Tuesday, February 7, 2012

Simulation of UREI LP/HP filter module

Maybe this is the last post about UREI parametric filter projects. On the previous post I finalized the design of modular parametric EQ. I have only one idea for the future, the stereo version of this project, but the biggest problem the potentiometer with 4 adjustable resistors, what is not impossible. The simulation of parametric EQ successfully done, but the original values of high pass/low pass filter have to be modified. This filter circuit with original values started too high, and the possible highest frequency is around 15 kHz what is too low.

This is the schematic of HP/LP filter module:

In this module the most important parts are C2 C3 and C4 C5 capacitors. Noted, this equalizer made for instruments amplification like guitar and vocal, not for home hi-fi, for these applications the middle range of audible frequency is good (better) solution than the full frequency range. But I wanted more, because this module is adjustable like all others in this project. I think, if the middle positions of potentiometers have good result, I have reserved range and more possibilities.

On the first version, the capacitor values like in the schematic, and the simulation result of LP filter is here:

This solution with the end position of potentiometer cutting the highest frequency around 40kHz what is too much. Not a big failure, because the value is adjustable, but not required - compared to the original EQ where the end value is around 15kHz. This is the reason why the capacitors changed to 5.6 nF.

The AC analysis of HP filter:

The start value is around 14Hz what is too low. The end value of this filter is around 326Hz what is too low again. This is the reason why decreased the capacitor value to 330nF.

The simulation of both LP/HP filter at one graph:

With the new simulation the capacitors of LP filter changed to 5.6nF, the capacitors of HP filter changed to 330nF. Here is the AC analysis of HP filter:

The LP filter with 5.6 nF:

Both analysis at one graph:

Not impossible to use this filter module alone, but I analyzed how modified the frequency response of parametric EQ modules. The potentiometers of LP/HP filters set to 50% middle positions:

  • The red line if all modules off (bypassed)
  • The blue is the result if only LP/HP filter on with middle positioned potentiometers
  • The green line if all EQ modules on but the LP/HP filter module off
  • The maroon line if all modules on.

See also:

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