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100W RMS/Channel Stereo Amplifier - Part 3

Final article covers the assembly of the remaining modules and includes the full wiring and setting up details.

By Greig Swain and Leo Simpson.

Power Supply Board Assembly

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This is the completed Power Supply module with the heatsink removed. Note the hot-melt glue around the four 8000μF filter capacitors - this prevents the cans from moving & placing stress on the solder joints.
The next step is to build the Power Supply Module - see Fig.18. This board carries the four large 8000μF 63VW filter capacitors for the ±52.5V rails, plus all the parts for the ±55V regulated power supply.

Begin by installing the eight 1W resistors (6 x 15Ω and 2 x 8.2kΩ), mounting them all about 1mm proud of the board. This done, install the remaining resistors, diode bridge BR2, the wire links, the two multi-turn trimpots and the two zener diodes (ZD2 & ZD3). Make sure that BR1 and the zener diodes are correctly oriented.

The male quick connects can be mounted next, followed by the electrolytic capacitors. Note that the two 470μF capacitors must be rated at 100VW - 63VW is not good enough since the input voltages to the positive and negative regulators are about +71V and -71V respectively.

Take extra care to ensure that the capacitors are installed with the correct polarity, especially the 8000μF units. Get one of these wrong and you'll soon know about it the first time power is applied.

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This view shows how the heatsink is fitted to the Power Supply module. Note that all the devices must be isolated from the heatsink - see text.
The TIP33B transistors (Q17 & Q18) and the two 3-terminal regulators (REG1 & REG2) can now be bolted to the flat aluminium heatsink. These devices must all be isolated from the heatsink using silicone washers and, where necessary, insulating bushes - see Fig.13 in Pt.2. Don't tighten the mounting screws just yet, though - that step comes after the assembly has been mounted on the PC board.

Note that the TIP33B (or TIP33C) transistors can came in either a TO-218 package (as shown in Pt.1, Fig.4, p29) or in a TO-3P package as used on the prototype. If you have the TO-3P package devices, then you don't need the insulating bushes. Conversely, if you have TO-218 package devices, the insulating bushes are required.

Make sure that you don't inadvertently swap REG1 & REG2 when attaching them to the heatsink.

The heatsink assembly can now be mounted on the PC board. Push the assembly down as far as it will go so that the plastic bodies of the regulators are about 3-4mm above the board surface and make sure that everything is square before soldering the leads. It's a good idea to tack solder a couple of leads on either side initially. That way, you can easily make any final adjustments before soldering the rest of the leads.

As before, use a multimeter to confirm that the device leads and metal tabs are all correctly isolated from the heatsink.

Hot-Melt Glue Time

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Fig.18: the parts layout for the Power Supply module. Make sure that all the electrolytic capacitors go in the right way around.
As shown in the photographs, we applied hot-met glue fillets to the bottom edges of the four 8000μF capacitors. This stops the large capacitor cans from moving and placing stress on the solder joints.

In short, the hot-melt glue treatment is a good idea but watch the hot tip of the gun - it's all too easy to damage the sleeving around one of the large electrolytic capacitors if you're not careful.

Loudspeaker Protector & Fan Control Module Assembly

The last board to assemble is the Loudspeaker Protector & Fan Control Module. Fig.19 shows the details.

Mount the resistors and diodes first, then fit the male quick connects. Be sure to use brass screws, nuts and star washers to mount the quick connects for the eight loudspeaker terminals, as shown on the overlay.

Next, install the transistors, electrolytic capacitors and bipolar capacitors, taking care to ensure that the correct transistor type is fitted to each location. Note that the four 47μF bipolar capacitors can go in either way around but watch the orientation of the 10μF and 1000μF electrolytic capacitors.

Regulator REG1 is bolted to a micro-U heatsink (25 x 30 x 12.5mm) before it is soldered to the PC board. A thermal washer isn't necessary here since the heatsink doesn't touch any other components - just secure the regulator using a 6mm M3 screw, flat washer and nut.

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Fig.19: follow this parts layout diagram to build the Loudspeaker Protector module. Note that brass screws, nuts and star washers are used to mount the eight quick connect terminals for the amplifier and loudspeaker connects.
The procedure for mounting REG1 is exactly the same as for Q8 & Q9 on the power amplifier modules - ie, insert the leads into the board, smear the mating face with thermal grease, attach the heatsink and push the assembly all the way down before soldering the leads and tightening the mounting screw.

Of course, you could fit the heatsink with the device off the board. However, if you do that, the heatsink will swivel when you attempt to tighten the mounting screw and you'll get thermal grease everywhere. Follow the steps outlined above and you'll end up with a much neater result.

Finally, you can complete the board assembly by fitting the two relays, the 2-way terminal blocks and the 2-way header.

By the way, our prototype board shows a couple of PC stakes in place of the pin header, which was a later refinement. Our final board also has four less quick connect terminals than the prototype board shown in the photographs. That's because we originally intended to drive the headphones via the Loudspeaker Protector Module. We later changed our minds and decided to drive the headphones directly from the Power Amplifier modules instead.

