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Author Topic: Modify Dolby CP55 for Center/Surround James Moore Film Handler Posts: 2 From: Derwood, MD, Montgomery Registered: Jun 2007 posted 08-13-2010 01:13 PM                     I am told there are backplane modifications that can be made to allow the CP55 to operate in Center/Surround config. If anyone has the info can you please email the file or post/email a link to the drawings? Any other suggestions are welcome!  |  IP: Logged Tony Bandiera Jr Film God Posts: 3067 From: Moreland Idaho Registered: Apr 2004 posted 08-13-2010 01:27 PM                     Hi James, welcome to F-T. A quick and dirty way to do it with some decent results is to take (2) 75k ohm to 100k ohm, 1/2 watt resistors. Tie one end of each resistor to the center channel output terminal, and take the other end of one resistor to the right output and the other resistor to the left output. When you set the levels using the CAT 85c pink noise card, you will set the center output to the usual 85dbC with the fader at "7" , and set the left and right channels to between 75-80 dbC (some experimentation with the left/right levels will be needed with actual films to determine the best balance.) As for the EQ, set the center channel using an RTA and simply set the Left and Right EQ cards' pots to the same position. (You don't need to run pink noise when setting the Left and Right cards, simply pull them and set all the pots the same as the Center card. Unless you want to run the pink..but it's not critical.) Low cost and reasonably effective.  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 08-13-2010 04:55 PM                     Bzzzt...nice try....Don, what do we have for our lovely contestant? The "slick" way of doing a Center/Surround on a CP55/65 is on the Cat 242 card. Is with just two jumper wires (or one if you are careful with your routing. Remove the Cat 242...locate IC8...Jump pins 5, 10 and 12 together. Tune center channel normally. Left and right will play at their proper levels out of center and also have the proper EQ. Note, the Cat 242 will have equal intensity LEDs for Left, Center and Right. This operation is completely reversible. With just two (6.8K) resistors, again with the Cat 242, one can have a Left/Right surround (no center). One resistor goes between pins 6 and 8 and the other goes between pins 12 and 8 of IC8. -Steve  |  IP: Logged Tony Bandiera Jr Film God Posts: 3067 From: Moreland Idaho Registered: Apr 2004 posted 08-13-2010 05:11 PM                     I'm not sure how I feel about Steve calling me lovely.. But he is right of course...his method is a good one, and better than what I suggested, maybe.... One advantage to the method I espouse is that you still have full control over the balance of left/right and center levels. My tests (also used this method on a CP-65 for my outdoor shows) has shown me that you need to attenuate the left/right feeds into that single speaker or the dialog will get buried under the cacophony of sounds. Now of course if Steve's method allows you to reduce the levels of left/right in relation to center then it is a good idea. Another plus to my method would be if/when you go to a full three speakers behind the screen, simply cut the resistors off. Someone down the road may not notice the mods to the 242 card..or worse, may damage it in either doing the mod or removing it. More than one way to skin this cat (242) for sure...  |  IP: Logged Frank Angel Film God Posts: 5305 From: Brooklyn NY USA Registered: Dec 1999 posted 08-13-2010 05:35 PM                        Tony -- There'll come a time when you'll be happy when anyone calls you lovely; I say, take it where ever you can get it!!  |  IP: Logged Tony Bandiera Jr Film God Posts: 3067 From: Moreland Idaho Registered: Apr 2004 posted 08-13-2010 05:39 PM                     LOL true Frank...very true.  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 08-13-2010 08:49 PM                     I was trying to spoof a game show host... In any event, the mod I mention has been done MANY times...always a winner (in fact, it works better than it should considering one is tying the outputs of three muxes together and only taking advantage of the internal resistance of the mux...over 100-Ohms). No phasing issues of by having multiple B-chains either. Can someone screw up...sure...but I never said this is for the incompetent. This still leaves the outputs low impedance to feed the amps too. If you ever see the mod...you'll see how trivial it is to do and undo (same with the L/R surround mod). Steve  |  IP: Logged Tony Bandiera Jr Film God Posts: 3067 From: Moreland Idaho Registered: Apr 2004 posted 08-14-2010 07:46 PM                     LOL Steve, yeah I caught the drift..I have been trying to remember which game show that was from... I'm not downplaying the simplicity or efficiency of your mod..but honestly, how many people do you know who are capable of doing that mod safely, without overheating the IC or damaging the card? Most people aren't technically capable of doing board-level mods without causing damage. A lot of people can however handle soldering to fanning strips, with the advantage of replacing a melted fanning strip is super cheap compared to replacing a damaged 242 card. But my question remains: Does the 242 mod allow you to trim the levels of L/R to balance them out? As for the impedance issue, most modern cinema amps can handle the higher impedance from my resistor trick with no audible/measurable issues.  |  IP: Logged Frank Angel Film God Posts: 5305 From: Brooklyn NY USA Registered: Dec 1999 posted 08-15-2010 09:48 AM                        quote: Steve Guttag ....Don, what do we have for our lovely contestant? It was that lecherous old fart on The Price Is Right.  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 08-15-2010 01:26 PM                     When using just a straight jumper wire, no the levels are fixed. One could use resistors to drop L/R some though I've never found that one needed to. As for the high impedance on your design...it simply isn't necessary and can definitely get one into a noisy situation. Better to use a proper summing amplifier (like an RDL ST-MX3) to get the loading right. Steve  |  IP: Logged Tony Bandiera Jr Film God Posts: 3067 From: Moreland Idaho Registered: Apr 2004 posted 08-15-2010 11:37 PM                     Yeah, the fixed levels can be a problem..maybe the situations you have encountered are different from mine but I have always found the attenuation of L/R signals makes a huge difference in dialog intelligibility which with the crappy mixes coming from H'wood is a big deal. The RDL ST-MX3 is very good, I have used them and they work great, but as you know a lot of folks won't spend the extra money (And they are pricey)...again I have not (so far anyways) encountered any noise issues with my simple trick.  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 08-16-2010 11:35 AM                     Perhaps the reason you had problems with the fixed levels is the method you are doing it. Note, my method is no different than a straight mono system with respect to the mix...including that center channel would be inherently 6dB louder than left/right. Steve  |  IP: Logged James Moore Film Handler Posts: 2 From: Derwood, MD, Montgomery Registered: Jun 2007 posted 08-26-2010 05:08 PM                     Thanks for the help guys. Dolby had suggested something similar to what Tony posted., The only difference being 15K resistors between L-C & R-C, & 27k between R-S & L-S. Seeing as how my soldering hand is steady as a surgeons I'll try Steve's mod first. It also seems I need to mod a CP50 as well. It's a new ownership at an OLD OLD theater (obviously!), and we are fixing what they have as best we can till they can afford upgrades. Thanks again!  |  IP: Logged

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