determine the cap value that I need to use to result in the desired shelf frequency boost.

fxC=const.
HF:
6000x330=12000x165=1980000
so Cx=1980000/fx (pF)
LF:
60x0.47=120x0,235=28.2
so Cx=28.2/fx (microF)

For the HPF you should use R42 and R24 also in the calculation. But if you know stated HPF frequency of the module,  you can easily find constant of the filter so you can find desired capacitor for another frequency using the formula above.

Thanks for that! So it is constant?  I didn't know if there was an exponential change.

 

[PRR]

If you double the cap(s), the place where the effect happens shifts down an Octave (2:1).