Adjustable Boost/Cut Mod
The above design is simple but offers no adjustability. The amount of boost and cut is limited by R1 and R2, so if these were replaced with 33k-ohm linear potentiometers the EQ would be quite flexible. This would give you almost 6dB boost at 3.4kHz and 8dB cut at 10.5kHz. You can also wind the pots back to zero for flat response (with no phase shift either). Somewhere around 12 o'clock should sound pretty good I think. The potentiometers have to be dual-gang for stereo.
Toko Inductors and Tuning
Jupiter, thanks again for the Toko inductor tip. I don't think this circuit will push them anywhere near magnetic saturation, and ferrite hysteresis is minimal so distortion should be very low. Moreover, they are tiny and magnetically shielded, meaning they can all live happily together in a much smaller box. The "J" type with 5% tolerance will be best.
Since they are not adjustable, the tuning will have to be done using pairs of smaller capacitors in parallel.
eg: Instead of C1 = 18nF
Use 15nF + 3.3nF in parallel to lower the frequency.
Use 15nF + 2.7nF in parallel to increase the frequency.
etc. etc.
Phase Shift
Has anyone looked at the phase graph above? The peak phase shift of this circuit is less than 25 degrees(!)
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