These are also known as “line boxes”. With regard to impedance and level, line boxes (in contrast to DI boxes) have an input/out-put transformer ratio of 1:1. Line boxes are used to solve problems relating to ground loops.
To provide protection against electric shocks, many devices have metal housings and a power plug fitted with a grounding pin. This ensures that the device housing is grounded. If a fault occurs, the grounding prevents users from coming into contact with dangerous voltages. If you connect up two such devices using a screened audio cable, this may cause a ground loop, which produces a (50/60 Hz) humming noise. The reason for this is as follows: In an ideal situation the ground potential should always be 0 Volts. Cable routes with different lengths and many other complications can cause the ground potential to deviate slightly. By connecting up two devices with different ground potentials the screening allows an equalizing current to flow between the devices. This current superimposes itself over the audio signal and causes the humming noise. Here, it is important that you do not disconnect the ground contact. It is your only guarantee that high voltage short circuit current can be grounded if a fault occurs. Screened audio cables cannot be used here because of their cross-sectional size and connector type.
The safest way of preventing a ground loop is to isolate the two devices galvanically, i.e. to ensure that the devices are not DC-connected in any way. The best way to do this is by using a high quality audio transformer. Here, the signals are passed through the transformer by inductive coupling. There is no DC-connection from the primary to the secondary winding.
This transformer must be designed for the intended purpose. Use of an incorrectly matched transformer can have severe effects on the frequency response and distortion of the signal. In the field of sound engineering, two types of line levels have become technical standards:
Professional line level varying between 0 and 6 dBu (0.775 V to 1.55 V), max. +20 dBu at 7.75 Volts with 600 Ω source impedance and a line level for semi-professional (consumer) equipment which is at -10 dBV nominal (approx. 0.3 V) lower, but which has a nominal impedance of 10 kΩ. As a transformer can operate in both balanced and unbalanced mode, line boxes are also suitable for converting unbalanced lines routes to balanced ones and vice versa.