Audio Engineering

Recording Consoles 781

For more demanding studio applications, a more complex microphone preamplifi er is
demanded. First, the stage illustrated in Figure 27.13 is only suitable for unbalanced
microphones, and the majority of high-quality microphones utilize balanced connection,
as they are especially susceptible to hum pick-up if this type of circuit is not employed.
Second, high-quality microphones are nearly always capacitor types and therefore
require a polarizing voltage to be supplied for the plates and for powering the internal
electronics. This supply (known as phantom power as mentioned earlier) is supplied via
the microphone’s own signal leads and must therefore be isolated from the microphone
preamplifi er. This is one area where audio transformers ( Figure 27.14 ) are still used, and
an example of a microphone input stage using a transformer is illustrated, although plenty
of practical examples exist where transformerless stages are used for reasons of cost.

Note that the phantom power is provided via resistors, which supply both phases on the
primary side of the transformer, with the current return being via the cable screen. The

inverting port but it also forms the dominating part of the emitter load of the fi rst transistor and consequently
the gain of the fi rst stage. As the value of Ra decreases, so the feedback diminishes and the closed-loop gain
rises. At the same time, the open-loop gain of the circuit rises because TR1’s emitter load falls in value.
Consequently, the performance of the circuit in respect of phase and frequency response, and consequently
stability, remains consistent regardless of gain setting.

Two way for

mic/level

10 K

22 K

22 K 47 K

470n

BC109

BC177

1N4148  2

BC107

100 K

100 K

510 130 K 9K 1

10 μ

10 μ 100 μ

220

100 Ω (may be

necessary with

inductive sources)

9v

1 μ

lin Ra

Figure 27.13 : Microphone preamplifi er.