Likely bc the "adder" is not an adder. I've analyzed it. It's composed of FETs with nonlinear gate voltage-drain current relationship (that's normal). Often

Likely bc the "adder" is not an adder. I've analyzed it. It's composed of FETs with nonlinear gate voltage-drain current relationship (that's normal). Often in radio circuits these are set up and operated in a small linear region. They work well enough in a small voltage range, but in SID they are used for larger amplitudes. Result can be modeled as a sum of Taylor series meaning fun, new partials like x^N get synthesized for free!