OK, now that all the PC board modules are completed, you're ready to mount them in the chassis and install the wiring.

Preparing The Case

As supplied, the case features a tough powder-coating that's also a good insulator. However, we must ensure that all sections of the case, including the side panels, lid and heatsink - are correctly earthed and that means ensuring they make good electrical contact with each other.

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Use an oversize drill to remove the powder coating from under the screw heads, so that all sections of the case are correctly earthed (see text).
There are two reasons for this: (1) all sections of the case must be connected to the mains earth to ensure safety; and (2) correct earthing is necessary to keep RF interference out of the audio circuitry.

The first job is to ensure that the side panels and the lid are earthed to the bottom section of the chassis. This is done by using an oversize drill to remove the powder coating from the countersunk screw holes (see photo). Use a drill that's slightly smaller in diameter than the screw heads and be sure to remove the powder coating right back to the bare metal.

Don't just do this for one or two holes - do it for all seven holes in each side panel and for all three holes in the lid. Provided you use a drill that's not too big, the bare metal will later be covered by the screw heads.

Next, scrape away the powder coating around the screw holes on the inside of the side panels, the underside of the lid and from the matching contact areas around the screw holes in the chassis. That way, when the case is assembled, earthing takes place via the screws themselves and via direct metal-to-metal contact between the various sections.

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Fig.20: mounting details for the mains earth quick connects. The second nut locks the first nut, so there is no possibility of the earth lugs coming loose.
The transformer mounting bolt should also be earthed and this means that you have to remove some of the powder coating from around the mounting hole on the outside of the chassis (ie, from under the bolt head).

The front panel doesn't require any special preparation. Instead, it's later earthed by running a lead from a quick connect terminal that's welded to the panel back to the main chassis earth (see photo).

Don't attach the front panel to the chassis at this stage. Leave it wrapped up so that it doesn't get scratched or damaged in some other way. It doesn't take much of an accident to spoil the panel's appearance.

Mounting Feet Screws

The screws used to secure the rubber mounting feet protrude about 5mm into the chassis and could easily pierce the 26-way cable insulation on the righthand side. To prevent this, you can either remove the sharp ends of the screws using a Dremel tool or screw additional rubber feet over them.

Do the same for the mounting feet screws at the front-left and rear-left of the chassis. Admittedly, they pose no real danger to the mains wiring but it's best to make absolutely sure (see photos).

You need a ratchet-driven crimper

One essential item required to build this amplifier is a ratchet-driven crimping tool, necessary for crimping the insulated quick-connect terminals to the leads (except for the power transformer, which is supplied with quick connect terminals already fitted).

Suitable crimping tools include the Altronics Cat. T-1552, Dick Smith Electronics Cat T-3535 and the Jaycar TH-1829. These all feature double-jaws so that the bared wire end and the insulation are crimped in a single action.

Don't try to use one of the cheap (non-ratchet) crimpers that are typically supplied in automotive crimp kits. They are not up to the job for a project like this, as the amount of pressure that's applied to the crimp connections will vary all over the place. By contrast, the ratchet-driven crimpers apply a preset amount of pressure to ensure consistent, reliable connections.

Chassis Earth Lugs

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Use cable ties to secure the mains wiring and fit rubber boots over both the mains switch (left) and the IEC socket (right). Note the additional rubber feet (arrowed) that are fitted over the ends of the mounting screws for the external feet.

Fig.20 shows the mounting details for the main chassis earth lugs. This assembly consists of two double-ended quick-connect terminals which are bolted to the chassis using a 10mm M4 machine screw, two star washers and two nuts. The second nut on top is a "belts 'n braces" measure to lock the first nut, so that there's no possibility of the earth lugs coming loose.

Altronics has indicated that the earth lug mounting area in kit versions of the chassis will be free of powder coating. However, if this hasn't been done, you will have to remove the powder coating yourself. To do this, temporarily bolt one of the double-ended quick connects to the chassis and use a pencil to outline the contact area. The quick connect can then be unbolted and the powder coating removed using a a sharp implement or a Dremel tool fitted with a grinding bit.

Be sure to do the nuts up nice and tight, to ensure a reliable earth.

Installing Hardware

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OK, you can now start installing the hardware in the case. Begin by mounting the IEC power socket, then smear the underside of the 35A bridge rectifier (BR1) with heatsink compound and bolt it to the chassis using an M4 screw, star washer and nut.

Fig.21 and the photos show BR1's mounting location. Orient it as shown, with its positive DC output at top left (as viewed from the front of the chassis).

The RCA input socket module can go in next and is secured from the rear of the chassis using three 6g x 10mm pan-head self-tappers. Don't overtighten these screws - you'll strip the plastic threads formed by the self-tappers if you do.

We initially mounted this board upside down (see page 23 of the November 2001 issue) but in the end decided to mount it component side up. This ensures that the board sits closer to the floor of the chassis and also makes it far easier to dress the 26-way connecting cable that plugs into the pin header.

