WhitakerAudio 60 W Stereo Power Amplifier, Руководство пользователя и по сборке, Страница: 11 / 154

11
60 W Stereo Power Amplifier
amplifier stages by the 5U4 full-wave rectifier. The rectifier feeds a capacitor-input filter (C51/
C52) followed by choke L51 and a second capacitor bank (C53/C54). The maximum rated load of
the power supply is 288 mA, set by the rating of transformer T51 (Hammond 300BX).
Bleeder resistors (R51/R52 and R53/R54) are included across each filter capacitor to load the
supply and reduce the possibility of electrical shock during maintenance. The main filter capacitor
banks consist of two 100 µF 450 V electrolytic capacitors in series (effectively ~50 µF). Resistors
of 100 kΩ 2 W are placed in parallel with each capacitor to equalize the voltage distribution and
bleed off the charge when power is removed. The series filter capacitors were chosen over higher
voltage single devices because of cost benefits and layout simplicity.
As shown in Figure 1.2 an ac line input voltage of 120 V nominal is assumed. The primary of
plate transformer T1 can be connected to accommodate other voltages, including 110 V and 220/
240 V. In the case of 220/240 V, special considerations apply and are addressed on an individual
basis. Note that typical residential line voltages can vary widely. The construction steps contained
in this manual assume 120 V ac input. If lower line voltages are present, the amplifier may not
reach the full rated output power of 30 W per channel.
Power thermistor VR51 is used to limit current inrush upon application of ac line voltage to
the transformers. VR51 is rated for 8 A steady-state current. When “cold” (room temperature) the
device exhibits a resistance of 5
Ω
; however, when loaded to 50 percent of rated current, the
resistance is just a fraction of an ohm. The benefit of having VR51 in the primary lead is found
only at power-up. During steady-state operation, the device performs no real purpose (other than
generating heat). Furthermore, the resistance of the device, while small, can impact the B+ supply
voltage under high power output due to the series resistance it adds to the primary circuit (0.5 V
drop across VR51 has been observed). For this reason, relay RYL51 is used to close the circuit
around VR51 after the power supply has reached its operating point. Power for RYL51 is derived
from one of the 5 V filament windings that are not otherwise used on the Hammond 300BX
transformer. The 5 V ac input is rectified by a bridge, filtered, and applied to RYL51 through
varistor VR52. Owing to the time constants involved, RLY51 typically closes about one minute
after power to the supply is switched on.
A varistor is included across the Power On/Off switch to minimize transient disturbance
resulting from removal of primary power from the amplifier. Switches normally exhibit some
amount of “contact bounce” when changed from one position to another. This can lead to noise