Description:
This simple circuit is a line-level audio signal muting switch based on a soft-action power on/off process. When the S1 switch is closed, R4, C1 and Q4 JFET quietly ground the signal to be muted in between 100 to 200 ms and the RED indicator light goes on. When the S1 is opened the signal is released and the GREEN led goes on.
Potentiometer R2 must be set to the value twice of the transistor's cut-off voltage so the on/off transition times become approximately equal. R2 and D3 discharge C1 swiftly and mute the signal during the power down. In this process, the signal path should remain stable to below 1/3 of the normal supply voltage (In this example below +/-4). Then Q1 finishes muting.
Source: http://www.edn.com/article/CA468422.html?spacedesc=DesignIdeas&taxid=10577&industryid=44217
