Oakley PCB Issue 2 User Manual

Oakley Sound Systems

Power Supply Unit (PSU)

PCB Issue 2

User Manual

V2.3.4

Tony Allgood B.Eng PGCE
Oakley Sound Systems
CARLISLE
United Kingdom

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Introduction

This is the User Manual for issue 2 of the PSU circuit board from Oakley Sound. The Oakley
PSU is a power supply unit designed to be used in conjunction with the Oakley Dizzy power
distribution system.

When built onto the 3U master panel the maximum peak current output is 520mA per rail
using the recommended Yamaha PA-20 power supply. When built onto the larger 4U 19” rack
panel and using the Yamaha PA-30 power supply the maximum current draw is 780mA per
rail.

It is possible to upgrade the power supply's maximum current capability using a different
power source and suitable smoothing capacitors and appropriate heatsinking.

As well as giving details as to what the PSU board does, this document contains instructions
on how to wire up your board to your source of power, any external switches and the optional
power indicators. We will also look at the Oakley power buss and how it differs from the
MOTM one yet retains complete compatibility with it. I will also give you some information
about suitable sources for your low voltage AC power supply.

The Oakley PSU allows for various options in the installation. You can use the unit either in
full wave rectification mode for connection to tapped linelumps or twin transformer
secondaries, or in half wave rectification for single phase AC output wallwarts and linelumps.
If all this sounds very confusing at the moment, do not worry, in this manual I will try to make
it clearer so that you make the right decision about what power source you will need.

If you are building the PSU as a complete DIY project please also download the latest
Builder's Guide. This can be found on the main PSU webpage along with our very useful
generic parts and construction guides.

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Safety Warning

This supply has been designed to work with isolated low voltage AC inputs only. Connection
to any other supply is done at your own risk. Low voltage is classified as being less than 25V
with respect to the ground potential. Voltages above this level can, and often are, lethal to
living creatures.

Oakley Sound Systems will not advise on building or modifying this board to allow for direct
connection to the mains, or other high voltage sources, further to what is provided in this
document. Please do not ask us for additional information pertaining to direct mains
connections or using internally mounted transformers as we will not give it.

For safety and legal reasons we cannot recommend powering this board from any other
supply than low voltage AC output mains adapters.

Oakley Sound Systems are not liable for any damages caused by the misuse of this product. It
is your responsibility to use this product safely. If you have any doubt about installing a safe
power supply, then please do not attempt to do so.

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The Oakley PSU module

The power supply board will allow the conversion of a suitable low voltage alternating current
(AC) to be rectified, smoothed and regulated for operation with the Oakley Modular. The
module is designed to be fitted to a suitably large metal panel which functions as a heatsink for
the series pass devices connected to the PSU circuit board. This metal panel should have
adequate airflow around it. An example of such a panel is our 3U wide master panel which
also contains a handy multiple and power switch. You can also use a 4U high 19” rack blank
panel. These are particularly suitable if you are mounting your modular synth in a 19” rack.
The greater surface area on the 19” blank panel allows you to mount not only the power
supply module but also up to two Dizzy PCBs. And the greater surface area also allows the
power supply's power devices to dissipate more heat safely and that means a greater current
output should you need it.

A standard 4U high 19” panel with an Oakley PSU and single Dizzy. This would be suitable for powering

up to 12 Oakley modules. Note the grounding point just below the main power inlet.

The voltage output of the power supply module is a split rail 15V supply. This means it
generates both +15V and -15V. That is, two power rails, one of a positive voltage, the other a
negative one. These voltages are measured with respect to a common ground which is
normally connected, via your house's wiring, to the earth that you stand on. The voltage
across both rails is 30V, with the common ground sitting exactly in the middle of this at 0V.

The output current capability is the maximum current you can draw out of the power supply.
The current taken from the supply is, for the most part, determined by the amount and type of
modules you are connecting to the power supply. However, the actual patch also has an effect
on the current draw – the more outputs that are connected to inputs increases the current
draw slightly. Also, some LEDs when lit may increase the current draw.

The Oakley PSU features current limiting. This sets the maximum current the power supply
will actually provide. If you try to draw more than the limit the output voltage will fall to
maintain the current at the limit value. The current limit is set by two resistors, R2 and R3, on
the PSU board. It is essential that you make the current limit resistors suit your application.
The next section, and also in the parts list of the Builder's Guide, will give details on what
values to use.

