470R controls the quiescent current flowing through the BD139/140
complimentary output transistors. Adjustment here, is a trade-off
between low distortion and low quiescent current. Typically, under
quiescent conditions, current is about 15 mA rising to 150 mA with a 50
mV input signal. The frequency response is shown below and is flat from
20Hz to 100kHz
This amplifier circuit is DC biased so that the
emitters of the BD139 and BD140 are at approximately half supply
voltage, to allow for a maximum output voltage swing. R9 and R10
provide a degree of temperature stabilization which works as follows.
If the output transistors are warm, the emitter currents will increase.
This causes a greater voltage drop across R9 and R10 reducing the
available bias current. All four transistors
are direct coupled which ensures: