Power Supply

Initially, I assumed that the outboard power supply would be a simple affair - perhaps built into a standard diecast aluminium enclosure, for example. As it was intended to be hidden out of site, there were no physical limitations placed on the electrical or mechanical design.

But once the design had been finalised, I began to realise just how compact it could be. It soon became apparent that I could fit everything into a box that matched the amplifier...

So the final solution uses birch ply side panels, two pieces of angle stock to form the chassis, and 1mm aluminium to form the top and bottom panels - these panels clamp the torroidal transformer in place.

Construction

The first step was to prepare the chassis. Two pieces of angle-stock were cut down and the round opening was cut to allow for the transformer. Fitting in a 120VA transformer into a 45mm high box was going to be tight - there was no question of recessing the rear panel to hide the connectors this time!

Chassis sections

Next the wooden sides were made. These were slightly easier because the 9.5mm rebate didn't have to stop part-way along the piece. You can see that the rebate continues along the rear panel.. Also, an area had to be removed to make way for the smoothing capacitors.

Side panels

Next I made a PCB for the supply. Like with the gainclone prototypes, this was made using a scalpel and excess copper was removed using heat from the soldering iron. You can't see it here, but I also made an insulator to sit between the PCB and the chassis. I used a very thin piece of FR4, necessary because space here is tight.

Bare PCB on chassis

This view shows the underside of the PCB - as you can see the layout is very simple. To conserve the height, the component leads were laid flat before soldering - while this also ensures a better contact, it does make future servicing more difficult. To act as a spacer, I used some plastic bushes - the sort normally used for insulating semiconductors from heatsinks.

View of the underside of the assembled PCB

And here's the completed PCB in situ. Note how the DC output XLR connector just clears the bridge rectifiers - this was a neat solution, but caused one or two sleepless nights! You can also see the spacers that the bottom panel are attached to...

PCB installed in the chassis

Next, I added a winding to the transformer to power the LED. This winding gives around 5 volts, from memory this needed around 30 turns. Because of the limited space, I used very thin enamel wire that didn't significantly alter the height. The windings were secured with insulating tape - the end of the tape runs were at the top of the transformer, so the transformer clamping will keep the tape secure...

Modified transformer

This is the LED rectifier. To stop the LED flickering, there is a smoothing capacitor.

Power supply LED

And this is how it fits onto the other half of the chassis. As with the amplifier, the 3mm LED has been filed down and locates into the hole. The LED holds itself in place, but is additionally retained by the transformer.

LED installed in chassis

Next was the final assembly. The wires from the XLR connector and the secondary windings from the transformer are attached to the PCB, and the primary windings are wired up. Note the green earth wires that securely connect the two halves of the chassis to the mains earth connector in the IEC socket - also the shell of the XLR socket is earthed.

I also added an insulator to the top of the capacitors. As they age, they might expand and touch the bottom panel, causing a short circuit. You can see the perspex front panel, which like the amplifier, has a laser-printed transparency to define the power LED (there's currently no text, but that might change). As there are no controls (not even room for a power switch), the front panel is held on with a pair nice stainless steel hex bolts.

Transformer installed...

And here is the final assembly, with both wooden side panels attached. These panels hold the two halves of the chassis together - the thin top panel helps, but it primarily holds the transformer in place... As you can see, it's a squeeze - the PCB only just fits. You can see the four spacers that the bottom panel attach to - these have been covered with neoprene "Hellerman" sleeves to protect the transformer should it move and come into contact with them.

Power supply complete

And finally, here's the top view, before the top panel is added. This panel is secured by four allen-bolts, and hides all the other fixings, including the low-tech wood-screws. This demonstrates that we really couldn't afford the 3mm thickness of the chassis...

Top view of complete power supply

The final construction worked well - as the insides are dominated by that 120VA torroid, the resulting unit is very heavy for its size. Always a good sign ;-)