Signal Chain Basics (Part 22): Phantom microphone power--the ghost in the machine

So long and thanks for all the fish. Now where have I heard that before ?

Hi Don, Does this also apply to lighting? At random times in the Dolphin theatre and also the Rechabites, I've switched on power and had a couple of lights shining at low levels even though everything was set to zero. Do you know what causes this?

If a theatre is fitted with old dimmers, they will usually have a top and bottom dimmer set. The convention back in the sixties was set the bottom of the dimmer at about 12 volts, not zero, so the tungsten filament on the old T1 lamps were kept warm, a condition called preheat, so when you ran the channel to full at a fast rate, the lamp filament did not fail due to thermal shock. This practice has been discontinued in most venues because most conventional theatre lighting is using tungsten halogen lamps which are designed to handle the inrush of current into a cold filament. The Dolphin has JTM dimmers with a top and bottom setting, the top is actually set to 125 volts with the fader at 50 percent so the lamp only gets 230 volts at 100 percent to extend lamp life. The bottom is set to zero, but the Strand analogue interface "bleeds" so the racks don't usually go to zero. No relation to microphone phantom power. Now standby for David to talk on preheat ?????????????