AN-108: Direct Box Transformer Applications and Benefits

Direct box transformers are specifically engineered to take high-level unbalanced instrument signals and convert them to balanced, low-impedance microphone-level signals. CineMag has long been an innovator in this category, with designs that emphasize maximum isolation and clean performance over long cable runs.

Dual Faraday Shields for Superior Isolation

One of the assets setting a true direct box transformer from another style is the method the input signal is isolated from the signal on the output of the transformer, which is done with Faraday shields.  These shields consist of  conductive layers (typically copper foil) that are interposed between windings, which are electrically connected to the signal grounds (there are two distinct ones).  Unlike a microphone input transformer with only one (1) Faraday shield, direct box transformers have two.  This allows the driving signal to be ultra-isolated from the the secondary of the transformer.   With this, the preamplifier can be totally isolated from the signal on the primary side, avoiding ground loops which will cause hum.

Signal Step-Down and Low Output Impedance Benefits

A direct box transformer takes a higher level signal, usually from an instrument, and reduces the signal to microphone level.  This accomplishes certain objectives:  The sent signal level now is compatible with the microphone preamplifier’s operational input level.  Signal level reduction with a transformer makes the impedance on the secondary very low, which inherently occurs when the transformer steps down the signal level.  That allows for excellent bandwidth when driving the cable even if the impedance of the signal on the primary side is high.