Trimmers
There are three trimmers on the issue 2 PCB which need adjusting correctly to get the best out
of the VCA module. It is beneficial to adjust these in your own modular as the settings are
affected slightly by the power supply voltages.
Allow the modular and VCA module to warm up for at least 15 minutes.
OFFSET:
Turn the GAIN pot to its maximum level. Ensure all other pots are at their
minimum settings. Measure the output voltage from the output socket with a good digital
voltmeter. Adjust OFFSET with a jeweller's screwdriver or equivalent until the output voltage
is 0.000V +/- 5mV.
LIN:
Insert a 5V peak triangle wave signal at roughly 440Hz to the INPUT(AC) input. Turn
the GAIN and the INPUT(AC) pots to their maximum. All other pots should be at their
minimum settings.
Now with a scope measure the output voltage and adjust LIN until the input and output
signals are the same level. It doesn't have to be that accurate – within 250mV will be fine.
If you do not have a scope, all you need to do is compare the input signal to the output signal.
Adjust LIN until the two signals are the same volume. The best way to do this is to use a
multiple. Connect the triangle wave from your VCO to the multiple, and then with another
cable into the VCA input as above. Connect up your monitoring system to the same multiple
and adjust your monitoring volume to suit. Then simply swap the monitoring system's jack
plug from the mult to the VCA's output and adjust LIN accordingly. It doesn't have to be that
accurate, just try to get the two signals sounding roughly the same.
EXP:
Insert a 5V peak triangle wave signal at roughly 440Hz to the INPUT(AC) input. Turn
the INPUT(AC) and EXPO CV pots to their maximum. All other pots, including the GAIN
pot, should be at their minimum settings. Connect a 5V source to the EXPO CV input. This
could be a fixed voltage source, a triggered envelope generator or midi-CV convertor –
anything that will give you a +5V output signal.
Now with a scope measure the output voltage and adjust EXP until the input and output
signals are the same level. Again it doesn't have to be that accurate – within 250mV will be
fine.
Likewise, if you do not have a scope then repeat the same procedure as the LIN trimmer, but
this time adjust EXP to ensure that input and output are the same volume.
If you have an issue 1 board you will have one additional trimmer to set. The NULL trimmer
is designed to reduce audio breakthrough. Thus NULL needs to be adjusted so that any signal
on the signal inputs are not passed onto the main VCA output when the VCA is off. However,
the original ARP documentation says any trimming procedure is worthless and that the
trimmer should be left as you find it. After various experiments I would have to agree with this
for my version too. Therefore, I would recommend that it should be simply turned fully
clockwise, ie. off.
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