Hy Baxter,
Here is the complete circuit:
spec
**broken link removed**
NOTES
(1) With no audio input the output will be 0V
(2) With 1.2V or more peak audio input the output will be 12V with a current capability of 1A (more if PMOSFET, Q10, is on a heatsink)
(3) R23 can be adjusted to alter the sensitivity. If R23 is 100K the turn on point will be 0.6V peak
(4) R27 and C3 control the turn off delay which will be around a second. C3 can be changed to adjust the turn off delay.
(5) C18 and C22 are only decouplers and should be a disk ceramic type of X7R dialectic
(6) The input impedance will be at least 10K so hopefully it will not load the audio output unduly
(7) Many PMOSFET types will be suitable
(8) R28 is a gate stopper to prevent the PMOSFET from oscillating at a high frequency. R28 should be mounted directly on the MOSFET gate terminal.
(9) The amplifier will turn on fairly quickly and turn off fairly slowly so that no audio should be lost and there should not be any clicks and thumps from the speaker. Also the power will not be continuously turned off and on for small gaps in the audio.