The drawback (if you could call it that) is that the inputs run in the opposite direction to the source switch labelling - ie, the CD inputs are to the right (as viewed from the front of the amplifier) while the CD switch position is to the left. But really, who cares which way the inputs run? As long as the CD player is plugged into the CD inputs, the DVD player into the DVD inputs and so on, they will all be correctly selected at the indicated switch position.

Now for the loudspeaker terminals. Begin by cutting two 120mm lengths of figure-8 speaker cable and solder these to the two terminal panels - see photo. This done, fit the other ends of the leads with blue quick connects, then secure the two terminal panels to the chassis using four 6g x 12mm countersunk self-tappers.

Next, fit four 20mm tapped spacers to the Loudspeaker Protector module (use 6mm x M3 screws and washers) and secure it to the rear panel. Similarly, mount the Power Supply module in position on 15mm spacers but leave the top screws loose for the time being - this will make it easier to lift the board up to run leads under it later on.

Cable-Tie Mounts

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Use this photo and the wiring diagram of Fig.21 as a guide when placing the cable-tie mounts and installing the supply wiring.
As shown in one of the photos, the wiring is all secured to the floor of the chassis using cable-tie mounts and cable ties. Before installing the mounts, it's necessary to mark out their locations.

To do this, temporarily secure the power amplifier module in the chassis and use a pencil to mark these positions:

(1) the location of the heatsink and its three wiring channels;

(2) the locations of the individual amplifier boards;

(3) the locations of the PC tracks carrying the class-B currents from the paralleled 1.5W resistors on each side of the two boards; and

(4) the locations of the central earth tracks that run back to the 0V terminal from the two 1000μF capacitors (see Fig.21).

The reason for this is that it's important to correctly route the supply wiring under the power amplifier boards, to ensure the lowest possible distortion. The ±55V and 0V leads are routed directly under the earth track in the centre of each amplifier, while the ±52.5V leads radiate out at right angles beneath the tracks carrying the heavy class-B currents for the output stages.

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Fig.21: the loudspeaker cables and the DC supply leads to the Power Amplifiers account for most of the wiring. Route all cables exactly as shown, to ensure the lowest possible distortion from the Power Amplifier modules.
Once you have everything marked out, remove the amplifier module and install cable-tie mounts at the following locations:

(1) on each side of the heatsink immediately opposite the three wiring channels.

(2) near the edge of the heatsink in line with the central earth track of the righthand power amplifier;

(3) at the central locations under the power amplifier boards where the ±52.5V leads branch off at right angles;

(4) on the floor of the chassis just inside the edges of the amplifier PC boards where the ±52V leads emerge to join onto their terminals;

(5) on the floor of the chassis, directly under the 0V terminals on the amplifier boards;

(6) directly under the centre two loudspeaker terminals of each amplifier board; and

(7) directly between the two amplifier boards but with the central tie point clearing the front of the boards by about 5mm.

Additional cable-tie mounts are installed along the righthand edge of the chassis to further secure the mains wiring. These are located behind the power transformer and towards the front of the chassis.

The accompanying photograph shows the locations of most of the cable-tie mounts.

26-Way Header Cable

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The rear panel of the amplifier carries the IEC mains socket, the loudspeaker terminals and the RCA input sockets.
The next step is to fit the headers to the 26-way flat ribbon cable. First, cut the 26-way cable to exactly 550mm, then fit a header socket to one end by feeding the cable through and squeezing the assembly together in a vyce.

There are a couple of points to note here:

(1) the lead with the red stripe must go to pin 1 of the header socket (indicated by a small arrow in the plastic moulding); and

(2) the cable must by fed through from the side opposite the arrow (ie, the end of the cable is on the side with the arrow).

Once the assembly has been "clamped" in the vyce, the cable is looped back over the top of the header and the strain relief clamp clipped into place.

The header at the other end is fitted exactly the same way. It's a good idea to test fit everything before fitting the second header - once the headers are clamped to the cable, they are virtually impossible to remove.

The cable can now be installed in the chassis. First, plug one end into the RCA input board, then fold the cable at right angles and then at right angles again so that it runs along the edge of the chassis (red stripe to the outside). The cable is then folded at right angles twice more at the front of the chassis, so that pin 1 of the header mates with pin 1 on the Preamplifier & LED Display module.

Preamplifier Mounting

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Fig.22: this diagram shows the colour coding used for the toroidal power transformer.
Before mounting the Preamplifier & LED Display module, it's necessary to attach the shielded audio output leads. These audio output leads will probably be supplied as a length of figure-8 shielded cable. Cut the cable to a length of 210mm and lightly tin the leads (ie, the braids and the bared ends of the insulated wires) before securing them to the terminal blocks.

It's a good idea to fit a 10mm length of heatshrink tubing to the figure-8 cable at the terminal block end. That way, when the figure-8 cable is later separated (so that the leads can go to the power amplifier inputs), the heatshrink tubing prevents them from separating right at the Preamplifier end.

This done, you can plug the input cable into the header pins and mount the Preamplifier & LED Display module on its standoffs. The insulated headphone socket can also be installed at this stage - it's mounted on the chassis "dimple" immediately to the left of the Preamplifier board, so that its front later sits flush with the front panel.

