You've probably heard the engineering terms open loop versus closed loop. It might be a bit of a mystery as to what we're referring to so, if you're curious, I'll try and explain.
The "loop" we're referring to is called feedback. Feedback is an engineering technique to improve the measured performance of a circuit. Essentially, we take the output of an audio amplifier and feed it back into the amplifier's input where the two signals are compared. Any difference between input and output (the definition of distortion) are deleted by this activity.
This deletion of unwanted differences between input and output is accomplished in the same way I have described to you in the past of how a balanced cable works. In the balanced XLR cable example, we have two signal paths, each 180˚ out of phase with each other (meaning that as one signal is rising in voltage the second signal is falling). These two opposite signals are compared to each other and any thing in common (noise and distortion picked up from the cable or circuit) is deleted.
This is called Common Mode Rejection where anything in common is deleted.
A similar differential technique (called differential because it is using the known difference between the two signals—each 180˚ out of phase with each other—as a baseline) is used to eliminate any difference between the input and output signals.
So, back to open and closed loops.
An open loop circuit has no feedback, while a closed loop has feedback of some amount.
Simple.
The trick in design is to create an amplifying circuit that works well and has good performance without closing the feedback loop (open loop), and then judiciously and by ear, closing the loop with as little feedback as is needed to measure well and sound great.
Now you know.
