Audio Engineering

354 Chapter 11

i

VV

(^1) R 1

2



 x

(11.3)

and i x

V

(^3) R 3



(11.4)

from Equations (11.1), (11.2), and (11.3) we have

i

VV

R

VV

(^312) R

2







xx

(11.5)

but it has been seen from Equation (11.4) that i 3  Vx / R 3

therefore xxx

2

123

VV

R

VV

R

V

R







.

(11.6)

If we insert the actual values for R 1 , R 2 , and R 3 , we can discover the difference in output
current fl ow in the load resistor ( R 2 ) between the O/C and S/C conditions of Q 1. For example,
if all resistors are 10 Ω in value, when Q 1 is S/C, Vx will be equal to V , and there will be no
current fl ow in R 2 and the change on making Q 1 O/C will be ( V /20)A. If R 1 and R 2 are 10 Ω
in value and R 3 is 5 Ω , then the current fl ow in R 2 , when Q 1 is O/C, will still be ( V /20)A,
whereas whenQ 1 is S/C, the current will be (–0.25 V /10)A and the change in current will be
(3 V /40)A. By comparison, for the push–pull system of Figure 11.8(b) , the change in current
throughR 2 , when this is 10 Ω and both R 1 and R 3 are 5 Ω in value, on the alteration in the
conducting states ofQ 1 and Q 2 , will be (2 V /15)A, which is nearly twice as large.

The increase in available output power from similar output valves when operated at the
same V line voltage in a push–pull rather than in a single-ended layout is the major
advantage of this arrangement, although if the output devices have similar distortion
characteristics, and the output transformer is well made, the even harmonic distortion
components will tend to cancel. Also, the magnetization of the core of the output
transformer due to the valve anode currents fl owing in the two halves of the primary
winding will be reduced substantially because the induced fi elds will be in opposition.

In addition, an increase in the drive voltage to the grids of the output valves, provided that
it is not large enough to drive them into grid current, will, by reducing their equivalent
series resistance ( R 1 , R 3 in the calculations given earlier), increase the available output
power, whereas in the single-ended layout the dynamic drive current cannot be increased
beyond twice the quiescent level without running into waveform clipping. However, there
are other problems, which are discussed later.