ASM-2 Construction Guide
ELBY Designs - Laurie Biddulph
9 Follan Close, Kariong, NSW 2250, Australia
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VCF - Voltage Controlled Filter (VCF 1)
Power up, Testing and Calibration Procedures:
There are three trimmers on the PCB:-
V/OCT (P201): This adjusts the scaling of the exponential inputs. Adjust this so that there
is an octave jump in cut-off frequency when the 1V/OCT input is raised by one volt.
Plug a 1V/octave source into one of the CV inputs. This may be your keyboard’s pitch CV
output, or from the CV output of a midi-CV converter. Set the Resonance pot P204 fully
clockwise to get the filter oscillating. Now listen to the output coming from the low pass
output. You may find it best to use the Frequency pot on the front panel to set the filter
oscillating at quite a high frequency tone. Somewhere around 1KHz will do. Now play a
note on your keyboard and then the same note an octave above. Repeat this again and
again and adjust the V/OCT trimmer to get the filter’s oscillations to jump an octave too.
Don’t worry about the actual pitch the VCF is producing. Just concentrate on getting
roughly one octave difference between the low note and the high note. It is a fiddly
adjustment and it takes a while to get it right. But remember that this is a filter and not a
VCO, so you don’t have to be too accurate.
OFFSET: The two OFFSET trimmers (P202 and P203) control the amount of DC on the
output. Use a scope or a voltmeter to measure the voltage at pin 1 of U204. Rotate the
Frequency pot until the voltage is approximately zero volts. Without any audio input,
measure the voltage at the BP output. Set OFFSET 2 so that the voltage at the BP output
is as close to zero as you can get it. You should be able to get it down to +/-5mV or less.
Now measure the voltage at the LP output. Set OFFSET 1 so that this voltage is as close
to zero as you can get it. Again aim for +/-5mV or less.
