Help with Geofex Parametric EQ

[grooveshysta]

Hello,

I am fairly new to all this but I am very pleased that I have managed to get the simple parametric EQ from the geofex website almost working on a breadboard. http://www.geofex.com/article_folders/eqs/paramet.htm

The only thing that seems to do nothing is the frequency pots. I am using 1m pots and looking at the schematic, do I connect pin 1 to the 51k resistor pin 2 to ground, and pin 3 to Vb+ in addition to connecting 2 and 3 together? If I do this, I get really bad distortion.

Also, if I were to make this 3 or more bands, I guess I add another op amp but then I am stumped. I cant figure out where.

Thanks in advance!

The values for C1 to 4 are pretty much what are discribed in the article. C1 is 0.1 C2 is 0.015 C3 is 0.022 and C4 is 0.01.

[slacker]

There's no connection to ground on the frequency pots, lug 1 goes to the 51k resistor and 2 and 3 go to +vb. If you want more bands  just add more 10k pots connected the same as the existing ones and duplicate everything connected to pin 2 of the pots ie: C1 the resonance pot and the simulated inductor.

[PRR]

> add another op amp .... where

Here is the canonical form:

[grooveshysta]

Great diagram! Thank you for all of the responses. This make much more sense now.

[grooveshysta]

Thanks for your help with this. i have just re done my breadboard and will now have no problem adding in bands. However, i cannot get the frequency pots to do anything.  have them connected as discussed. The reason I asked about grounding the middle pin was due to the little arrow pointing up on the middle pin. Also, i have the middle pin of the 10k resonance pot connected to ground. If i dont do this, I get sound going through but non of the pots do anything.

What am I doing wrong??

thank you!

[The_Armadillo]

The little arrow simply indicates the direction of pot rotation. Up-CW Down-CCW
The middle lugs on both pots should be connected to each pot's lug 3, so they act as variable resistors.

[grooveshysta]

Aha! It lives!

Thanks for your help guys!

[The_Armadillo]

Glad to help!