Sometimes you are trying to fix a problem with an amp or other equipment that you're concerned might have a power fault. The worst thing that can happen is that when you turn it on, it immediately pops a fuse - or worse yet a valuable part you've just installed! If you don't have a variac with a current meter so that you can make power on gradual, the next best thing is this widget.
The circuit is able to sense a mains load as low as 50mA, so even the smallest preamp will trigger it reliably. Using a light dependent resistor (LDR) as the sensor.
The 2N3392 transistor is a low-noise type in a TO-92 housing and can be replaced by a NTE199 or ECG199.
A stereo power amplifier is limited in its output power by two main factors - the impedance of the load and the internal power supply voltage. To obtain more power, one has very limited choices - other than the purchase of a more powerful amp.
This is a simple circuit to control the speaker output relay.