Wiring Up

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The powder coating must be removed from around the mounting holes that are used to secure the lid (and from matching contact areas on the lid itself).
Now for the internal wiring. It's not nearly as intimidating as it appears at first sight, since most of it consists of supply wiring to the power amplifier boards plus the loudspeaker wiring.

Note that all the supply leads and the loudspeaker leads should be tightly twisted together. This not only keeps the wiring neat but also minimises hum pickup since the hum fields are effectively cancelled out.

There's an easy way to twist leads together and that's by using a hand drill. All you have to do is secure one end of the leads in a vyce and the other end in the drill chuck. You then rotate the drill handle until you get a nice even twist along the full length of the leads.

Make the twists reasonably tight but don't overdo it - the wire will break through the insulation if you do. Once the leads have been twisted, trim the ends to remove any damaged insulation and fit quick connect terminals to one end only.

The leads at the other end are also later fitted with quick connect terminals after they have been run to their destination and cut to the correct length.

Fig.21 shows the wiring routes. Note that all the supply wiring goes through the lefthand wiring channel in the heatsink. The loudspeaker leads and the leads to the thermal switch go through the channel next to the fan.

Here are the leads that you have to run:

(1) Three twisted heavy-duty (7.5A) leads from the +52.5V, -52.5V and 0V connections on the Power Supply board to each Power Amplifier board.

(2) Three twisted heavy-duty leads from the +55V, -55V and 0V connections on the Power Supply board to each Power Amplifier board. Important: the power supply ends of one set of leads must be fitted with piggyback in-line quick connects. The other set of leads is fitted with "ordinary" quick connects which then plug into the piggyback terminals.

(3) Two twisted leads from the Loudspeaker Protector module to each of the Power Amplifier boards (use the heavy-duty 2 x 90/0.18 Whopper speaker cable fitted with blue quick connects).

(4) Two twisted heavy-duty leads from the +12V and 0V terminals on the Loudspeaker Protector module to the +12V and 0V terminals on the Preamplifier module.

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The loudspeaker indicator LED is mounted on the front panel using a modified cable-tie mount (see text). A dab of superglue or hot-melt glue can then be used to hold the LED in place.
(5) Two twisted medium-duty leads from the 2-way header on the Loudspeaker Protector module to the headphone socket switch and from there to the "Speakers" LED on the front panel. These leads are fitted with a matching 2-way header plug at one end and are directly soldered to the headphone socket terminals and the LED leads at the other.

Fitting the 2-way header plug isn't difficult - just lightly solder and crimp the leads to the small header pins that are supplied, then push them down into the plug body until they lock into place.

(6) Two twisted medium-duty leads from the Loudspeaker Protector's TH1 terminals to thermal switch TH1 on the heatsink.

Once you've done all this, the power amplifier/heatsink module can be mounted in the chassis. The ends of the cables are then cut to length, fitted with quick connects and connected to the power amplifier boards, the thermal cutout and the preamplifier board.

The headphone socket wiring is next on the list. Route the leads slightly in front of the amplifier boards, along with the 15VAC and 12V DC wiring to the preamplifier. The leads can all then be secured using cable ties.

Finally, the amplifier wiring can be completed by connecting the shielded audio leads from the preamplifier to the screw terminal blocks.

Power Transformer Wiring

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All wired up and ready for action - use cable ties to secure the transformer secondaries and other wiring to the Power Supply module.
The toroidal mains transformer can now be bolted into position and its secondary leads connected. Note that the transformer is supplied with two neoprene rubber washers - one sits under the transformer while the other sits on top, with the metal cup washer stacked on the top of that.

The mounting bolt passes through the centre of the transformer and the assembly secured by fitting a nut to the top. Do the nut up firmly but don't overtighten it, otherwise you'll distort the metal chassis.

As mentioned last month, the transformer leads are all the correct length to reach their destinations and are pre-fitted with female quick connects. All you have to do is twist the various lead pairs together and plug them into the relevant quick connect terminals - ie, on BR1, the Power Supply module, the Loudspeaker Protector module and the Preamplifier & LED Display module.

It's important to connect the two 35V windings (red) with the correct phasing, otherwise you'll get 0V out of the bridge rectifier (BR1). This simply means connecting the leads as shown in Fig.21 - ie, leads 1 & 4 go to BR1, while leads 2 & 3 go to the two centre (0V) terminals on the Power Supply module.

The same goes for the two 50V windings (white & brown). Leads 5 & 8 go to the 50VAC terminals, while leads 6 & 7 go to the adjacent 0V terminals.

Two short leads fitted with quick connects at either end are used to make the DC connections from BR1 to the Power Supply module. Use heavy-duty cable for this wiring and be careful not to get these two connections transposed.

Once all the leads have been connected, they can be anchored by fitting the cable ties. Additional cable ties can also be fitted at various points between the cable-tie mounts, to keep the wiring tidy.


All transistors with "MJL" and "MJE" type numbers in the power amplifiers (ie, transistors Q10-Q16) must be genuine Motorola or On Semiconductor devices.

Do NOT substitute devices from other manufacturers - they do not have the same characteristics as the specified devices and can cause instability.

