High Pass Switch for Parametric EQ? - diyAudio
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Old 17th March 2011, 03:16 AM   #1
miallen is offline miallen  United States
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Default High Pass Switch for Parametric EQ?


I'm creating a parametric EQ. The following link is the schematic (for one of 3 filters):


My question is, how does one implement a graceful switch between regular bandpass (bell) mode and highpass/lowpass (shelf) mode?

In the current circuit, S7 switches between the bandpass output and the lowpass output of the state variable active filter. I used the LP because that is the only arrangement that I found that yielded the desired result. Are there other methods for switching between HP / BP or is this the conventional method?

Note that I also found that the series resistor on the wiper (R24 in the schematic) had to be exactly the same value as the cut / boost inverted input resistors (R9 and R11 respectively) or the filter would not produce a good highpass / lowpass characteristic.

Also, the current circuit as-is is unstable when the 100K level control (R12) is turned all the way CCW when acting as a lowpass. Meaning when R12 is delivering all LP signal to the cut op amp, the output is distorted.

Finally, I would appreciate any general opinions or comments regarding how the circuit might be improved.

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Old 17th March 2011, 05:30 AM   #2
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Have you considered the carlrec eq from gyraf audio? the hi/ low section has both peaking and shelving function this might help Gyraf Audio - DIY Calrec EQ

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Old 17th March 2011, 06:47 AM   #3
miallen is offline miallen  United States
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No, I am not familiar with any existing EQ circuits other than the J. Audio Eng. paper I based my circuit on. But just from looking at the page you cite, that Caltrec looks like it's using a 3 pole switch to adjust the level into the SVF, adjust the Q resistor network and knock out an integrator. That is a lot more complicated than just swapping in the LP using a 1 pole switch. The LP is the same as the HP just inverted so it at least makes sense to me why what I'm currently doing works. Although the extreme -dB shelving instability is a wart. Maybe it would improve stability if I litter the circuit with caps to cut everything above and below the outer ranges of interest.

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Old 25th April 2011, 07:22 AM   #4
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Hope this thread ain't dead yet - did you come along with your design? been trying to bring a similar design to life - for a bass guitar preamp, that is - and brooded over several different para eq methods (2 of which have added q factor). will swap schems if you like. still not sure about a switchable high cut
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