FILTER 8
MULTIMODE FILTER/8-PHASE OSCILLATOR
6
14 VOLT PER OCTAVE TRIMMER
This trim potentiometer is used to calibrate the
module’s pitch tracking. Since it is accessible
from the front panel, calibration can be easily
performed without removing the module from
the system. Each module is individually
calibrated during production; do not adjust this
trimmer if not needed.
Should you find your Filter 8 to be out of tune, set
it to oscillator mode (no input signal, maximum
resonance, range switch to audio, compensation
enabled). Set the coarse frequency knob to
about 20 % of its range (9 o’clock), and the fine
knob in the centre position.
Make sure Filter 8 has been powered for at least
20 minutes at a stable ambient temperature.
Now connect any output to a calibrated digital
tuner. The volt per octave input signal should
switch between 0 V and a pre3 V source,
toggled automatically or by hand. Using a
dedicated trimming tool or standard 2.5 mm flat
screwdriver, adjust the trimmer until the interval
between both states is exactly 3 octaves.
15 RESONANCE TRIMMER
The second trim potentiometer sets the self-
oscillation amplitude at maximum resonance.
Use the same settings as for the V/oct. calibration
procedure and display the BP4 output signal on
an oscilloscope. Adjust the trimmer until the sine
wave’s amplitude is exactly 10 V
pp
.
Although the volt per octave tracking is
temperature-compensated, the resonance
amplitude is not. It will change slightly with
ambient temperature, as in any other analogue
VCF. If an accurate 10 V
pp
amplitude is required,
recalibrate this trimmer whenever the
temperature changes.
At low settings of this trimmer, Filter 8 will not be
able to achieve self-oscillation. If your module
can’t self-oscillate, an improperly adjusted
resonance trimmer is probably the reason why.
