Chapter 16 " Active Baxandall "

(Head Wize Project 及び National Semiconductor LF353 Application Notes より参照、引用。)
  The active version of the 2-band baxandall shown in fig.1b incorporated the frequency shaping circuit into the feedback loop of the op-amp. The pots are linear taper , and the double capacitors in the bass and treble sections have been simplified to one per section. The input stage serves as both an impedance buffer and as a phase correcter, so that the output of the EQ is in phase with the input. The equations for this circuit are shown in fig. 1c. This equalizer has true voltage gain ±10 (±20dB).

  A 2-band equalizer is limited in its ability to correct trouble spots in headphones. The circuit in fig.2 is an active version of the Baxandall with an adjustable resonant filter acting as a midrange control………..

  As with the 2-band version, the treble and bass filters are shelving equalizers (fig.4) – although the “shelves” occur outside the audible range with these component values. A better shelving characteristic can be obtained by moving the shelving frequencies closer together. With small modifications to the center frequencies, this circuit could generate a broad facsimile of the biophonic curve. One important characteristic of the biophnic curve is the “dip” at 7.5kHz to simulate the ear canal resonance of normal hearing. If diffuse-field headphones are used, they probably already have a response curve with the ear canal resonance compensation, and then the EQ will help adjust it for the individual listener.
   The treble and mid bands then are the most critical. They must be spaced far enough apart to simulate the response dip at 7.5kHz. Moving the treble shelving frequency higher to 20kHz will help. Because the operating range of the midrange resonant filter overlaps with those of the bass and treble controls, no simple equations can describe how changes in component values will exactly affect the response curves. Apply the equations for the 2-band version to the 3-band, they appear to predict the bass and treble crossover frequencies on the 3-band, but not the gain for the treble control. The midrange control parameters must be set by experimentation.