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I'm planning on building a simple 5 band EQ using the BA3812L chip but changing the frequencies to more suitable values. The frequencies that I'll be using are the ones from Mesa Boogie DC-5 amp. Here is the schematic:

And here is a copy of the Mesa Boogie DC-5 owner's manual about a similar EQ.

The capacitor values were obtained using a simple GNU/Octave script:

#1;
# Calculations for a simple EQ: BA3812L
# All resistors in Ohm and capacitors in F

R0=1.2e3;
R1=68e3;
Q=.7;

f=[80 240 750 2200 6600];

C0=1./(2*pi*R0*Q*f);
C=Q./(2*pi*R1*f);

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])

One can relate the octave bandwidth N to quality factor Q by the expression or inversely This is important because one would like to be able to control the complete frequency range of the guitar signal.

For the EQ frequencies 80, 240, 750, 2200 and 6600, roughly a factor of 3 for consecutive frequencies, and a Q factor of order 1 one gets a 1.5 band width in octaves, so because of the ratio of the EQ frequencies one can, with this Q=1, control the full interval of frequencies between 40Hz to 10kHz.

I think I'm getting it right?!

Here are the capacitors values fitted to the standard values freq real_freq real_Q C*1 C*2

f    f    Q   C*2    C*1
80   83   1.1 1.5u   .030u
240  257  1.1 .47u   .01u
750  830  1.1 .15u   3n
2200 2137 .9  .068u  1n
6600 6260 1   .022u  .36n

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For attribution, please cite this page as:

Charters, T., "Simple guitar 5 band EQ using the BA3812L chip": https://nexp.pt/simple-eq.html (30-11-2024 [23:02])

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2024 Tiago Charters (cc-by-sa)