Hey... to compensate for DC winding resistance in the input stepdown transformer, you may find that you have a better boost/cut range by running the EQ circuit into a 500-ohm load instead of 600, unless your input transformer has very low DCR (or, if you're not using a stepdown transformer, your source impedance is less than 60 ohms).
I still recommend using a stepdown transformer unless your source impedance is very low--again, less than 60 ohms.
The reason this came to mind is because I was looking at some DCR measurements I made on some transformers in my collection, and it occurred to me that this appears in series with the impedances reflected by the "ideal" transformer. So, the impedance reflected to the secondary would be the source impedance PLUS the primary DCR, divided by the square of the turns ratio, and then in series with the secondary DCR.
So, say your source impedance is 1K (pretty common for "semi-pro" equipment). Suppose the DCR of your windings is 200 ohms for your primary and 40 ohms for the secondary. The 1K source impedance plus the primary DCR would be reflected to the secondary as 1200/16 = 75, in series with the DCR of the secondary = 75 + 40, or 115 ohms. That's quite a bit higher than the 60 ohm figure that you'd expect from an ideal transformer with no copper loss. And then when you add in the real-world DC resistance of the inductors in the filter circuits, your maximum boost is only about 8dB instead of 10. Changing the output load from 600 to 500 brings the differential between flat and boost to 9.5dB, which is much closer to where it should be.
The resistor values in the attenuator network could be adjusted to compensate for this. But calculating them in the first place was a very tedious exercise that I don't intend to repeat anytime soon, so changing the output load to 500 ohms is the solution I intend to use. It only increases the insertion loss by about 1.5dB.
