WhitakerAudio 60 W Stereo Power Amplifier, User And Assembly Manual

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8
WhitakerAudio
1 Circuit Description
A schematic diagram of the 60 W stereo power amplifier is shown in Figure 1.1. The power
supply is shown in Figure 1.2.
A high-gain pentode voltage amplifier (V1) is used as the input stage for the audio power
amplifier. The output of this stage is direct-coupled to the control grid of a triode split-load type
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), 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. The potentiometer in the input circuit of the 6U8A
pentode section functions the volume control for the amplifier.
A linear taper potentiometer is specified for the volume control. A linear taper was chosen
over an audio taper under the assumption that the power amplifier will be fed by a preamplifier
with an integrated volume control. As such, the function of the volume control is only to set the
nominal input level of the amplifier and would not typically be adjusted during use.
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 cathode-connected 7027A beam power tubes operating in a
class AB
1
push-pull output stage (V2 and V3). Fine adjustment of drive balance is made with
potentiometer R20. The 7027A output tubes are biased for class AB
1
operation by the fixed
negative voltage applied to the control grid circuit from the rectifier circuit. Fixed bias is used
because a class AB amplifier provides highest efficiency and least distortion for this bias method.
Transformer T1 couples the audio amplifier output to the speaker. The taps on the secondary
of the transformer match the plate-to-plate impedance of the output stage to the voice coil
impedance of an 8
Ω
loudspeaker. Negative feedback of 18.5 dB is coupled from the secondary of
the output transformer to the cathode of the input stage to reduce distortion and to improve circuit
stability.
1.1 Power Supply
The use of an oversized power transformer and choke ensure ample energy for power output
stages. The transformer-coupled ac input power is converted to dc operating power for the