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120W public address amplifier - 2 Last month we presented the circuit for our new 120W PA amplifier and showed you how to assemble the PC boards. This month, we show how to assemble everything into a rack-mounting case. By LEO SIMPSON All the parts for the 120W PA Amplifier mount neatly into the high energy rack mounting case as sold by Altronics (Cat. H041B). We will assume that you have a completely finished chassis with all holes drilled. The order of assembly is to wire up the power supply, then the input sockets, install the mixer board and potentiometers and then the power amplifier module. To make the wiring job easier, we suggest you remove the front panel by undoing the four retaining screws. Not only will this make access to the chassis easier but it will lessen the chance of scratches to the front panel while assembly is in progress. In fact, it is a good idea to put 44 SILICON CHIP masking tape over the edges of the front panel, to avoid damaging the anodised finish. Power supply wiring The first step in wiring the power supply is to install the power cord, mains fuseholder and insulated terminal block. The power cord should be installed using a cord-grip grommet, to firmly anchor it. Then fit the mains fuseholder and wire it and the power cord to the 4-way insulated barrier strip. We used a neon-lit miniature double-pole rocker switch for the power switch although a larger type probably would be better for ruggedness and reliability. We suggest that both the mains switch and fuseholder be fitted with heatshrirtk sleeving to cover up the live terminals. Since we have recommended that the front panel be removed from the chassis for the initial wiring, leave out the mains switch at this stage and substitute two short loops of insulated hookup wire across the insulated terminal block. Later, when the front panel is installed, the loops can be removed and the mains switch wired into circuit. The toroidal power transformer is bolted to the rear panel of the chassis. It is sandwiched between two large neoprene washers and retained by a 90mm diameter steel plate. Then install the bridge rectifier and two chassis mounting BOOOµF 63VW electrolytic capacitors. The main earth point is via two solder lugs adjacent to the insulated terminal block. Install the earth wiring exactly as shown in the photos and in the wiring diagram. Now before proceeding further, MIC 1 ~~- 'D R ~AUX1 ~ A R C MIC 2 ~AUX2 1OOV LINE TRANSFORMER POWER TRANSFORMER E PREAMPLIFIER BASS Fig.1: this diagram shows how everything is wired together in the chassis. Be sure to use shielded cable where shown, to keep noise to a minimum. The supply wiring to the power amplifier is run using heavy gauge tinned copper which is sleeved with plastic tubing. Arrange the wiring as shown in the photos to minimise distortion. JA NUARY 1989 45 This view into the right rear corner shows how the lOOV line transformer and power transformer are mounted. Install the mains wiring in a professional manner and insulate the exposed mains connections at the switch with plastic sleeving. check the power supply operation. Carefully check your wiring and then apply power. The supply rails should be close to ± 50V DC. Switch off and wait for the B000µF capacitors to discharge to below 5V before proceeding with further work on the chassis. Disconnect the power plug from the mains. Caution: remember that the power supply puts out a total of 100 volts DC. This is a potentially dangerous voltage. Make sure you don't come in contact with it. Now proceed to install the rest of rear panel hardware. This includes the toroidal line output transformer which mounts the same way as the power transformer. PCB installation The mixer board can now be installed. It can be mounted on plastic PCB mounts or using screws, nuts and spacers, as in our prototype. You can then install the shielded 46 SILICON CHIP cables from the input sockets to the mixer board. The line inputs are wired in using figure-8 shielded cable while the balanced microphone inputs use twin-shield cable (ie, two insulated wires in a common shield). At this stage we suggest the power amplifier module be installed. Smear heatsink compound over the mounting surface of the heatsink bracket and secure it to the lefthand heatsink with four screws and nuts. You can now wire up the power supply to the amplifier module. Now while these connections look straightforward they must be done exactly as shown in our photos. The three supply wires are run in stiff heavy gauge (say 16 gauge) enamelled or tinned copper wire which is then sleeved. The reason for arranging the wiring as shown is that small variations in the layout can lead to very large increases in harmonic distortion, particularly at the high frequencies. Using