I have a circuit in which I want a pre-built stereo amplifier kit (with + and - outputs for each speaker) to drive a speaker and an LED in parallel. The circuit works, however is there any way to stop the LED flashing with the audio output? In the circumstances the LED drive has to come from the amp, and therefore has to have an audio signal input. Will some kind of transistor do it? Any help is much appreciated. EDIT: I want the LED to be constantly on, instead of flashing. Sorry for confusion.
Ultimately, no, you can't keep the LED on steady. Think about it. When there is no sound at all, there is no power available to run the LED from.
You can harvest power from the audio signal and store any excess that isn't needed to immediately run the LED. When the audio level gets too low to provide enough power to run the LED, you then use some of the stored energy. However, any stored energy will be finite, so this can't go on indefinitely.
As Olin mentioned in his answer, the LED will be off if there is no audio output. The best compromise I can think of involves inserting a 1N4007 diode in series with a speaker (+) output. The end of the diode with a white line goes to a 1K ohm 5 watt resistor, then to the (+) wire of a 470uF 150vdc capacitor.
From this connection a 10K 5W resistor connects to the LED (+) terminal. The LED (-) lead goes to the capacitor (-) lead and ties to the speaker (-) lead. This circuit simples stores energy long enough to keep the LED from flickering, however, low volume levels will dim the LED, high volume levels will make the LED bright.
If the sound cuts off suddenly, the LED will slowly become dark. It is not a perfect solution but it does remove the flickering effect.