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This topic comprises 2 pages: 1 2
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Author
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Topic: Homebrew sound changeover?
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Frank Angel
Film God
![](http://www.film-tech.com/ubbmembers/248b.jpg)
Posts: 5305
From: Brooklyn NY USA
Registered: Dec 1999
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posted 08-25-2005 07:24 AM
Of course if you are still using exciter lamps, you can do what they did years ago which was simple but effective: just switch power between the two exciter lamps. Use a dropping resistor to keep the filament of the "off" projector hot, but not lit. This minimizes the time it takes for the bulb to go from off to full brightness. This method can be accomplished with nothing but two DPDT switches in a toggle configuration and the two power resistors, one at each projector. If you want indication, add an LED and dropping resistor for it at each machine to tell you if the sound is ON on that projector; then you will need two 3PDT switches or a relay. Either way, relay or switches, nothing is being switched in the audio chain so it is totally silent.
Note: Even with a pre-heat bias, you will still get a very slight drop in sound at the moment of change-over, but it is nearly imperceptible and way better than a pop.
As for the bias pre-heat resistor, you want to bias it with as much voltage as you can just so it barely lights. The best way to determine the resistor value that you need is to use a resistor substitute box and just dial up the voltage until the filament just begins to glow. Read the value and that's what you need, but be sure you get a high power ceramic power resistor (at least 10 watts). These bulbs draw high current. Be sure your substitution box uses at least 5 watts resistors and only leave it on just long enough to see when the filament glows. Leave it on any longer and it might burn out the resistors which in sub boxes are typically only one watt or even less.
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