Not content to build a Pultec copy, I've been thinking of my own circuit for a passive EQ. My criteria are low loss and lack of undue loading on the device preceding the EQ. I also wanted something that could be built small, with an uncluttered control layout--suitable, perhaps, for building into a console. Here's the general outline of what I've come up with: As you can see, this is more of a theoretical construct than a DIY project, unless you like to do the math yourself. The insertion loss of a passive equalizer is equal to or greater than the maximum amount of available "boost"--in other words, you're not really boosting, you're just decreasing the insertion loss in a particular frequency band. I reasoned that since I rarely use anything near 20dB of boost when equalizing, it would be wasteful to throw away that much signal in a passive EQ implementation. This EQ is unusual, I believe, in that it uses the input impedance of the device connected after it as part of the voltage divider. Or, if running into a very high impedance (such as the grid of the tube), RL could be a resistor included in the EQ itself. The input impedance of the EQ never falls below the value of RL as long as the resonant frequencies of Z1 and Z2 are spaced far enough apart. In this condition, the input impedance ranges from RL (at maximum boost) to 4RL (at "flat"). The unusual "two part" construction of the series and shunt resistance arms is because with a conventional voltage divider (one continuous string of resistors), RL would have to be 10X or greater the total value of the shunt arm. This circuit allows RL to be of a much lower value, about one-third that of the series arm. The resistances in the shunt arm are selected to give the appropriate attenuation when placed in parallel with RL. The formula for attenuation at frequencies where the reactance of Z is very low compared to RL is: 20 * log (Rshunt / Rshunt + Rseries) (Rshunt = shunt arm in parallel with RL). I'm too much of a lazy **** to calculate actual resistor values now. You get the idea, right? The basic operation of the EQ is thus... When in boost, the reactive element (Z1 or Z2) shunts some or all of the series arm and reduces the insertion loss in that frequency band. In "flat" the reactance is out of the equation, and the circuit acts as a simple 10dB voltage divider. When in cut, the reactance acts as a frequency-dependent shunt across the lower arm of the voltage divider, increasing the insertion loss beyond 10dB in the appropriate band of frequencies. Since I never seem to find the time to actually build this thing, I thought I'd put the basic circuit concept out there for review and comments. Thanks.