ClariTy 2x300W Class-T Amplifier Part 3: power supply assembly and alignment （2）

Published:2011/8/4 1:02:00 Author:Amy From:SeekIC

Ton Giesberts

Auxiliary voltage and switch-on delay

The 10-V supply for the output stage is also powered by a separate trans­former (TR2). After rectification and fil­tering, the voltage is stabilised using a standard positive 10-V regulator. If a 10-V regulator is difficult or impossible to obtain, a 9-V type can also be used. Two different types of transformer are shown on the schematic diagram for this supply. The PCB is designed to accommodate a transformer with two separate windings (2 x 6 V) or one with a single winding (1 x 12 V). In either case, a 12-V ac voltage is thus made available. Just as with the ana­logue supply, the filter is placed after an extra diode, rather than directly fol­lowing the bridge rectifier.

The voltage provided by rectifier B2 is used to power the relays of the mains switch-on delay circuit. This voltage also has minimal filtering (C19). This causes the relays to disengage as quickly as possible when the mains voltage drops out.

The switch-on delay circuit consists of two relays. The first relay (RE1) switches on power to the amplifier via a set of high-power resistors in order to limit the magnetizing current of the transformer and the charging current for the electrolytic capacitors of the main power supply. These high-power resistors consist of five 10-W, 220-fi resistors connected in parallel. Two of them are mounted above the other three, separated by a certain amount. The peak load capacity must be taken into account in dimensioning these resistors. The transient power dissipa­tion is around 1200 W, and the absolute peak dissipation is actually more than 2 kW!

The second relay (RE2) shorts out the resistors and connects the main power supply transformer (a 1000-VA type in our prototype) directly to the main volt­age. This allows the amplifier to man­age with a relatively small mains switch (6-A rating). With the delay cir­cuit, the effective value of the switch-on current does not exceed 5.2 A.

The drive circuit for the switch-on delay is a standard design. Voltage divider R6/R7/R8 ensures that the volt­age at the base of Tl is not high enough for it to conduct enough cur­rent to energize RE2 until the supply voltage has reached two thirds of its nominal value. The time required for this voltage to be reached is delayed by the charging time of C20. The value of C20 can be kept to a minimum by using equal values for R6 and R7 to set the delay time. When the mains volt­age drops out, D5 causes C20 to be rapidly discharged. With this arrangement, the delay time remains as nearly as possible constant if the supply is switched off and then quickly switched on again.

The required mains power indication is provided by LED D7. It must therefore be clearly visible on the front of the amplifier.

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