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Author Topic: Homebrew sound changeover? Matthew Taylor Expert Film Handler Posts: 186 From: Essex, UK Registered: Mar 2004 posted 08-22-2005 04:42 PM                     Is it very difficult to make a changeover circuit for just swapping the sound from one projector to another? I imagine just using a switch would cause a 'pop' or some other noise when the sound is switched.  |  IP: Logged Jack Ondracek Film God Posts: 2348 From: Port Orchard, WA, USA Registered: Oct 2002 posted 08-22-2005 05:40 PM                        There are several ways to switch sound from one machine to another. The best ways depend on what projectors you're running, and what kind of sound electronics are involved and where they're located. More info, please.  |  IP: Logged Bruce Hansen Jedi Master Film Handler Posts: 847 From: Stone Mountain, GA, USA Registered: Dec 1999 posted 08-22-2005 05:57 PM                     You may be able to do away with the pop by connecting a resistor to ground to each output line, if the output uses ICs or transistors. If it uses a transformer, then there should be no problem. The pop is caused by the caps. on the output having to "charge" through the input to the amp. If there is another path for the caps. to charge there should not be a pop. If your preamps have LO Z outputs, try 10K resistors, if HI Z try 100K resistors. There could be a very quick (about 1/4 sec.) interruption in the audio as the switch goes from one set of contacts to the other. If this is a problem, you could get a "make before break" rotatory switch. Of course, you could get a sound processor that is set-up to do change overs, if you have the bucks.  |  IP: Logged David Buckley Jedi Master Film Handler Posts: 525 From: Oxford, N. Canterbury, New Zealand Registered: Aug 2004 posted 08-23-2005 02:55 AM                        All you need is audio switching, it wont click or pop as long as there are no DC offsets hanging around. In this old thread is a picture from our old "processor" system, built by persons unknown.  |  IP: Logged Dieter Depypere Master Film Handler Posts: 343 From: Deutsch-Wagram, Lower Austria, Austria Registered: May 2005 posted 08-23-2005 07:47 AM                     We use a relay which tilts a mercury switch. This doesn't cause a "pop" when changing over (The mercury flows slowly to the opposite electrode).  |  IP: Logged Marin Zorica Jedi Master Film Handler Posts: 671 From: Biograd na Moru, Croatia Registered: May 2003 posted 08-23-2005 04:46 PM                     What you have there? You probably have two sound head output and one input in cinema amplifier(processor)? If is it use HEF4066 IC to switch sgnlas from reader and 5-12V voltage to switch inputs os HEF4066. HEF4066 is quad electronic switch (it has got 4 switch inside it) and you need about 5V on every switch to activate it, so on switch 1 you conect output of reader 1, and on switch 2 conect reader 2 output, outputs of switch conect together and pas thru amplifier (processor) input, when projector 1 activ, take 5v power from lase/exciter lamp suply thru couple of KOHM resistor to swtich 1, and when projecot 2 activ take them laser/exciter lamp power to switch 2. This will work if you have setup when laser/exciter lamp is only burning during show, not always like in reverse scan. Another thing i saw cople of times is taht you just put solar cells in paralel thrue 1KH resistors and thing works!  |  IP: Logged Gordon McLeod Film God Posts: 9532 From: Toronto Ontario Canada Registered: Jun 99 posted 08-23-2005 06:39 PM                     The 4066 is a single ended device and the audio must be biased up to +6v to pass through it undustorted with the chip running on a +12vdc supply  |  IP: Logged Marin Zorica Jedi Master Film Handler Posts: 671 From: Biograd na Moru, Croatia Registered: May 2003 posted 08-24-2005 03:02 AM                     Yes i forgot to say it it must be biased, or except biasing you may powerit from simetrical supply +/- 8V and it works!  |  IP: Logged Matthew Taylor Expert Film Handler Posts: 186 From: Essex, UK Registered: Mar 2004 posted 08-24-2005 10:31 AM                     Thanks for your advice. I have used 4066's in other applications so I will probably go for that. It sounds better than just using a relay.  |  IP: Logged David Buckley Jedi Master Film Handler Posts: 525 From: Oxford, N. Canterbury, New Zealand Registered: Aug 2004 posted 08-25-2005 02:51 AM                        Actually, a cmos switch will not sound better than a relay, due to (a) distortion, and (b) crosstalk. Whether either of these are significant, of course, is another matter :-)  |  IP: Logged Marin Zorica Jedi Master Film Handler Posts: 671 From: Biograd na Moru, Croatia Registered: May 2003 posted 08-25-2005 03:40 AM                     quote: Actually, a cmos switch will not sound better than a relay, due to (a) distortion, and (b) crosstalk. Whether either of these are significant, of course, is another matter :-) I already work with CMOS switch and i didn't get any distortion or crosstalk (what kind crosstalk we talking about)?? Only thing that is mater is that be properly biased input signal or power CMOS with simetrical supply.  |  IP: Logged Frank Angel Film God Posts: 5305 From: Brooklyn NY USA Registered: Dec 1999 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.  |  IP: Logged Louis Bornwasser Film God Posts: 4441 From: prospect ky usa Registered: Mar 2005 posted 08-25-2005 07:57 AM                        If you switch the exciters in this way, you could use a 3-way, (single pole, double throw) and a 4-way (double pole,double throw with jumpers already installed). After you get proper operation in terms of on-off; you then add the resistor from the on lamp to the off lamp. These switches are easy to find, cheap and easy to wire with no soldering, and most important, will probably never wear out. They are made for 15-20 amps Louis  |  IP: Logged John Pytlak Film God Posts: 9987 From: Rochester, NY 14650-1922 Registered: Jan 2000 posted 08-25-2005 11:16 AM                        The old RCA sound system used at the Grandview Drive-in where I started in projection used the exciter lamp changeover method. Once played two identical trailers in sync and switched back and forth with no perceptable change in loudness, and certainly no "pops". Of course the method depends on the "thermal inertia" of the thick filaments used in those old exciter lamps.  |  IP: Logged Darryl Spicer Film God Posts: 3250 From: Lexington, KY, USA Registered: Dec 2000 posted 08-25-2005 11:30 AM                    last changeover booth I ran had both exciter lamps stay lit. However it did use dolby equipment so the automation was changing it on the preamp boards in the processor.  |  IP: Logged

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