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ADSR/VCA "OFF" trimpot
MUFF WIGGLER Forum Index -> Oakley Sound Systems  
Author ADSR/VCA "OFF" trimpot
Sparrows78
What exactly is this trimpot suppose to do? When doing the calibration, turning the trimpot gives a slight increase/decrease of the volume of the square wave. Fully clockwise is low and fully cc is high. At no point the volume goes near zero. Nor does it change the timbre of the sound.

Is this the correct behaviour or is there something wrong with my build?

Thanks in advance!
Sparrows78
Also, the off trimpot affects the VCA OUT level - turning it clockwise (while calibrating the VCA output) raises the level.
Synthbuilder
OFF adjusts the unwanted small voltage, called an offset, found on the VCA output when the VCA is turned fully on by the envelope's output voltage. OFF is adjusted so that the offset is as close to zero as possible.

One way to do this is to put the module in loop mode, turn the sustain up to full. Put a short jack lead into the VCA IN and leave the other end unplugged - this essentially makes the VCA IN voltage 0V. Then adjust OFF so that the voltage on the VCA output is as close to zero as possible.

It may be easier to do the square wave thing as detailed in the manual but if you can't get it to minimise then the LM13700 is probably out of spec or there is another issue. What you should get is quiet buzzing on either side of a obvious quieter point somewhere near the middle of the trimmer's position.

OFF should have no effect on audio output volume in normal use but it will have a very small effect on the output voltage.

Tony
Synthbuilder
I've just rewritten the calibration section in the user manual. It was OK but I think this version is simpler to do.

Calibration

There are just two trimmers to be set before you are completely finished. These are the GAIN and OFF trimmers. Ideally both should be trimmed after the unit has been fitted into your modular as slight changes in power supply voltages do have an affect on the calibration settings.

OFF:

The OFF trimmer attempts to compensate for the unwanted input offset voltage of the VCA circuitry. The trimmer needs to be adjusted to minimise the amount of control voltage, from the raw ADSR envelope, leaking into the output of the VCA.

Set all four pots to their minimum positions and set the two switches to LOOP and SHORT respectively. Insert a short patch lead into the VCA IN socket. Leave the other end of the patch lead unconnected. Now attach another patch lead to connect the VCA OUT socket to your final mixer or amplifier. You should hear a faint oscillation at around 400 to 500Hz. Adjust the OFF trimmer until the oscillation volume is minimised. You may need to turn your monitoring amplifier up to fine tune the trimmer. You won’t get it absolutely silent but on either side of the correct position for OFF the oscillation will be louder.

GAIN:

The GAIN trimmer allows you to trim the output of the VCA to the desired gain. With no jack inserted into the VCA IN socket, the VCA OUT socket should be trimmed to give +5.0V peak attack and sustain levels.

Keep both switches in LOOP and SHORT. Ensure that Attack, Decay, and Release are still at their minimum positions, but now set Sustain to fully clockwise. Remove any patch lead from the VCA IN socket. Adjust GAIN until you get +5.0V from the ‘VCA OUT’ output.
Sparrows78
It was a faulty LM13700 causing my problem, as you suspected.
In my case it was a 78ZHR04E3 LM13700N. I don't know if its specs are off or if it is actually broken, but after replacing it with a 73ZKC24E3 LM13700N everything works as it should!

Thanks again for the clarification and help! thumbs up
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