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and DOWN points in the MOTM-820. These allow the MOTM-820’s own pots to be used as
input attenuators whilst being able to set the absolute rise and fall times with the OMS-820.
Feel free to make any changes to the circuit to improve or alter the performance. If you do, do
drop the Oakley-Synths mailing list an e-mail and tell us all what you have done.
Components
Most of the parts are easily available from your local parts stockist. I use Rapid Electronics,
RS Components and Farnell, here in the UK. The OMS-820 module was designed to be built
mostly from parts obtainable from Rapid Electronics.
The BC550 and BC560 devices are discrete low noise transistors. The former is NPN, while
the latter is PNP. You can replace the NPN with BC549, and the PNP with BC559. Quite
often you see an A, B or C suffix used, eg. BC550C. This letter depicts the gain or grade of
the transistor (actually hfe of the device). The OMS-820 is designed to work with any grade
device although I have used BC550C and BC560C throughout in my prototype.
The board mounted pots are Spectrol 248 conductive plastic types and are held onto the board
with specially made Oakley pot brackets. Three pot brackets and an extra set of nuts are
required and these are provided with the ‘pot bracket kit’. You could use any pot type you
want, but not all pots have the same pin spacing. Not a problem, of course, if you are not
fitting them to the board.
In the UK, Farnell, CPC and Rapid Electronics sell the Spectrol pots for a reasonable price.
For the resistors ordinary 5% 0.25W carbon types can be used for all values. In other Oakley
modules I recommend the use of 1% 0.25W metal film types and because 5% types use a
different colour code to the newer 1% types it may be prudent to use only 1% types to avoid
confusion.
All the electrolytic capacitors should be radially mounted. The working voltage (WV) of the
capacitor is not critical, but chose one that will fit on the board. I would use 25V for the 22uF
and 63V for the 2u2 capacitors.
The pitch spacing of the polyester capacitors is 5mm (0.2”). These types come in little plastic
boxes with legs that stick out of the bottom. Try to get ones with operating voltages of 50V,
63V or 100V. They may be called polyester film or metallised polyester capacitors.
L1 and L2 are leaded ferrite beads. These are little axial components that look like little
blackened resistors. They simply consist of a single piece of wire going through the middle of
a ferrite cylinder. They are available from most of the mail order suppliers. Find them in the
EMC or Inductor section of the catalogues. Farnell sell them as part number: 108-267.
All ICs are dual in line (DIL or DIP) packages. These are generally, but not always, suffixed
with a CP or a CN in their part numbers. For example; TL072CP. Do not use SMD, SM or
surface mount packages.
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