Audio Engineering

670 Chapter 22

return circuit is through the shield on the microphone cable at pin 1. Conductors 2 and
3 have the same DC potential and hence there is no direct current in the transformer
windings. In order to accomplish this, the resistors denoted asR must be carefully
matched to be equal to within 0.1%. This precision is required not only for DC balance
but also to maintain a large common mode rejection ratio. Commonly encountered
voltage and resistor values are listed here.

Supply voltage Resistor value

1 2 V 680 Ω 0.1%

2 4 V 1200 Ω 0.1%

4 8 V 6800 Ω 0.1%

There is a trend by some designers to employ electronically balanced inputs in the mixer
input microphone circuitry. In such instances, blocking capacitors must be employed
to isolate the differential mixer input from DC while maintaining continuity for the
microphone signal. Such an arrangement appears in Figure 22.17.

The phantom power circuits of Figures 22.16 and 22.17 work well but both have an
undesirable feature. The necessity of the employment of matched balancing resistors in
both instances limits the current that may be supplied to power the microphone circuitry.
This limitation can be removed through the employment of transformers that are center
tapped on the appropriate windings. Such transformers would be quite expensive because
of the necessity of very accurately having both an equal number of turns on either side

2

1

3

Mic signal

Power return

Phantom supply

To preamp





Mic power

Figure 22.17 : Phantom power circuit when electronically balanced inputs are employed.