POWER

A power distribution board to fit a double-row 126HP case for 30-40 typical modules.

There is no live mains in the case, the unit is powered by an external DC power supply:

Originally a 19V power brick from a retired laptop.
More recently a 15V power supply that will live by itself in its own box.

Two busses of 20 power sockets across the full width of the case; each bus has its own +12V 1A supply and shared -12V and +5V supplies. A third bus of 5 power sockets with a separate +12V 0.6A supply allows voltage-sensitive modules such as VCOs to be isolated from potentially noisy modules.

INPUT – terminals for 15V-25V DC external DC power supply
+12V OUTPUT – three separate supplies to each bus, max total 2.6A
+5V OUTPUT – max 1A shared by all three buses
-12V OUTPUT – max 0.6A shared by all three buses

I measured 22 typical ShedSynth modules in a single case: the +12V rail supplied 400mA in total, the +5V rail is not used, the -12V rail supplied 100mA. Most of the analog ShedSynth modules draw very little current; the heavy users are mostly Arduino or CMOS with LEDs, mainly powered by the +12V rail – and in fact it is the 20-30mA consumption per LED which is significant.

This is a larger version of an earlier design.

Circuit

A P600A diode prevents damage if input polarity is reversed.

Three voltage regulators supply the main +12V and +5V rails; circuits are adapted from the L7812 and L7805 datasheet.

A MeanWell NSD15-12D12 DC-to-DC regulated power supply provides the -12V rail and the third +12V rail. Two 82µH inductors and a 66µF capacitor form a filter described in the datasheet, to suppress any high-frequency noise from the switch-mode power supply.

Yellow, red and blue LEDs confirm operation of each rail and shine out of the gaps between panels.

Construction

A 500mm x 100mm stripboard panel fits across the whole width of a 126HP case.

The copper tracks are rated for several amps, but I reinforced that by soldering single-core wire along the course of each track between sockets.

A large flat bit was used on the soldering iron to solder the larger joints.

Keyed 16-way boxed headers prevent misconnecting your modules. The top two tracks are not used for GATE & PITCH buses, so those 4 pins were removed from each header to reduce insertion force.

The 13°C/W heatsinks were chosen for maximum efficiency when fitted to a vertically-mounted board.

+5V update

While building the SheDX7 module it turned out that 300mA was too much for the 7805 voltage regulator on a +19V supply; the regulator overheated and repeatedly dropped out.

I upgraded with a pre-built LM2596 DC/DC convertor for 5V. The LED should be disconnected while adjusting the trimmer down to the correct voltage.

The original reasoning behind using simple 7812 and 7805 voltage regulators was to avoid noise from buck-convertor oscillators – although these work at very high frequency, there was a danger of an audio-frequency beat between multiple DC/DC convertors. My hope is that 5V supply to typical modules is unlikely to affect the audio output.

Y-cables

To make best use of the limited number of backplane sockets, ribbon cables are made with two 10-way module headers sharing a single 16-way backplane header.

Breakout gadget

This breakout gadget was made from stripboard to test breadboard circuits and incomplete modules lying on the bench.

There is a 10-pin socket on the right for +12V and -12V input from the case. A pair of 100mA self-resetting polyfuses protect against short-circuit, and a pair of LEDs shows power connected.

Three options to connect to a circuit under test:

The 16-way header (with only 10 pins) allows the use of a normal ribbon cable.
The three pairs of header pins are for jumper wires to a breadboard.
The screw terminals are for solid jumper wires to a breadboard.

The exposed copper strips on the base are protected by a strip of Velcro, with a plan to anchor the gadget to something else on the bench.