API 553

This is one of the projects i'm working on. It's a 3 band swinging input eq from API. On GDIY you can find a kit in 500 series format designed by "PeterC". Since i don't own a lunchbox (YET!) i decided to make a non 500 series PCB for it.

below you will find the original schematic of the 553 eq:


As you might notice the input stage is unbalanced. Since it's nicer to have BOTH the input as the output balanced PeterC on GDIY edited the original schematic with a balanced Differential input stage. See Schematic below.


As you can see from this schematic PeterC added switchable frequencies for all bands. You could calculate your own freqencies. Below at the references you will find a calculator from the opamp labs website to calculate your own frequencies from certain L (inductance) and C (capacitance) values. Note that the hi band only uses a capacitor and no inductor. Do realize that the bands interact with each other so you can't go tottally overboard with the values. Also higher L values mean tighter q. but i'll come back to that in a later post.

The "swinging input" topology is great for making graphic type eq's and various incarnations can be found on the web. A few good ones in particular can be found on the opamp labs website and links to those can be found below at the references.

So on to my version. Below is the schematic, layout and PCB in pdf of my current prototype. Sadly i have not been able to rack it up just yet. so this schematic has not been tested yet. As you can see i omitted the pots and associated components for the eq part and i just made a pcb for the active stuff. Since i wanted to leave this project totally open ended. You could make it fixed band or add switches and make it semi-parametric.

Schematic
Layout
PCB

On the schematic you will see a few changes opa627 instead of opa604 and some resistor value changes. The resistors were changed to compensate for the use of a 1:2 output transformer instead of a 1:3 tranformer to allow for unity gain. Further more i had some 627 opamps and no 604 opamps but pinout is the same so either should work (627 might even need different compensation values). Also eagle doesn't have a footprint for the 604 build in so i used a 627 instead for now.


References:
api 553 500 series kit
http://www.prodigy-pro.com/diy/index.php?topic=12161.0

opamp labs schematics and LC calculator
http://www.opamplabs.com/cfl.htm
http://www.opamplabs.com/325eq.htm
http://headfonz.rutgers.edu/NYDave/opamplabs-109eq.jpg
http://home.mindspring.com/~redbugg/OpampEQ2.jpg
http://headfonz.rutgers.edu/NYDave/opamplabs-325eqapplication.jpg
http://headfonz.rutgers.edu/NYDave/opamplabs-425-eq-app.jpg

discrete swinging input graphic eq QuadEight 712
http://www.prodigy-pro.com/diy/index.php?topic=42413.0