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In professional audio, analog interconnections between devices typically use "balanced" interfaces. At the source, a balanced output stage differentially drives two signal wires (theoretically with equal and opposite signal voltage waveforms). At the receiver, a balanced input stage extracts the difference between the two signals.
So long as the source and load impedances remain balanced between the two pairs of signals, the (desired) audio signal appears out of phase in the two wires, while the (undesired) hum and noise appears in phase. Thus, the subtraction yields signal without noise. This is especially important when analog audio signals are sent over long distances, or where the ground references for the send and receive ends are different.
Traditional balanced audio output stages
In years gone by, audio output stages often included transformers that provided the two signal phases without additional cost. Among the advantages of transformers is that the output signal can "float." Either end of the transformer (or a center tap) could be grounded without upsetting the drive circuitry. For many years now, though, designers have avoided the costs (budgetary and sonic) of a transformer, opting instead for active balanced output stages.
Simple active balanced output stages use two low-output-impedance amplifiers, one to drive each of the positive and negative output lines. This can increase headroom by 6 dB (compared to a single output) because the load sees two signals, each of which swings to the limit of each individual output stage.
However, while these stages perform well into balanced loads, unlike transformers, they run into trouble driving single-ended loads where one output terminates in a short (or near-short) to ground. This typically results in large currents flowing into the short: currents that must be sourced by the power supply and eventually returned to the output stage's ground system. As well, if the two outputs are independently driven, when one is shorted to ground the system gain drops because only one half of the output signal remains present.
The cross-coupled output stage
In August of 1980, George Pontis (of HP) described a "floating" and "cross-coupled" output stage. (A few months later, Tom Hay of MCI described a similar circuit in an AES preprint.) Pontis' design took feedback from both output ports, effectively allowing the circuit's outputs to "float" much like a transformer. This also avoided the large ground currents into the short, so long as the output remained unclipped. The main drawback to this approach was the need for precision resistors to maintain stability.
In the late 1980s, the SSM division of Precision Monolithics (since acquired by Analog Devices) introduced a monolithic implementation of Pontis' concept in the SSM2142. This part used laser trimming to ensure tight resistor matching. Since then, Texas Instruments (DRV134 and 135), and THAT Corporation (THAT1606 and 1646) have offered their own IC output stages. Like ADI, TI and THAT use laser trimming. The availability of a range of largely pin-compatible parts has made cross-coupled output stages a popular choice for professional audio applications.
Yet, despite the convenience of integration, some problems remain. Perhaps most obvious is that when driving single-ended loads, the headroom boost of the balanced configuration is lost. Signals which do not clip a balanced load may well clip driving a single-ended load. As well, when one of the IC's outputs is shorted to ground, the load for the shorted output becomes a low-resistance internal resistor (50 Ω from ADI and TI, 25 Ω from THAT). Unless IC makers take precautions, the system loses feedback when the driven leg reaches clipping, resulting in large currents flowing into the short.
THAT Corporation licensed Chris Strahm's US patent 4,979,218 for use in its "OutSmarts" balanced output stage. Strahm's concept was to separate the common-mode and differential feedback loops to, among other things, reduce the need for precision resistors. Gary Hebert (THAT's CTO) took this a step further (US patent 6,316,970) to maintain equal signal currents even when clipping into a single-ended load.
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