352 Chapter 11
The primary:secondary turns ratio of this component would need to be (10 k/3) 58:1. It
is diffi cult to design transformers having such high turns ratios without losses in performance;
consequently, when higher audio quality was required, the LS manufacturers responded by
making loudspeaker drive units with higher impedance speech coils. Before the advent of
transistor-operated audio amplifi ers the most common LS driver impedance was 15 Ω.
With regard to the amplifi er design shown, the input stage ( V 1 ) uses a simple directly
heated triode, with grid-current bias developed across the 4M7 Ω grid resistor, R 1. This
is resistor/capacitor coupled toV 2 , a small-power beam tetrode or pentode, operated with
fi xed bias derived from an external DC voltage source.
Because both V 1 and V 2 will contribute some distortion to the signal—in the case of V 1 ,
this will mainly be second harmonic, but in the output valve ( V 2 ) there will also be a
substantial third harmonic component—the output signal will sound somewhat shrill due
to the presence of these spurious high signal frequency components in the output. The
simplest and most commonly adopted remedy for this defect was to connect a capacitor
(C 4 ) across the primary of the output transformer (TR1) to roll-off the high-frequency
response of the amplifi er as a whole to give it the required mellow sound. The HT line
decoupling capacitor ( C 2 ) serves to reduce the amount of spurious and distorted audio
signal, present on the V supply line, which will be added to the wanted signal present
on theV 2 grid. An amplifi er of this type would have an output power of, perhaps, 0.5 W ,
a bandwidth, mainly depending on the quality of the output transformer, that could be
150 Hz 6 kHz, 6 dB, and a harmonic distortion, at 1 kHz and 0.4 W output, of 10%.
The amplifi er shown in Figure 11.7 uses a circuit of the kind that would allow operation
from batteries, and it was accepted that such designs would have a low output power and
a relatively poor performance in respect to its audio quality: this was the price paid for the
low current drain on its power source. If, however, the amplifi er was to be powered from
an AC mains supply, the constraints imposed by the need to keep the total current demand
low no longer applied, which gave the circuit designer much greater freedom. The other
consideration in the progress toward higher audio power outputs was the type of output
stage layout, in that this infl uenced the output stage effi ciency, as examined later.
11.4 Single-Ended Versus Push–Pull Operation ...........................................................
These two options are shown schematically in Figure 11.8 , in which Q 1 and Q 2 are
notional amplifi er blocks, simplifi ed to the extent that they are only considered as being