Audio Engineering

356 Chapter 11

Some local negative feedback from anodes to grids in V 2 and V 3 is applied by way
ofC 4 / R 4 and C 6 / R 7 in an attempt to reduce the third, and other odd-order, harmonic
distortion components generated by the output valves. Since the designer expected that
the output sound quality could still be somewhat shrill, a pair of 0.05- μ F capacitors, C 7
andC 8 , has been added across the two halves of the output transformer primary windings
to reduce the high-frequency performance. These would also have the effect of lessening
the tendency of the output valves to fl ash over if the amplifi er was driven into an open-
circuited LS load—an endemic problem in designs without the benefi t of overall negative
feedback to stabilize the output voltage.

The anode voltage decoupling circuit ( R 3 ,C 3 ), shown in Figure 11.9 , is essential to
prevent the spurious signal voltages from the  V supply line to the output valves being
introduced to the output valve grid circuits. This would, in the absence of the supply line
decoupling circuit, cause the amplifi er to oscillate continuously at some low frequency—
a problem that was called motorboating, from the sound produced in the loudspeakers.

Various circuit arrangements have been proposed as a means of generating a pair of low
distortion, low phase shift, push–pull drive voltages. Of these, the phase inverter circuit
of Figure 11.10 is the simplest, but does not offer a very high-quality performance. It
is, in principle, a bad thing to attenuate and then to amplify again, as is done in this

C 1

0.1μF

Vcc

Output 1

Output 2

R 5

27 K

R 4

200 K

R 6

470 R

R 2

470 R

R 1

1M0

R 3

27K

C 6

C 4

C 3 0.05μF

C 5

25 μF

0.05μF

V1/26SN7 (^1) 0.05μF V1/26SN7 2
C 2
25 μF
RV1
10 K
0V 0V
Input
Figure 11.10 : A simple phase inverter.