WhitakerAudio J C Whitaker 20 W Stereo Audio Amplifier, Manual

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WhitakerAudio
1 Circuit Description
The 20 W Stereo Amplifier is built around two main modules—the power amplifier and power
management system. Each module is described in the following sections.
1.1 20 W Stereo Amplifier Module
A schematic diagram of the 20 W stereo power amplifier and accompanying power supply is
shown in Figure 1.1. Note that power supply functions are generally divided between the right
and left channel boards as follows:
• Primary power distribution and control on the right channel board
• Power supply filtering on the left channel board
These functions are separated in order to maximize the available PWB real estate.
1.1.1 Amplifier Circuit
A high-gain pentode voltage amplifier is used as the input stage for the audio amplifier. The
output of this stage is direct-coupled to the control grid of a triode split-load type of phase
inverter. The use of direct coupling between these stages minimizes phase shift and,
consequently, increases the amount of inverse feedback that may be used without danger of low-
frequency instability.
A low-noise 6U8A tube (V1/ V4), which contains a high-gain pentode section and a medium-
µ triode section in one envelope, fulfills the active-component requirement for both the pentode
input stage and the triode phase inverter. Potentiometers R35 and R36 function the volume
controls for the amplifier.
The plate and cathode outputs of the phase inverter, which are equal in amplitude and opposite
in phase, are used to drive a pair of pentode 6973 beam power tubes in a class AB
1
push-pull
output stage (V2/V5 and V3/V6). The 6973 tubes are biased for class AB
1
operation by the fixed
negative voltage applied to the control grid circuit from the power supply. Fixed bias is used
because a class AB amplifier provides highest efficiency and least distortion for this bias method.
Transformer T1/T2 couples the audio amplifier output to the speaker. Taps are provided on
the primary windings for connection to the screen grid of each output tube. This mode of
operation reduces distortion of the amplifier, albeit with a small power output penalty. The taps
on the secondary of the transformer match the plate-to-plate impedance of the output stage to the