764 Chapter 27

and meter are fed from the collector of TR a , which is a current source in parallel with
Re. Because Re is a high value in comparison with the emitter load of TR a the voltage
gain during the part of the input cycle when the rectifi er diodes are not in conduction is
very large. This alleviates most of the problem of the Si diodes ’ offset voltage. From the
circuit it is clear that a VU meter is an indicator of the average power of a waveform;
it therefore accurately represents the apparent loudness of a signal because the ear too
mathematically integrates audio waveforms with respect to time. However, because
of this, the VU is not a peak-reading instrument. A failure to appreciate this, and on a
practical level this means allowing the meter needle to swing into the red section on
transients, means that the mixer is operating with inadequate system headroom. This
characteristic has led the VU to be regarded with suspicion in some organizations.

 20

 5

0

 3

VU

(a)

Figure 27.2(a) : VU meter.

18 K

82 K

200 K 10 K

47 K

BC 107

220

220 μ

10 μ

VU meter

fsd  200 μA

approx.

Si

diodes

TRa

Re

470 n

(b)

Figure 27.2(b) : VU meter circuit.