Editor's note: Part 1 can be found at Design of High-Performance Balanced Audio Interfaces
The transformer floating source consists of a transformer whose primary is driven by an amplifier whose output impedance is effectively zero by virtue of conventional negative feedback. The common-mode output impedances CCM1 and CCM2 consist of the interwinding capacitance for multi-filar wound types, or the secondary to shield capacitance for Faraday shielded types. Differential output impedance ROD is the sum of secondary and reflected primary winding resistances. For typical bi-filar transformers, CCM1 and CCM2 range from 7 nF to 20 nF each, matching to within 2&percent;. Typical ROD range is 35 Ω to 100 Ω. System CMRR will be 110 dB to 120 dB at 20 Hz, decreasing at 6 dB per octave since the unbalances are capacitive, to 85 dB to 95 dB at 500 Hz, above which it becomes frequency independent.
If, instead of the active receiver, a Jensen JT-10KB-D input transformer is used, its full CMRR capability of about 125 dB at 60 Hz and 85 dB at 3 kHz is realized with any of the sources and conditions described above.
The GROUNDED LOAD behavior of these three sources is an important consideration if unbalanced inputs are to be driven. Of course, for any line driver, either output should be capable of withstanding an accidental short to ground or to the other line indefinitely without damage or component failure. This is best accomplished with current limiting and thermal shutdown features.
The GROUND REFERENCED source will output abnormally high currents into a grounded line. Hopefully, it will current limit, overheat, and shut down. If not, at the system level, it will be forcing high, and probably distorted, currents to a remote ground. These currents, as they return to the driver, will circulate through the grounding network and become common-mode voltages to other devices in the system. The usual symptom is described as "crosstalk."
The ACTIVE FLOATING source compromises CMRR, output magnitude balance, and high frequency stability in quest of a "transformer-like" ability to drive a grounded or "single-ended" input. However, to remain stable, the grounded output must be carefully grounded at the driver.  Since this makes the system completely unbalanced, it is a serious disadvantage.
The TRANSFORMER FLOATING source breaks the ground connection between the driver and the unbalanced input. Because the transformer secondary is able to "reference" its output to the unbalanced input ground, power line hum is reduced by more than 70 dB in the typical situation shown in the following schematic
Because the ground noise is capacitively coupled through CCM1
, reduction decreases linearly with frequency to about 40 dB at 3 kHz.