Mains Wiring

Take extra care with the mains wiring - it must be installed exactly as shown in Fig.21.

The best place to start is at the IEC socket end. Begin by stripping about 100mm of the outer sheath from the mains cable, then feed the brown and blue leads through the rubber boot that comes with the IEC socket. That done, crimp red quick connects to the ends and connect the Active (brown) and Neutral (blue) leads to their respective IEC socket terminals. The Earth lead (green/yellow) from the mains cable goes directly to the chassis earth - see Fig.21.

A second green/yellow mains-rated earth lead (about 240mm long) connects the earth terminal on the IEC socket to a chassis earth. Be sure to also feed this lead through the rubber boot before fitting quick connects to both ends.

The third chassis lug is connected via a 250mm-long lead to a 0V terminal on the Power Supply module.

Now double-check your wiring to the IEC socket, to make sure it is correct. That done, slip the rubber boot over the IEC socket and fit a cable tie to the leads as close to the boot as possible (this prevents the boot from coming off).

Additional cable ties can now be used to secure the wiring and to secure the mains cable to the cable-tie mounts. You will find this job easier with the side panel removed.

Important: you must secure the mains cable so that it is well clear of the self-tapper screws that are used to secure the side panel. In particular, note that the mains cable sheath should start well forward of the bottom rear self-tapper (see photo). This will allow the leads to be routed well away from this self-tapper and secured using cable ties.

Note also that the righthand heatsink channel has been positioned so that the heatsink mounting screws cannot possibly penetrate the mains wiring. The channel position also ensures that the leads take a natural path so that they clear the self-tapper that's just in front of the heatsink.


High DC and AC voltages are present on the Power Supply and Power Amplifier modules when power is applied. In particular, make sure that you don't get across the two 50VAC terminals. The 50VAC transformer windings that connect to these terminals are wired in series, so there's 100V AC between them!

Similarly, the two 35VAC windings are in series, so there's 70VAC between the AC terminals on bridge rectifier BR1.

In short, don't touch any of the high-voltage AC or DC (52.5V DC& 55V DC) terminals otherwise you could get a very nasty shock which could even prove fatal.

Speaker LED Mounting

The Speaker LED mounts directly on the front panel and the best way to go about this is to hold it in place using a modified cable-tie mount.

First, drill a 5mm hole in the top of the cable tie mount, then use a sharp knife to cut a clearance hole in the adhesive contact area (leave the backing paper in place during this procedure). The cable-tie mount is then affixed to the front panel (directly in line with the LED Speaker hole), the LED pushed in and a dab of superglue or hot-melt glue used to hold it in place.

Don't try using superglue to attach the LED directly to the front panel - it's all too easy to make a mess and spoil the panel's appearance. Hot-melt glue won't work here either because it doesn't adhere properly to the powder-coated surface..

Once the glue is try, the leads from the headphone socket can be soldered directly to the LED. Cut the LED leads short, so that they cannot foul other parts on the preamplifier board.

Mains Switch Wiring

Now for the mains switch wiring. This switch should be mounted on the front panel with terminals 1a and 2a towards the top. It's then simply a matter of cutting a 10mm-long slit in the back of the protective boot, feeding through the various leads and connecting them to the terminals.

This done, the protective boot is pushed over the switch body (it's a tight fit) and is secured by fitting cable ties to the wiring. The earth lead from the mains cable connects to an earth lug on the front panel (note: the front panel is supplied with this earth lug attached).

Leave enough slack in the switch wiring so that the front panel can be opened out to provide access to the nut that secures the volume control pot. Also, take great care not to scratch the front panel when working on the amplifier. It's supplied wrapped in bubble plastic and this makes a handy "mat" to lay the front panel on when it's detached from the chassis.

Once all the switch wiring has been completed, the front panel can be slipped over the LEDs and fastened in position. Don't forget to fit the earth lead.

Preamplifier Modification
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Fig.22 (left): the modified preamplifier board includes an extra 82kΩ adjacent to each LM3915 display driver IC.

We recently discovered that our prototype Preamplifier board had a problem during wet weather, with some of the LEDs in the bargraph displays lighting when the amplifier was turned on from cold (ie, with no audio signal applied). The LEDs would then progressively go out over a period of several minutes.

This turned out to be due to moisture on the PC board, which was allowing leakage between the copper tracks. As the amplifier warmed up, the moisture evaporated and the LEDs behaved normally, even if the unit was switched off and then back on again.

The Altronics kit will be supplied with solder masked PC boards and this by itself should eliminate the moisture problem. However, we have decided to modify the circuit just to make sure.

The "cure" is to connect 82kΩ resistors between D3's cathode and ground and between D5's cathode and ground, to shunt this leakage resistance. We have modified the Preamplifier PC board to accept these extra resistors and this new board will be supplied with the Altronics kit.

Fig.22 shows the affected section of the PC board. The additional 82kΩ resistors are adjacent to pin 9 of each LM3915 display driver IC (IC3 & IC5).

Switching On The First Time

Don't do it - at least, not yet. There's a step-by-step power-up sequence that must be observed, so that something doesn't fry.

