WhitakerAudio JC Whitaker 20 W Stereo Integrated Audio Amplifier, Manual

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substantially reduces hum. Zener diodes are provided in this circuit and the bias supply to prevent
excessive voltages due to component failure.
Power thermistor VR51 is included in the primary ac circuit to limit the inrush current when
power is first applied to the amplifier. When at room temperature, the resistance of VR51 is about
10
Ω
but as it heats due to current through the device, the resistance drops to a fraction of an ohm.
Relay RYL51 takes VR51 out of the circuit after the amplifier has warmed up, thereby
eliminating a source of voltage drop in the primary circuit (and heat). The time delay for this
action is determined by a 5 V dc supply on the left channel PWB.
Filter FL51 conditions the input ac to remove noise present on the line. In addition, metal
oxide varistor (MOV) devices VR53 and VR54 suppress transient disturbances that could degrade
reliability of the system.
1.2 Transistorized Stereo Preamplifier Module
A schematic diagram of the preamplifier circuit is shown in Figure 1.2. Two PWBs a1re used.
The main board shown in Figure 1.2 a contains the phono preamp, tone-control, and power supply
filter circuits. The daughter board shown in Figure 1.2 b contains a buffer amplifier for the Front
Aux input.
The phono preamp is a three-stage high-gain circuit intended for use with a magnetic
phonograph pickup device. A nominal load of 47 kΩ is assumed for the phono cartridge. An input
signal of 0.0055 V rms at 1 kHz provides approximately 1 V output from the stage, which is 13
dB below the clipping point of the preamp and about 70 dB above the unweighted noise level. The
maximum output of the preamplifier is approximately 4.5 V rms, which is achieved with an input
level of 0.027 V rms at 1 kHz. At typical operating levels, THD is below 0.3 percent for the phono
preamp stage.
The equalized output of the phono amplifier stage is within 1 dB of the standard RIAA curve.
The equalization circuit includes C103/C104 and R107/R108 for the right channel and C107/
C108 and R117/R118 for the left channel. The output level of the preamplifier is matched to the
line and front panel auxiliary input levels through adjustment of R110 and R120.
All inputs (Phonograph, Line, and Auxiliary) are routed through front panel Input switch
SW101 to Balance control R121 and Volume control R122 before being applied to the tone
control circuit. The nominal input level from the preamplifier is 1 V rms for maximum output