|A "universal" balanced input circuit allows transparent insertion of components into any audio system using virtually any pin configuration. *
John Meyer developed the original version of this circuit to meet the unique demands of Meyer Sound's SIM source independent measurement system. Because SIM is designed for transparent insertion into an active sound system, the SIM unit had to accommodate the cabling schemes of the system under test while in operation. As SIM quickly became a worldwide standard for sound system testing, the need arose to find a way to "look at the system lightly" without altering the gain structure, regardless of the source pin configuration and without use of cumbersome external adapters. The circuit in the original design employs two magnetically isolated, serially connected transformers. The three pins of the input XLR jack are connected to the transformer's primary windings in a fashion such that any two of the three input pins will be coupled — without change in gain — through one or both transformers.
The second patent embodies a refinement of the same concept. It describes a variation of the circuit that uses a single center-tapped transformer with matched windings, rather than a matched pair of separate transformers. It also provides additional details on how the active circuitry forces the transformer to operate in current mode (no voltage drop across its secondary and primary windings), and on how signal current from the secondary winding are converted to an output voltage. Circuits based on the revised patent were incorporated into later SIM machines in order to provide greater reliability at reduced cost. The patented input circuit also was used to a limited extent in some Meyer processors. However, since most sound system racks are wired to a single standard, the need for such extraordinary input flexibility rarely justified the added cost.