[sdiy] Request brain dump on balanced lines

[Grant Richter]

>>> (To that end, I'll even claim that they typical opamp-style transistor
>>> amp is not an amplifier at all, it's a servo.)
>> 
>> And the servo works nicely if it is fast enough, and as long as it
>> does not hit its limits of operation.
> 
> But at the same time the output is the gain times the error.  :-)
> 

You can see, or even listen to the error, just by monitoring the
differential between the inverting and non-inverting terminals. (difference
= servo failure).

In the case of an inverting summer, the non-inverting terminal is grounded.
So you can just monitor the inverting terminal to measure the dynamic error.

For a modern TL074 style op-amp, the fast slew rate and wide bandwidth allow
the servo to keep up just fine.