Math expression for UREI546 parametric EQ design

While I found math expression easy for gyrator EQ project, I cannot found mathematical help to customize UREI 545 and 546 EQ circuits. For the EQ module I found nothing, the biggest problem that all parameters can be adjusted by resistors. Therefore I made expression with excel, to count out frequencies to make more than 4 band parametric EQ. The expression is not the best, but working well with several C(a) and C(b) values where C(a) = C(b). If somebody design more than 4 channel UREI 546 EQ, the potentiometes of Q adjustment must be changed too. For example, if the Q adjustment is 10kOhm for 4 bands, for 8 bands must be half (5 or 4,7 kOhm). I hope it's enough and useful for customized parametric EQ design.

Look at the expressions for UREI 546 band-frequencies:

C(a) = C(b), the adjustable resistors for frequency 55kOhm stereo, the permanent resistors are 4,7 kOhm like in the schematic. You have to change the value of C(a) and C(b) capacitors. In the last expression you can get the fpot, which is the frequency depend on the value of potentiometer. The variable Rpot can be set between 0 and 55000 like in the "real life". The result getting in Hz.

The module contains EQ circuit, most be placed to the mainboard PCB:

See also:

• Simulation of UREI LP/HP filter module
• Modular, expandable parametric equalizer based on UREI546
• Simulation of 8 band parametric equalizer
• Simulation and software analysis of UREI 546
• UREI546 parametric EQ like URS VST EQ bundle
• 6 and 10 bands for UREI 546 parametric equalizer
• Parametric EQ Project - Modular UREI545 and 546
• Modular equalizer with gyrator filter
• PDF manual of modular gyrator EQ
• PDF manual of UREI 545 and 546 EQ clone