Before applying power, check that the 8000μF and 470μF electrolytic capacitors on the Power Supply module are all correctly oriented and that the connections to BR1 are correct. Electrolytic capacitors have a very nasty habit of exploding if installed the wrong way around or if power is applied with reverse polarity.

Basically, you have to go through three main procedures to power up and adjust the amplifier: (1) check that the power Supply Module is delivering the correct voltages; (2) apply power to each of the Power Amplifier modules in turn and adjust the quiescent current; and (3) apply power to the Preamplifier and Loudspeaker Protection modules.

Here's the full step-by-step procedure:

STEP 1: disconnect all the ±52.5V & ±55V leads from the Power Supply module, the 15VAC leads from the Loudspeaker Protector module and the 15VAC & 12VDC leads from the Preamplifier module.

STEP2: connect an IEC power cord to the amplifier and use a multimeter to check for continuity (ie, 0W) between the earth pin of the plug and chassis earth.

STEP 3: install the 3A mains fuse in the IEC socket, apply power and check that the unregulated ±52.5V rails are correct (to within about a volt or so). Important: this should be done with the multimeter leads running out the back of the amplifier and with the lid sitting in place (this is a safety measure to protect your eyes in case something is wrong).

If the meter reads 0V, switch off immediately, recheck the connections to BR1 and check the phasing of the 35V transformer windings. If the phasing is incorrect, the two windings will operate in anti-phase and there will be no output voltage.

STEP 4: check the regulated ±55V rails using the procedure outline in Step 2. If you get "ballpark" figures (they won't be exact at this stage), the power supply is working OK although it's a good idea to wear safety glasses while making adjustments. Adjust trimpots VR2 & VR3 to obtain exactly ±55V DC.

Check the phasing of the 50VAC transformer windings if there is no voltage at the output of bridge rectifier BR2.

STEP 5: switch off and restore all the connections to the Power Supply module. Connect the ±52.5V & ±55V supply wiring to one Power Amplifier only. Disconnect all supply wiring from the other Power Amplifier.

STEP 6: remove the fuses and solder 220W 5W resistors between the two sets of PC stakes on the board. Now wind VR1 fully anticlockwise, apply power and check the voltage at the output of the amplifier (ie, the voltage between the loudspeaker terminals). It should be less than ±30mV.

If not, check the base-emitter voltages of the transistors; they should all be 0.6-0.7V. Check also that the correct transistor is installed at each location and that all transistors and other parts are the right way around.

STEP 7: monitor the voltage across one of the 220W 5W resistors and wind VR1 slowly clockwise until the meter reads 4.4V. This is equivalent to a quiescent current of 20mA (ie, 10mA through each output transistor).

Let the amplifier run for about 10 minutes, then readjust VR1 if necessary. Finally, switch off, remove the 220W 5W resistors and fit the fuses.

STEP 8: connect the ±52.5V & ±55V supplies to the second power amplifier and repeat steps 5 & 6 to set the quiescent current for this module.

STEP 9: connect the 15VAC leads to the Loudspeaker Protector module. Apply power and check that the relays turn on after about four seconds. Check that the Loudspeaker LED lights when the relays turn on and that the relays turn off when headphones are plugged in.

Next, check that the DC fault protection works by disconnecting the leads from one amplifier and connecting 3V DC (2 x 1.5V cells) between the terminals instead. Do the same for each channel - in each case, the relay should immediately turn off.

Now remove one of the leads from the thermal cutout (TH1) on the amplifier heatsink. Check that the fan immediately starts running and that it stops when the lead is reconnected.

STEP 10: check the +12V supply from the Loudspeaker Protector then switch off and connect the 15VAC and 12V DC leads to the Preamplifier module. Be sure to get the 12V leads the right way around - transpose them and smoke signals are guaranteed.

Apply power and check that the Power LED lights (the bargraph LEDs may also flash briefly). Check the outputs of the two 3-terminal regulators - you should get +15V from REG1 and -15V from REG2.

Where To Buy A Kit Of Parts

A complete kit of parts for the Ultra-LD 2 x 100W Stereo Amplifier will be available from Altronics, 174 Roe St, Perth. Phone (08) 9328 1599. You can order on-line from

Listen To The Music!

Now for the big moment. Connect your CD player and loudspeakers, switch on and listen with your ear close to the loudspeakers but without any music playing. Even with the volume control turned right up, there should only be a barely perceptible "hiss" from the speakers.

Finally, turn the volume control back to "normal", place your favourite CD in the tray, hit the "Play" button and sit back to enjoy the sound. Try varying the volume - you should see the bargraph LEDs "jump" up and down in response to the music.

