My first Optical Organ, described in the following page, has a problem that it makes annoying click noises at key on/off.
I’ve analyzed the issue, and identified that switches to turn sound on/off has the problem.
Here is the schematic of the switch I currently use. The organ has this switch with every key. The circuit is very simple. The switch is in between an optical sensor that constantly generates sound signal and the output summing amp. This switch has a problem in connecting/disconnecting sound signal. Analog switch has very steep onset/offset characteristics, so the waveform at key on is too steep. It makes click noises. We cannot avoid it even if we carefully cancel the DC offset since the switch shapes the input signal itself.
In the next version of the optical organ, this is a point that needs improvement.
The circuit above gives the best result so far. This is a traditional semiconductor switch for gating sound signal, used by many effectors.