I recommend that you use a Yamaha PA-20 or PA-30 power supply. These are neat tidy
external power supplies that keep the dangerous mains voltage away from your modular. The
PA-20 will allow a maximum current of 520mA (0.52A) to be drawn from each rail. While the

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PA-30 will allow up to 780mA (0.78A). Remember though you can't use a PA-30 with the 3U
master panel and expect to draw 0.78A out of it without the panel getting too hot. If you are
using the 3U master panel then you should draw no more than 0.52A per rail.

If you are using a 500mA (or 0.5A) AC wallwart* then the most current you can take from
this power supply module is 125mA from each rail. That is, take no more than 125mA from
either the +15V supply or the -15V supply. Most Oakley modules take around 20 to 30mA
each from both the +15V and the -15V rails. That means you should restrict yourself to
driving a maximum of five or six synthesiser modules in your system if using 500mA wallwart.
However, you must remember that some modules may take more that 30mA. So check with
the module's User Manual or webpage to see how much current each one takes.

Various companies make linelumps** with a greater capacity than 500mA. If you can get a
1A output one than this will be able to drive up to 250mA per rail.

Some linelumps, like the Yamaha PA-20 and PA-30, use a split AC output or a centre tapped
AC output. This means it has three wires coming from it and will use a different plug from the
usual round barrel one you see on the single phase AC wallwarts. Single phase mains adapters
have only two wires coming from them so they can use a simpler connecting plug.

The Oakley PSU is recommended to be used with full wave rectification. This allows it to
utilise split AC outputs very effectively. The Yamaha PA-20 supply is rated to give an output
voltage of 35Vac (with a centre tap) at a load of 0.94A. Once rectified and smoothed this
means that a maximum current of 0.52A can be drawn from the PA-20.

Two sets of screw terminal blocks are provided for connecting the low voltage AC power
source to the board and the optional power switch. If you are using a single phase wallwart to
power the PSU module than you need only to use two terminals per terminal block.

The board has four mounting holes for stable placement onto your modular case. Care should
be taken so that the board's various board mounted components do not come into contact with
any part of your modular's enclosure. One of the holes, the bottom right hand one, is
connected to the ground pins of the two output terminals via a 'resistor', R13. In most
situations R13 is a wire link thus ensuring that the panel the PSU module is fitted to is
connected to 0V or ground of the power supply.

The power supply has two integral fuse holders in case of a problem with the power supply
circuitry itself. Two fuses are needed if you are using full wave rectification, but only one, F2,
is required for ordinary half wave rectification. The fuse type should be a slow blow or anti­surge type. The size is 20mm. It should be rated at between one and two times the maximum
current of your wallwart. Thus a 500mA AC output mains adapter should have a fuse that is
rated between 500mA and 1A, ideally 750mA. A 1A linelump should have a fuse that is
between 1A and 2A, ideally 1.5A.

Three on-board LEDs, one for +15V, one for -15V and one for the AC input, provide a quick
visual reference that all is well. All three LEDs could be fitted externally to the board and be
mounted on a front panel. However, the recommended way is to mount only the AC indicator
on the front of the synthesiser along with the AC power standby switch.

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As we have seen the standard circuit provides two outputs, one at +15V and one at -15V.
Both output voltages can be finely adjusted with just one trimmer. The -15V will
automatically track the output voltage on the +15V.

The output voltages are available from two screw terminal blocks. It is expected that each
terminal block will be connected to one Dizzy board. However, you can with care connect
more than one Dizzy board to each output block. We do not recommend that multiple Dizzy
boards be connected in daisy chain fashion. That is, each Dizzy board should go back to the
PSU separately.

* A wallwart is the vernacular term for a low voltage mains adapter that plugs directly into the
wall. These take the form of a black plastic block that is shaped like an oversized mains plug.
It is called a wart simply because its appearance is somewhat uglier than a normal slimline
plug.

** A linelump does the same job as a wallwart but it generally can handle greater currents.
Because of its increased size it cannot be made so that it will safely fit into a plug socket
directly. Thus the adapter sits in a black plastic box and connects to the wall via a cable and
traditional mains plug. It is therefore a black plastic lump connected to a line. The Yamaha
PA-20 and PA-30 are such linelumps.

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