Open Amplifier

An Open or Uncommitted Amplifier is an operational amplifier that had its feedback resistor removed, leaving only the input resistor in-circuit.

This allows different feedback elements (XIR networks, a Capacitor, Diode and so on) to be connected across the summing junction labeled SJ and the output of the amplifier. Doing this in turn modifies the transfer function of the amplifier, making it potentially more complex and more useful.

Uncommitted Amplifiers in THAT

In THAT, only a Summer can be converterd to an Open Amplifier. This is accomplished by ground the FB terminal to the ground post directly under it.

This pulls a resistor to ground, activating an Analog Switch which in turn opens the feedback path between the Summing Junction and the Output of the amplifier in question.

For Summers 1 to 4, the associated resistors to be pulled to ground are 10k (Version 1.3!) in value, and numbered R46, R49, R52 and R55. The Analog Switches associated are labelled IC29X, with X being a letter A to D, being part of the DG212B-type IC used throughout THAT.

With feedback removed, the amplifier will immediately overload, lightning the central Overload OL light. To correct this, proper feedback components must be connected between SJ and Out jacks.

Feedback in Open Amplifiers

Proper feedback is achieved by patching a component in place of the original resistor.