Table 1: Resistor Colour Codes

Power Supply Module

No. Value 4-Band Code (1%) 5-Band Code (1%)
2 8.2kΩ grey red red brown grey red black brown brown
2 6.8kΩ blue grey red brown blue grey black brown brown
2 180Ω brown grey brown brown brown grey black black brown
2 47Ω yellow violet black brown yellow violet black gold brown
6 15Ω brown green black brown brown green black gold brown

Loudspeaker Protection Module

No. Value 4-Band Code (1%) 5-Band Code (1%)
1 220kΩ red red yellow brown red red black orange brown
2 56kΩ green blue orange brown green blue black red brown
4 22kΩ red red orange brown red red black red brown
2 22kΩ red red orange brown red red black red brown
1 2.2kΩ red red red brown red red black brown brown
1 1.5kΩ brown green red brown brown green black brown brown
1 22Ω red red black brown red red black gold brown
Parts List For Ultra-LD Stereo Amplifier

Hardware & Miscellaneous

1 custom-made rack-mounting case with heatsink & 80mm fan (case supplied drilled & with all cutouts made; heatsink supplied drilled, tapped & with channel cutouts in base)
1 front-panel (screened lettering)
1 heatsink for power supply
1 300VA toroidal power transformer, 2 x 50V, 2 x 35V, 2 x 15V secondaries
1 mounting kit for transformer
1 pushbutton DPST 250VAC switch (Altronics Cat. S3245)
2 knobs to suit rotary switch and potentiometer
1 IEC male power socket, chassis mount with insulating boot (Altronics P8324)
1 3A slow-blow M205 fuse
1 insulating boot for power switch
2 panel-mount gold-plated binding post terminal pairs (Altronics P2016)
1 6.35mm chassis-mount insulated stereo headphone socket (Altronics P0074)
1 pack 25 adhesive cable tie mounts (Altronics H4120)
50 100mm cable ties (Altronics H4012)
2 26-pin IDC in-line crimp sockets with strain-relief clips (Altronics P5326)
4 screw-on rubber feet (Altronics H0902

Preamplifier & LED Display Module

1 PC board, code 01112012, 246 x 166mm
1 26-way DIL pin header
2 2-way mini PC terminal blocks (Altronics P 2038) - 5mm pitch
1 2-pole 6-position switch (Altronics S 3022) (S1)
1 10kW 16mm stereo log pot (VR1)
2 F29 ferrite beads


2 NE5534AN op amps (IC1,IC2) (Altronics Z2792 - do not substitute NE5534N)
1 TL072 op amp IC (IC2)
2 LM3915 display driver ICs (IC3,IC5)
1 7815 3-terminal regulator (REG1)
1 7915 3-terminal regulator (REG2)
2 1N4004 diodes (D1,D2)
4 1N914 diodes (D3-D6)
16 green thru-panel LEDs (LEDs 1-8, 11-18) (Altronics Z0711)
2 yellow thru-panel LEDs (LED9, LED19) (Altronics Z0713)
3 red thru-panel LEDs (LED10,LED20,LED21) (Altronics Z0710)


2 1000μF 25VW PC-mount electrolytics
2 100μF 25VW PC-mount electrolytics
9 10μF 35VW PC-mount electrolytics
4 10μF 50VW bipolar electrolytics
2 0.22μF MKT capacitors
2 0.01μF MKT capacitors
2 390pF ceramic capacitors
2 33pF ceramic capacitors
6 10pF ceramic capacitors

Resistors (0.25W, 1%)

2 680kΩ
2 2.2kΩ
2 330kΩ
2 1.8kΩ
2 220kΩ
3 1.2kΩ
2 150kΩ
4 150Ω
2 100kΩ
2 100Ω
2 82kΩ
2 33Ω
2 6.8kΩ
1 10Ω
2 4.7kΩ

RCA Input Module

1 PC board, code 01112013, 110 x 32mm
3 2 x 2 PC-mount gold-plated RCA sockets (Altronics P 0214)
1 26-way DIL pin header

Power Amplifier Modules

2 PC boards, code 01112011, 176 x 108mm
8 M205 PC mounting fuse clips
4 M205 5A fuses
2 coil formers, 24mm OD x 13.7mm ID x 12.8mm long (Philips 4322 021 30362)
2 200W multi-turn trimpot, Bourns 3296W series (VR1)
3 metres 1mm diameter enamelled copper wire
4 micro-U heatsinks, 19 x 19 x 9.5mm (Altronics H 0630)
8 TO-3P insulating washers (silicone)
4 TO-220 insulating washers (silicone)
2 TO-126 insulating washers (silicone)
4 TO-220 insulating bushes
2 2-way mini PC terminal blocks - 5mm pitch (Altronics P 2038)
8 PC stakes


4 Motorola MJL1302A PNP power transistors (Q13, Q14)
4 Motorola MJL3281A NPN power transistors (Q15, Q16)
2 Motorola or On Semiconductor MJE15030 NPN transistors (Q11)
2 Motorola or On Semiconductor MJE15031 PNP transistors (Q12)
2 Motorola MJE340 NPN power transistors (Q10)
2 BF469 NPN transistors (Q8)
2 BF470 PNP transistors (Q9)
6 BC546 NPN transistors (Q5-Q7)
8 BC556 PNP transistors (Q1-Q4)
2 3.3V 0.5W zener diodes (ZD1) - do not substitute 1W device


