PA

Fig. 1: this diagram shows how the Mosfet output transistors are mounted.

High-power amplifier module

The Mosfet Module uses the lefthand heatsink from the Playmaster Series 200 amplifier.

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ELECTRONICS Australia, October, 1985

TRANSISTOR

FIG. 1 : TO-3 MOUNTING METHOD

arc and this is likely to be maintained even when the relay contacts are fully open. By connecting the moving contact to the 0V raił, the arc is shorted to deck so that it cannot maintain the damaging current through the speaker.

Under this condition, if the arc is shorted to ground, the resulting very heavy current will soon blow the amplifier’s fuses. Thus the amplifier may be damaged but the speaker is protected.

Power supply

The suggested power supply is shown on the Circuit diagram. It consists of a Ferguson PF4363 transformer, wired for a centre-tapped secondary, 47V a side. This feeds a 6A bridge rectifier and two 8000/iF/75VW electrolytic capacitors to produce balanced supply railsof ± 68.1V, at no signal (with the mains supply at 240V AC).

The transformer also has two 15V windings which are connected to feed a full-wave rectifier (consisting of two diodes) and a 470^iF capacitor. This supplies 21V DC to feed the protection

Circuit. The Iow voltage supply is included on the printed board. The 6A bridge rectifier and 8000/iF capacitors will have to be chassis mounted components.

Construction

Wiring layout is a very critical aspect of a Mosfet amplifier design such as this so the printed Circuit board design we have produced is mandatory.

The printed board measures 153mm by 202mm and is coded 85ma9. It is teamed with a large heatsink which is identical with those used for the Playmaster Series 200. In fact, it is the lefthand side heatsink from the aforementioned amplifier. A smaller heatsink should not be used unless the amplifier is to be used at lower supply voltages.

Assembly of the board is a relatively straightforward process but it should not be hurried. First, make a visual check of the PCB to see if there are any shorted tracks or open circuits in the copper pattern.

Start with the links, resistors and diodes. Take care with the latter, making surę that you don’t confuse the lN914s with the 9V zeners. Make surę that all diodes are connected the right way into Circuit. The same goes for the electrolytic capacitors but the two 47/zF bipolars can go in either way.

The fuse elips, relay and trimpot can be mounted next and then all the small-signal transistors.

The 6.8ąiH choke at the output of the amplifier is wound with 24.5 tums of 1 mm enamelled copper wire on an 1 lmm plastic former. This will involve winding on three layers such that the coil start and finish leads are on either side of the plastic former, at the bottom edge. Bend the leads at 90° so that the choke mates naturally with the holes on the board. Don’t forget to clean the enamel coating from the choke leads (by scraping them with a razor blade) before soldering.

Notę that the 0.15/xF capacitor in the output RLC network is a dual dielectric type, rated at 250VAC. We recommend the Philips type MKT-P unit: 2222 330 40154. Other types are liable to fail.

The four Mosfet power transistors must be isolated from the heatsink using mica washers and plastic bushes, as shown in Fig. 1. We used 5mm lengths of 3mm fibreglass tubing for the insulating bushes. Smear the mating surfaces of the Mosfets and the heatsink with heatsink compound before assembly. The transistors are secured to the heatsink using 12mm 6BA screws and nuts. The nuts (brass) should be soldered to the PCB to ensure reliable long-term contact between the cases of the transistors and the copper tracks.

As each transistor is mounted, use your multimeter (set to a Iow “ohms” rangę) to check that its case is insulated from the heatsink. If the case is shorted, check the insulation around each screw by removing them one at a time. When everything is OK, the gate and source leads can be soldered to the PCB.

Setting up

Assuming that you have completed the amplifier and its power supply, check the voltage on the two supply rails before making finał connection to the amplifier board. Wait for the 800(VF capacitors to discharge below 5V before making connections. Remove the negative supply fuse clip and connect a multimeter set to measure up to 1A across it. It is most important not to connect a load to the amplifier at this stage.

The output stage quiescent current can now be set. Rotate the 500Q trimpot fully antieloekwise and apply power. The trimpot should then be adjusted for a lOOmA reading on the meter.

Switch off the power and wait for the