Soft-Action Mute Switch

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.

2-20 Watt Stereo Audio Power Amplifier by LM1875

Description:

Here is a reliably high sound quality audio power amplifier project based on the National Semiconductor’s LM1875 IC. The circuit that we give is a similar form of the application circuit in the datasheet that NS provides. It is really easy to build and low cost. The voltage gain of the each monolithic channel is 27dB but you can change this value by adjusting the resistors at the feedback path that connects the pin-2 and pin-4.

The supply voltage for the maximum power output (25 Watts) is +/- 25V. It mustn’t exceed +/- 30V at any time. We recommend you to use metal film resistors and electrolytic capacitors that are rated at 50V.