Audio Engineering

Recording Consoles 791

negative reactance is, almost always, an excellent way to destabilize output circuits.
Output “ padding ” resistors, such as R132 and R133, help a great deal in securing a stable
performance into real loads.

Line-level input stages present different problems. Note that the function performed
by the circuit in Figure 27.21 , over and above its duty to drive the output cables, is to
derive an equal and opposite signal so as to provide a balanced audio output from a
single-ended, unbalanced input signal. This is a common feature of professional audio
equipment because, although balanced signals are the norm outside the equipment,
internally most signals are treated as single ended. The reasons for this are obvious;
without this simplifi cation all the circuitry within, for example, a console would be twice
as complex and twice as expensive.



 

C78

100 n

R138

10 K

R139

1M

R137

NF

Left

unbal

in

Left

balanced

out

3

2

8

V

V

V

V

4

U26A U26B

(^167)
5
R136
R134
R135
R133
TL072 TL072 330R
10 K
10 K
10 K
330R
R132
1
2
3
1
2
3
XLRMale
C76
10 μ
C77
10 μ

 
C81
100 n
R146
10 K
R147
1M
R145
NF
Left
unbal
in
Right
balanced
out
3
2
8
V
V
V
V
4
U25A U26B
(^167)
5
R144
R142
R143
R141
TL072 TL072 330R
10 K
10 K
10 K
330R
R140
1
2
3
1
2
3
XLRMale
C79
10 μ
C80
10 μ
PL2
PL1
Figure 27.21 : Line output circuit.