Simple EQ using the BA3812L chip

[Mark Hammer]

Slots are a whole lot easier to machine when working with a small flat plate than when working with a box.

The sensible thing to do is to cut out a bigger hole in the box where the plate will go, machine the slots in the plate itself, install and affix the sliders to the plate, and then attach the plate, sliders and all to the box.

Of course the simplest thing to do is to use 9mm rotary pots and slender knobs.  Those little daughterboards that SBE now sells, with pads for 5 pots and an associated ribbon cable, would be primo. http://www.smallbearelec.com/servlet/Detail?no=1290  They may be made for the Bare Box, but that doesn't mean you have to use them with one.

[Crontox102098]

You don't necessarily need the specific chip, as you can do the same thing with a dual opamp chip and individual transistors. The individual transistors approach is flexible in that you can add more channels as you like, one at a time. But the single chip approach does get all the semiconductors down on the board in one clot.

See http://geofex.com/Article_Folders/EQs/paramet.htm at geofex.

National Semiconductor had a great article on graphic EQs, the Q of the filters, and how to do a constant-Q version of the graphic EQ (which the resonator-style EQ as used in the BA3812, KA2223, and opamp versions does not) in their Audio Handbook from the mid-70s. I think it's on line now, and a search might turn it up. Rane also has a discussion of graphic-EQ filter Qs and their ideas on why constant Q is better.  I've always thought the resonator style sounded fine.

R.G. thanks for your thoughts. I've already read your article about EQs and I do understand the flexibility of using transistors but I really wanted a much simpler and less part number version of an EQ. I also wanted an EQ that is freely available (as in free speech, not as free beer ), i. e., the schematic and the PCB, and I could not find one (only schematics). This is why I've started to work on this project and learned a lot on the way.

Thanks for the other refs, I have to look for them.

Cheers.

P.S.
Here is my work in progress PCB:

Which is the software?

[tca]

Which is the software?

Ola Carlos,
the software is this: http://pcb.geda-project.org

I don't know if there is a Windows version. I'm using the version from GNU/Linux distribution Debian (version 20091103).

I didn't have the time to build the EQ, yet.

P.S.
There is Windows version.

[anechoic]

thanks so much for posting this GNU Octave/Matlab code!!
muy perfecto!!

kc

Hi,
  I'm thinking on using circular pots for the first run on this EQ.

The schematic is correct, it was taken directly from the datasheet. The values of the capacitors for each band were also calculated from the data of the datasheet.
C0=1./(2*pi*R0*Q*f);
C=Q./(2*pi*R1*f);
where
R0=1.2e3;
R1=66e3;

All you have to do is to fit the capacitor values to the standards values. For completeness here is the GNU/Octave - Matlab code to get all the capacitors values for a set of given frequencies:

#1;
# Calculations for a simple EQ: BA3812L
# All resistors in Ohm and capacitors in F
# 2012 (c) Tiago Charters de Azevedo
# Verbatim copying and redistribution of this code
# are permitted provided this notice is preserved.

R0=1.2e3;
R1=68e3;
f=[80 240 750 2200 6600];
Q=1;
C0=1./(2*pi*R0*Q*f);
C=Q./(2*pi*R1*f);

disp("Q=1")
disp("        Freq      C0 (uF)    C (uF)")
disp([f' floor(C0'*10^12)/10^6 floor(C'*10^12)/10^6])