Preferred Alternative To Impedance Matching
The preferred alternative to impedance matching is ridiculously easy—turn the level control down 6 dB. Of course this means the unity gain mark, or detent position, loses its meaning, but this is far better than losing half your signal.
Many users do not view this issue as a problem. There are so many other variables that require turning level controls up or down that this just becomes part of the overall system gain setting. Most units have sufficient headroom to allow for an unexpected 6 dB of gain without hurting anything.
Besides, the unity gain mark/detent is only a reference point. The whole reason manufacturers give you level controls, is to allow setting the gain you need for your system. If it were important for them to remain at unity, they would not be there. They are yours. You paid for them. Use them.
Summary
Unity gain and impedance matching: a strange dichotomy. One solves the other, but badly.
Impedance matching is not necessary and creates many ills. It reduces signal levels and dynamic range by 6 dB (and possibly signal-to-noise by the same amount). The large currents necessary to drive the low matching-impedance usually degrades total harmonic distortion. And the extra current means excess heat and strain on the power supply, creating a potentially unreliable system.
Simply turning the level control down (or up, as the situation dictates), is the best solution for unity gain disparities.
Cross-coupled output stages have been around for a long time [1]. So has their marketing rhetoric. Some of the many grand claims are even true.
Understanding cross-coupled output stages begins with the following: The only purpose of cross-coupling techniques is to mimic an output transformer under unbalanced conditions. They offer no advantages over conventional designs when used balanced.
Understanding cross-coupled circuitry begins with an understanding of output transformers (Figure 4). Here we see a typical configuration. The output amplifier drives the primary winding of the transformer (with one side grounded), and the secondary winding floats (no ground reference) to produce the positive and negative output legs of the signal. An output transformer with a turns ratio of 1:2 (normal), produces a 2 volt output signal for a 1 volt input signal, i.e., there exists a difference of potential between the two output leads of 2 volts.
The diagram shows how a 1 volt peak input signal produces ±1 volt peak output signals (relative to ground), or a differential floating output of 2 volts peak. (Alternatively, two op-amps could differentially drive the primary; and use a turns ratio of 1:1 to produce the same results.)
So, 1 volt in, produces 2 volts out—a gain of 6 dB. Simple. Note that because the output signal develops across the secondary winding, it does not matter whether one side is grounded or not. Grounding one side gives the same 2 volts output. Only this time it references to ground instead of floating. There is no gain change between balanced and unbalanced operation of output transformers .