the layout shown, we were able to keep the harmonic distortion to below 0.3 % across the whole audio frequency range at up to full power. This is a good result for a powerful amplifier with a line transformer. Setting up the power amp module Now remove the negative supply fuse from its clip and connect a multimeter set to measure up to lA across it. Do not connect a load or the output transformer to the amplifier at this stage. The output stage quiescent current can now be set. Rotate the 5000 trimpot fully anticlockwise and apply power. Now rotate the 5000 trimpot for a current of 200mA. Switch off the power supply and There is plenty of room inside the chassis which means that the amplifier is easy to work on. We suggest that you bind the shielded input leads with plastic cable ties to prevent them from fraying and coming adrift. it to zero; ie, to less than ± lmV. Leave the amplifier with power connected for an hour or so and then check the settings for DC output and quiescent current. Reset if necessary. It is normal for both settings to drift slightly. Now you can connect the 100V line transformer or a loudspeaker and check for the presence of hum or any other signal. With no signal applied the amplifier should be absolutely quiet. Touching your finger to the input should cause the speaker to emit a small "blurt". Completing the wiring Follow this photograph closely when installing the supply wiring to the power amplifier. The leads are run about 2.5cm above the edge of the board and drop straight down to their respective PC stakes as shown. wait for the voltage across the supply capacitor to drop. The multimeter can now be removed from circuit and the 5A fuse replac- ed in the negative supply line. Reapply power and measure the DC voltage at the output of the amplifier. Rotate trimpot VRl to set You can now fit the front panel and power switch. Install the potentiometers and wire them to the mixer printed board. This wiring is run in shielded cable for the Mic 1, Mic 2, Aux 1, Aux 2 and Master Volume pots and in ordinary hookup wire for the Bass and Treble controls. Do not connect the shielded output lead from the mixer board to the power module at this stage. Connect the positive and negative JANUARY1989 47 ,/' Here's a close-up view of the wiring to the auxiliary sockets and the balanced and single-ended input microphone sockets. If you want to save money, it may be possible to leave out two of the microphone sockets (see text). power leads from the BOOOµF filter capacitors to the respective terminals on the mixer board. The OV lead from the power amp module then connects to the OV input on the mixer board. Now you can check voltages on the mixer board. Apply power and check the voltage across each of the 33V zeners. It should be 33 volts or less. Then check the 3-terminal regulator outputs. They should be + 15V for the 7815 and - 15V for the 7915. If you don't obtain these voltages, switch off and check your work carefully before proceeding further. Now check for the presence of + 15V at pin 8 of ICl, IC4 and IC6 and at pin 7 of IC2, IC3 and IC6. Then check for - 15V at pin 4 of all six ICs and the emitter of Q12. Finally, switch your multimeter to the lowest available DC voltage range. Check that the output voltage at each of the op amps is close to zero; ie, less than ± 20mV at pins 1 and 7 of ICl, IC4 and IC6 and at pin 6 of IC2, IC3 and IC5. Finally, connect the short shielded lead between the mixer output and the power amplifier input. Note The rear panel of the 120W PA Amplifier is clean and uncluttered. Two plastic binding terminals (above the mains fuse) are used for the 100V line output, while all the input sockets are grouped together at right. 48 SILICON CHIP SC01112881 ~,,____0. J ,. .,. I I 0 I °1' ~ ..... ~· 0 Fig.2: above is the full-size PC pattern for the mixer board (code SC01112881). Fig.3: this is the full-size pattern for the power amplifier board. The pattern is critical and must not be altered. that the shield is not connected at the power amp input. Your new public address amplifier is now ready for work. Options If you want to save some money, there are some features of the PA Amplifier that could be left out. For example, if you only intend driving 80 or 40 loads, the 100V line transformer can be omitted. The load is simply connected to the output side of the 40 resistor and its parallel 4.3µH inductor. For further information about lOOV lines in PA work, see page 17 of the November issue. Similarly, if you only intend using unbalanced microphones, you can save a few dollars by leaving out the XLR sockets. But note that you must still use shielded cable and include all the connections shown on Fig, 1. If you intend using balanced microphones, then leave out the single ended input sockets. lf; JANUARY 1989 49