Synthesis Technology MOTM-485 GX-1 Diode VCF Assembly Instructions & Owner'S Manual Download Page 16

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f) If this is properly operating, switch to HP mode and rotate FREQ back and forth.

Now you should hear the “opposite” effect: only the higher harmonics are passing
through the filter.

g) Put the filer back into LP mode and play with both RES and FREQ. You should be

able to hear the effects of increasing resonance (the volume will drop some, but the
filter will “squeal” more).

Limitations, Hacks and General Weirdness of the MOTM-485

The MOTM-485 is unlike any other MOTM VCF because it uses a pair of diode “rings” to
act as the variable element. The diode ring topology has several issues: they have smaller
dynamic ranges for the control current (the VCF cannot sweep as broadly), the diodes
introduce lots of IM distortion (especially in HP mode) and in this particular
implementation, the resonance characteristics are based in part on the harmonic content of
the audio output (this will be evident when using narrow pulses in HP mode).

So, why use diodes at all? Because, Yamaha needed to build a very inexpensive filter for the
GX-1, which is an organ and so has 1 pair (a HP/LP set) for every note! Of course, the
MOTM version uses matched pair transistor arrays and has temperature-compensation for
the control voltage. Also, the OTA in chip form was very expensive in the mid-1980s
compared to the diode version.

Even though the MOTM-485 is “faithful” to the original Yamaha overall design and
topology, the GX-1 was a “controlled” environment. The waveforms were limited; the
amplitudes under tight control and many hours were spent in trimming (remember, this
thing cost like $50,000 when it came out, as much as a large house). In updated modular
format (as open-ended as it gets), the, how shall we say, “quirks” of the design can be seen.
So, what I’m saying is, don’t be surprised if the MOTM-485 “doesn’t sound like my other
filters” or “puts out all sorts of strange, distorted sounds” because that’s part of its charm.

Quirk #1: HP mode has all sorts of IM (intermodulation) distortion at high resonance

This is will be based on the input level (see Quirk #2) and the setting of the Q PEAK
trimmer. But in general, once the RES pot is past 4 or so in HP mode, all hell can break
loose. It’s not broken, OK? It can almost sound like an overdriven ring-mod at times. If you
look with a ‘scope, you will think something is terribly wrong with the output (sine wave
turns into odd ramps, etc).

Quirk #2: the filter is sensitive to input levels

You will be tempted to jam 2 full-level VCO waveforms into the 2 IN jacks. Prepare for all
sorts of messed-up tones. If you want “tamer” outputs, run a mixer before the inputs (like a
MOTM-890). Again, HP mode will be more wacky than LP mode with large inputs.

Quirk #3: limited sweep range

The filter cannot sweep 20Hz-24Khz like say a MOTM-440. The MOTM-485 is more on the
order of 80Hz to 6Khz. Again, this is a function of the diode control currents.