Audio Engineering

780 Chapter 27

So, the fi rst stage of any audio mixer is a so-called microphone preamplifi er, which
boosts the signal entering the mixer from the microphone to a suitable operating level.
The example given here is taken from a design (Brice, 1990), which was for a portable,
battery-driven mixer—hence the decision to use discrete transistors rather than current-
thirsty op-amps. Each of the input stage microphone amplifi ers is formed of a transistor
“ ring of three. ” The design requirement is for good headroom and a very low noise
fi gure. The fi nal design is illustrated in Figure 27.13.^1 The current consumption for each
microphone preamplifi er is less than 1 mA.

Tune

250 Hz 7 kHz

Cut Boost

Low High

Q

Cut Cut

Boost

Boost

Cut Cut

Boost

Boost

Tuning frequency

Cut

Boost

Low Q High Q

HF

MF

LF

Figure 27.12 : Equalizer controls.

(^1) The amplifi er shown has an input noise density of 3 nV per root hertz and a calculated input noise current
density of 0.3 pA per root hertz ignoring fl icker noise. The frequency response and the phase response remain
much the same regardless of gain setting. This seems to go against the intractable laws of gain-bandwidth
product: as we increase the gain we must expect the frequency response to decrease and vice versa. In fact,
the ring-of-three circuit is an early form of “ current-mode-feedback ” amplifi er, which is currently very
popular in video applications. The explanation for this lies in the variable gain-setting resistor Ra. This not
only determines the closed-loop gain by controlling the proportion of the output voltage fed back to the