Sensei Tim wrote
What does the comment mean that all power supply pin the decouples?
Since the schematic calls for bipolar power and two single op-amp chips, they are suggesting that you filter the power directly on both power pins of each IC (ie. pins 7 and 4 for both chips) with both 10u electros and 100n film caps (so a total of 8 filter caps). Since this is an EQ and not a high-gain circuit, that may be overkill, or maybe not. You can test on the breadboard whether the extra filtering is necessary.
I know 9v in will have a 10u cap to ground but I'm not sure about the 100n caps? Is that just the input cap?
The input cap is C1, it's 1u.
Is pin 4 on both ic's to ground?
That depends on whether you intend to keep the bipolar power, or convert to single-rail 9V supply. With bipolar power, pin 4 gets -9V or -15V according to the schematic. If you convert to single-rail, pin 4 gets ground.
Should this circuit have an output cap as well as pulldown resistors even if it's going to be always on?
Pulldown resistors are unnecessary if it's going to be always on. The output cap depends on whether you convert to single-rail supply, though it won't hurt to have it even if you use bipolar power.
Note that if you do choose to convert to single-rail supply, you have to provide a reference voltage in
the usual way. Attach Vr from this link to pin 3 of both IC's.
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