OMF-1
Analog filter Rack
A bit of history...
In 1998 the Oakley Sound Systems VCF-1 was my first PCB project to be released. It
proved to be a moderate success and paved the way for the development of a whole range
of PCB projects designed for the audio DIY hobbyist. After the very first strip-board
prototype, two printed circuit board issues were made. The first issue used Alps 16mm pots
and 30 boards were sold. The second issue corrected a few things and allowed the use of
the then popular Omeg P16 plastic pots and brackets. Just sixty issue 2 PCBs were sold.
In 2002 I decided to update the design and improve certain aspects of it that I thought could
be done better. Three things stood out as being key areas to improve.
1. The VCF-1 created smooth sweeps that warmed up any harsh digital sound. But it
couldn’t do the opposite. It couldn’t be nasty, well, not nasty enough anyway. Enter
the OMF’s one pole or Fizz output. This created a bright bubbly sound that had a
whole new character. If you then mixed that in with the ordinary four pole or smooth
output you would get a band pass response.
2. The envelope follower (EF) in the VCF-1 was quick responding and worked well with
most program material, especially keyboards and drums. But it didn’t sound right with
guitars. The problem was a decay time that was too fast. Experimenting with the
MOTM-820 lag processor and the Oakley EFG on a modular synthesiser allowed me
to try out various topologies. In the end I settled for a complex, yet simple to operate,
envelope follower with fully variable rise and fall times. This combines the functions of
an attack-decay/attack-release envelope generator (EG) with a controllable envelope
follower. Rise and fall times can now be set independently which made the OMF-1
more useful than its predecessor.
3. The lack of a bypass function on the VCF-1 made the unit difficult to use without a
mixing desk. The OMF-1’s Thru function allowed the ‘wet-dry’ level to be set.
Although in practice finding the control's middle null point was somewhat tricky. It did
however allow for complex filter responses. Mixing the filter’s output with the
unfiltered audio input interacts in such a way as to create new filter and phase
responses.
The last OMF-1 was sold in 2005. There had been a second issue of the PCB in 2004 but
this was to be the last run of boards for a while. However, repeated requests for the board to
be reinstated finally had their desired effect. The first issue of the OMF-1 rolled off the
production lines in November 2009.
The OMF-1 is more of a refinement of the design ideas introduced with the OMF-1 rather
than a totally new module. OMF-1 boards are different only in the following things:
1. Uses 16mm Alpha/ALPS pots instead of the Omeg P16 types.
2. The 'thru' reversible attenuator has been changed to a dry/wet pot. This gives you the
same functionality but makes it a lot easier to find 100% wet.
3. Uses the excellent THAT300P instead of the now obsolete SSM2210 NPN pairs.
4. Resonance can now be turned completely to zero.
5. Changed output driver circuitry.
6. The PCB's are blue instead of green.
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