Sv: [sdiy] parametric eq schematic ?

[cfmd at swipnet.se]

Dear Mark,

> someone has asked me to make a three-band parametric EQ optimized for 
> guitar. as i am not skilled enough to design one, i am having trouble 
> finding a suitable design to work from.
> 
> could anyone direct me toward a schematic (preferably with explanation or 
> instructions) that is public domain or that i could license for this 
> purpose ?

A fully parametric EQ can you build by using a number of parametric bandpass filters, such that you can adjust the frequency and Q value. State variable filters have become popular in professional EQs. If you look at my page

http://home.swipnet.se/cfmd/synths/schematics/

You will find at the bottom a simple SVF which can be used. The schematic is over at

http://home.swipnet.se/cfmd/synths/schematics/svf1.pdf

Feel free to use! It's a crude schematic which isn't optimized. I put it up since people where nagging that these things where SO difficult to make, when they infact are not. I've cooked up things according to similar design with no major problem.

To make a 3-band full parametric EQ you take three SVF, use only the bandpass output. Feed all three of them with the same input signal. Feed the bandpass output to the central tapper of a pot, and each end is wired through a resistor to either the positive or negative mix-bus. The mixbuses is built by using resistor in negative feedback over an op-amp and the input of this buffer is from the three bands of each side. You feed the output of one buffer through a resistor to the other buffer, so a diffrentiation is achieved. The input signal is feed to one of the buffers through a resistor. An input buffer should naturally be used.

The pan-pots on the output of each band will set the peak/dip amount for that band, the Q-pot will naturally set the Q-value for the range and the double-ganged frequency pot of each band naturally sets the peak/dip frequency. The resistors at each end of the pan-pot will naturally set the maximum peak/dip values.

I am not back at home, so I can't draw a fancy schematic, but if you can hold your horses some, I could calculate some good values for you at least.
I'll be back at home at least for the weekend.

Cheers,
Magnus

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