WhitakerAudio J C Whitaker 20 W Stereo Audio Amplifier, Manual

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20 W Stereo Audio Amplifier
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.
1.1.3 Preamplifier Power Supply Module
A schematic diagram of the Preamplifier Power Supply PWB is shown in Figure 1.2. This board
provides filament and B+ voltages to the optional companion stereo preamplifier chassis. In
addition, 115 V ac is provided for the pilot lamp on the preamp front panel.
An auxiliary power connector is included on the back panel of the amplifier for connection to
the preamp. The pinout for the connector is detailed in Table 1.1.
As listed in Table 1.1, Pins #7 and 8 serve as an interlock. When the umbilical cable is
connected from the amplifier to the preamplifier, Pins #7 and 8 are shorted, which activates relay
RYL301. The relay energizes the filament supply and pilot lamp voltage. It also connects the B+
supply line to the connector. The benefit here is that no voltages (other than 24 V dc) are present
at the auxiliary power connector when the preamplifier is disconnected.
The nominal B+ supply voltage for the preamplifier is +250 V dc. Because the input voltage
to the preamp power supply PWB will vary depending on the amplifier type, R301 is used to set
the approximate operating voltage. Following R301, the B+ voltage is filtered and applied to SCR
Table 1.1 Preamplifier Power Connector Pinout
Pin No. Function Connects To Notes
1 Ground PIN-1 on the PWB Ground
2 Pilot lamp PIN-2 on the PWB 120 V ac
3 Pilot lamp PIN-3 on the PWB 120 V ac
4 Auxiliary B+ PIN-4 on the PWB Approxi250 V dc
5 Filament PIN-5 on the PWB 6.3 V ac for heaters
6 Filament PIN-6 on the PWB 6.3 V ac for heaters
7 Interlock PIN-7 on the PWB Connected to Pin #8 at load
8 Interlock PIN-8 on the PWB Connected to Pin #7 at load
9 Ground PIN-9 on th PWB Chassis ground