> This one was mentioned in PRR's original preamp design thread: http://www.thatcorp.com/1200desc.html
I didn't mention that.
It is however a good implementation of the circuit Pease is discussing (plus a frill due to Whitlock, not mentioned in Pease).
If you take off the input buffers and CM driver, just one opamp and four resistors, this is THE most common "balanced input" in today's world.
> It looks like the description of the Valley People/dbx solutions, but I could be wrong.
No. One design used widely, including my DBX 160 and several Valley boxes, is two opamps. One is a simple inverter. The other is a simple 2-input summing amp. The summer gets one input direct from the jack, and the other input is the inverted signal from the other side of the jack. Impedances are balanced every which way (not so for the 1-amp "diff" input). Both forms have the objection of large resistors and hence significant noise, but generally not a problem at line level. The resistors do protect things from abuse, always an important if non-theoretical consideration in the real world.
No, I'm objecting to Pease's apparent vindication of the one-opamp diff-input. He writes third-person, so maybe he is trying to stimulate our thoughts instead of simply being Pease's Word. The fable's conclusion is correct: IF you buffer both inputs, the one-opamp diff-input (total three opamps) is essentially perfect. But as DBX et al show, there is a two-opamp circuit just as good, some ways better. (Interesting that Pease works for a chip-maker....) And in any box under $999, what you usually get is the one-opamp form, without buffers. It is easy to show "ideal" results... if you drive it with ideal test signals. But in many real situations (as simple as just taking an unbalanced output into the "balanced" input) the CMMR really goes away (though not as bad as a plain unbalanced input).
The Smith Chart is fun, but I couldn't find my butt on it with a trail of breadcrumbs, and I can't think of any audio reason I would want to.