4 1000μF 63VW electrolytic
2 220μF 63VW electrolytic
2 100μF 63VW electrolytic
2 100μF 16VW electrolytic
2 2.2μF 25VW electrolytic
2 0.15μF 400VW MKC, Philips 2222 344 51154 or Wima MKC 4
10 0.1μF 63V MKT polyester
2 .0012μF 63V MKT polyester
2 100pF 100V ceramic

Resistors (0.25W, 1%)

4 18kΩ
2 330Ω
2 12kΩ 1W
4 150Ω
2 3.3kΩ
6 120Ω
2 2.7kΩ 5W
8 100Ω
2 1.2kΩ
4 47Ω
2 1kΩ
2 10Ω
2 390Ω
2 6.8Ω 1W
2 330Ω 1W
16 1.5Ω 1W
4 220Ω 5W (for current setting)


Heatsink compound, tinned copper wire for links, insulation tape

Power Supply Module

1 PC board, code 01112015, 199 x 93mm
2 2kW multi-turn trimpots, Bourns 3296W series (VR2,VR3)
1 drilled aluminium heatsink, 100 x 67mm (1.5-2.0mm thick)
2 TO-3P insulating washers (silicone)
2 TO-220 insulating washers (silicone)
4 TO-220 insulating bushes


2 TIP33B NPN power transistors (Q17, Q18)
1 LM317 adjustable positive 3-terminal regulator (REG1)
1 LM337 adjustable negative 3-terminal regulator (REG2)
1 PA40 or KBPC3504 400V 35A bridge rectifier (BR1) (Altronics Z0091)
1 KBPC604 or PW04 400V 6A bridge rectifier (BR2) (Altronics Z0082)
2 33V 5W zener diodes (ZD2, ZD3)


4 8000μF 63VW chassis-mount electrolytic capacitors (Altronics R6720). Do not substitute Altronics R6722 as cans will foul chassis lid.
2 470μF 100VW electrolytics
2 100μF 63VW electrolytics

Resistors (0.25W, 1%)

2 8.2kΩ 1W
2 47Ω
2 6.8kΩ
6 15Ω 1W
2 180Ω

Loudspeaker Protector & Fan Control Module

1 PC board, code 01112014, 193 x 68mm
2 12V SPDT PC-mount relays, 16A contacts (Altronics S4197)
1 micro-U heatsink, 25 x 30 x 12.5mm (Altronics H0635)
1 thermal circuit breaker, 60ºC, normally closed (TH1) (Altronics S5600)
1 2-way PC-mount pin header (Altronics P5492)
1 2-way header plug (Altronics P5472)
2 2-way mini PC terminal blocks - 5mm pitch (Altronics P2038)


5 BC547 NPN transistors (Q1, Q3, Q4, Q6, Q7)
2 BC557 PNP transistors (Q2, Q5)
1 BC327 PNP transistor (Q8)
1 BC337 NPN transistor (Q9)
7 1N4004 1A 400V diodes (D1-D7)
1 LM7812 3-terminal regulator (REG1)
1 red thru-panel LED (LED1) (Altronics Z0710)


1 1000μF 25VW PC electrolytic
1 220μF 25VW PC electrolytic
4 47μF 50VW non-polarised PC electrolytic
1 10μF 35VW PC electrolytic

Resistors (0.25W, 1%)

1 220kΩ
1 10kΩ
2 56kΩ
1 2.2kΩ
4 22kΩ
1 1.5kΩ
2 22kΩ 1W
1 22Ω 5W

Wire & Cable

1m 250VAC 3-core mains flex
2m 2 x 90/0.18 figure-8 "Whopper" speaker cable
3m red heavy-duty (7.5A) hook-up wire
3m black heavy-duty (7.5A) hook-up wire
3m white heavy-duty (7.5A) hook-up wire
1m blue heavy-duty (7.5A) hook-up wire
1m green heavy-duty (7.5A) hook-up wire
250mm figure-8 shielded cable
2m red medium-duty hook-wire
2m black medium-duty hook-up wire
100mm 3mm-dia heatshrink tubing
100mm 5mm-dia heatshrink tubing
750mm 26-way IDC cable (Altronics W 2626)

Screws & Nuts

4 25mm tapped brass spacers
8 20mm tapped brass spacers
4 15mm tapped brass spacers
1 M4 x 16mm screw
42 M4 x 10mm screws
43 M4 nuts
100 M4 star washers
12 M4 x 10mm brass screws
12 M4 nuts
12 M4 star washers
24 M3 x 10mm screws
40 M3 x 6mm screws
15 M3 nuts
50 M3 flat washers
10 M3 star washers
3 6g x 10mm pan head self-tappers (to secure RCA socket pairs to chassis)
4 6g x 12mm countersunk screws (to secure the loudspeaker terminal pairs)
2 No.4 x 9mm universal head self-tapping screws (to secure IEC mains socket) (Altronics H1139)

Quick Connects

53 doubled-ended male quick connects (Altronics H2261)
50 red female in-line quick connects, 6.3mm (Altronics H2001A)
25 blue female in-line quick connects, 6.3mm (Altronics H2006A)
3 red piggyback in-line quick connects, 6.3mm (Altronics H2011A)

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