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Ultra-LD Mk.3 200W Amplifier Module, Pt.3

Power supply & quiescent current adjustment

By Nicholas Vinen


High DC and high AC voltages are present in this circuit. The power supply uses a total of 80V AC and the amplifier power supply rails are a total of 114V DC. Do not touch any part of the amplifier circuitry when power is applied otherwise you could get a severe electric shock.

The two LEDs on the power supply board indicate when power is present. If they are alight, the power supply and amplifier boards are potentially dangerous.

THE POWER SUPPLY circuit is virtually identical to the Ultra-LD Mk.2 power supply described in September 2008. The changes are in the PCB, which is longer and narrower. This allows it to fit between two amplifier modules mounted on either side of a rack-mount case.

In this configuration, the two DC output connectors line up with the power supply sockets on each amplifier, simplifying the wiring. The AC input terminals face towards the back of the case, where the transformer is normally situated.

Note that if a single power supply is shared between two amplifier modules, the continuous output power is reduced when both channels are driven. However, the music power will not drop so much.

Power supply circuit

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Fig.17: the power supply is based on a toroidal transformer (T1) with two 40V windings and two 15V windings. The two 40V windings drive bridge rectifier BR1 and six 4700μF filter capacitors to produce the ±57V rails.

Fig.17 shows the circuit details of the power supply. It’s based on a toroidal mains transformer (T1) with two 40V windings and two 15V windings.

The two 40V windings are connected together to give 80VAC centre-tapped and this arrangement drives bridge rectifier BR1. This in turn feeds six 4700µF 63V electrolytic capacitors (ie, 14,100µF on each side) to provide balanced ±57V DC rails to power the amplifier.

Two LEDs are connected in series with 3.3kΩ 5W current-limiting resistors across these ±57V supply rails. These serve two purposes: (1) they provide a handy indication that power is present on the supply rails (or when it is not present) and (2) they discharge the filter capacitors when the power is switched off (see warning panel).

The two 15V windings are also connected together to provide 30VAC centre-tapped. These drive bridge rectifier D1-D4 and two 2200µF filter capacitors to derive unregulated rails of about ±20V. These rails are then fed to 3-terminal regulators REG1 & REG2 to derive regulated ±15V supply rails to power a preamplifier module.

The +20V rail is also made available as an output, along with a 30VAC output. The +20V rail is used to power the “Universal Speaker Protector & Muting Module” (described next month), while the 30VAC output is connected to the “AC Sense” input of this module. This latter input is used to quickly disconnect the speaker when the power goes off, to avoid switch-off thumps.

Power supply assembly

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Fig.18: install the parts on the power supply board as shown here, taking care to ensure that all the electrolytic capacitors are mounted with the correct polarity. Be sure also to use the correct regulator at each location. The two LEDs indicate when power is applied and remain lit until the 4700μF capacitors discharge after switch-off.

Start by checking the PCB (code 01109111) for defects such as hairline cracks or under-etched areas and repair if necessary.

Fig.18 shows the parts layout on the PCB. Begin by fitting the two wire links using 0.71mm or 1mm-diameter tinned copper wire (1mm diameter is better but you may need to enlarge the holes slightly). Follow with the four 1N4004 diodes (D1-D4), orientating them as shown.

Install the two 3-terminal regulators next. You will need to bend their leads down by 90° so that they fit the PCB pads with the tab mounting hole lined up correctly. Attach each regulator to the board using an M3 x 6mm machine screws, shakeproof washer and nut, taking care not to get the two different types mixed up. Solder the leads after the screws have been tightened.

The two LEDs can now be installed. These sit flush against the PCB with the flat side of the lenses orientated as shown on the overlay. Follow these with the two 3.3kΩ 5W resistors. These should be stood off the board by about 2mm, to allow the air to circulate beneath them for cooling (use a card­board spacer during soldering).

The two 5-way screw-terminal connectors are made by dovetailing 2-way and 3-way blocks together. Be sure to fit these assemblies with the wire entry holes facing towards the adjacent edge of the PCB.

The two 3-way terminal blocks for the ±57V outputs can then go in. Alternatively, instead of fitting these blocks, you can solder the DC supply leads directly to the PCB pads if it will be mounted right next to the amplifier modules.

The three Quick-Connect (spade) terminals are next on the list. If you are using PCB-mount connectors, simply push the pins through and solder them in place. It will take a while to heat the connectors so that the solder will “take”. However, be careful not to overdo it, as the solder could “wick” through the hole and onto the spade section.

If you are using 45° chassis spade lugs instead, screw them down tightly using M4 machine screws, nuts and washers – see Fig.19. If you can’t get single-ended chassis lugs, cut one side off double-sided lugs.

Finally, fit the electrolytic capacitors, starting with the two 220µF units and finishing with the six large 4700µF units. Be sure to orientate them correctly and make sure that they all sit flush with the PCB.

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