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The reason this works is because we're modulating the amplitude to always be -inf dB. Only when the LFO is above 0 will the synth amplitude open and make sound. This is what most beginners expect to happen when modulating the volume with an LFO.
When I was a beginner something so trivial as this was so difficult to do. Hope this helps you out there!
\!/ NOTE: This is also a post volume control, so should tame any noise that would be induced by FX in Vital. (Very useful if you like using Phat Racks or lots of OTT-like compression)
For Serum, global Amp is actually Pre-FX, so you'll need to sacrifice the volume knob on one of your FX by clicking the "Mix" and do the exact same thing we did before except just -50 modulation instead. Put that effect at the end of your FX chain and voila.