U1B is a summing amp, and it takes its input from the combination of the input, and the output signal from the CUT bus - this comes from the pots used as the level control for each frequency band. The combined signal is summed again by U2A, this time with the signal from the BOOST bus added. The signal drive to all filters is performed by U2B, the gain of which determines the maximum boost and cut allowed. As shown, the circuit will provide about +/-14dB, and the response is completely flat with all pots centre. Reduce the range by reducing the value of R18 - a value of 10k gives 6dB of boost and cut.
The actual operation of the circuit relies primarily on the amplitude and phase of the selected frequency, and it is beyond the scope of this article to cover it in great detail. The inverted signal drive is compensation for the fact that a standard multiple feedback filter is inverting at the resonant frequency - I shall leave it to the reader to work out exactly what happens (assuming you care, of course. For full details of the circuit look at this figure 1;
All unmarked capacitors (C1, C2, C7 and C8) are op-amp supply bypass caps, and should be 100nF ceramic types for best performance. This is critical if high speed op-amps are used, but still important if using the recommended TL072 op-amps. There is little or nothing to be gained in using "audiophile" grade op-amps for a subwoofer, since the TL072 has more than sufficient bandwidth for the job. Naturally if it makes you feel better, then OPA2134s or similar work beautifully.
The filters are repeated - two are shown in Figure 2, and this functional block is then repeated 4 times to get the eight bands used. Again, unmarked caps C3 and C9 (and those for the additional 4 filter sections) should be 100nF ceramic bypass types. Figure 2;
Figure of supply circuit;
Finally, the DC input conditioning section. The 10 ohm series resistors can be omitted, but they do no harm, and help to isolate the supply from other circuitry that shares the same supply module (the power supply is recommended for this unit). Small inductors can also be used if interference is a major problem, but these are unlikely to be needed in 99% of cases.
Although C41 and C42 are shown as 10uF 63V caps, I suggest that they be 100uF 25V - this is provided for on the PCB, which is now available.
