[sdiy] feedback configuration - WAS: 1/8th inch questions

[Grant Richter]

> Rarely do I see this done, but it seems to make so much sense.  I always
> have a resistor on the amp output going to jacks to protect the amp from
> short circuits. (usually 470 to 1K).   So, why would one not ALWAYS take the
> feedback resistor from the load side of this presumably relatively small
> resistor for the greater accuracy over varied loads?  Is this OK for
> inverting and non-inverting configurations? Any disadvantages?

This is standard on the Buchla 200 series. The output current limiting
resistor is inside the feedback loop for the op-amp. The limits the current
but the voltage drop is compensated for. You also need to use a current
limiting resistor in series with the op-amp negative terminal, to limit
current into the protection diodes from static discharge.

An op-amp with the output 1K resistor OUTSIDE the feedback loop can be
resistively mixed with another output at a multiple (just short together, 1K
resistors form passive mixer).

With the current limiting resistor INSIDE the op-amp loop this passive mixer
trick will not work.