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Author Topic: Refurbishing Altec Voice of the Theater Speakers? Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 09-06-2009 03:23 AM                     Actually, yes! If you were to take as many plot points as possible; that is say, starting just below 500Hz...say 300Hz (you need not nor should you do this at High Power!). Use your generator and verify constant voltage amplitude at each frequency, measure the current in the system. Do this at many frequencies and then plot them on graph paper, you could come up with the driver's impedance chart. Here is an example of what a typical response chart is versus impedance: Note, this is a driver mounting just on a tube (for loading), not a horn. The lower line is the impedance and its values are on the right side of the graph. You'll notice that the value is rarely at the 8 or 16-Ohm mark. In fact, the driver impedance is typically the LOWEST it should be within its desired operating frequency range. Note too, near the crossover frequencies (500-1.5K, depending on the system)...that value may vary GREATLY. For the 16-Ohm version of the driver this plot was done for, it is at 50-Ohms! Something may seem amiss...so lets put a horn on the driver and see what things look like: Now things don't look so smooth (welcome to reality) but also the impedance dropped near the crossover region. It is closer to 35-Ohms. So even your measurement conditions will affect things. This is one reason why passive crossovers are so tricky to get right. A driver's impedance number alone tells one very little. One really needs to design the crossover around the intended speaker system including all of its components in order to get a smooth transition. The more generic the crossover is, the less likely it is going to fit any particular system. A great advantage to bi-amping is that you remove all of this from the equation. The frequency response of the electronics (analog or digital) is going to be much more uniform through the crossover region. Most passive crossover systems (though certainly not all) today are really just accommodation crossovers that protect the drivers...they rarely, if ever, provide for a smooth transition in either level or even phase. At least in yesteryear, many of the crossovers had multi-tap inductors to try and better match things through crossover and also some adjustability on the HF response. Nowadays, things are pretty much "preset." About the only way a manufacturer can do a spec sheet though is basing the frequency response on an anechoic chamber, which as zero resemblance to a theatre. Which is why what a speaker's response looks like on paper often differs when it is in operation. Steve  |  IP: Logged Sam D. Chavez Film God Posts: 2153 From: Martinez, CA USA Registered: Aug 2003 posted 09-10-2009 09:22 PM                       (Sam is partially correct. If the system has a passive crossover and is expecting a 24-Ohm impedance, then you will alter the crossover frequency by not using the correct part. If it is an active crossover system (bi-amped) then you have many choices...including going down to 8-Ohms for better power efficiency from modern amps.) Steve, Of course you are pedantically right as usual, but I was trying to deter the tech from putting a resistor in series with the driver and trying to offer practical advice. Now, we don't really know if this was really a 24 Ohm system as the diagram or HF driver may have been changed a number of times over the years. We also don't really know what kind of crossover is being used here. Strange things do happen when you misload a passive crossover and the absolutely correct impedance match may require any number of add-on auto formers and only two people I know will go the whole way to get this right.  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 09-11-2009 01:14 AM                     Of course, I would never recommend using a resistor to alter the "impedance" of the system...I think my point was more that the correct part exists so why would you put in the incorrect one? Or why would one presume the system has been compromised (different components have been changed out). If anything, I was trying to point out the importance of the driver and crossover being matched as well as possible. Now, as others have noted, Altec is one of those brands that I have taken a particular interest in and we support as well as anyone. In fact, we were pretty instrumental in getting Great Plains to continue the 20221 diaphragm. Steve  |  IP: Logged Jarret Chessell Master Film Handler Posts: 288 From: London, Ontario, Canada Registered: Jan 2009 posted 09-11-2009 01:27 AM                     I do appreciate the explanations and examples you posted Steve. It is a good read. I'm not trying to put the issue on hold.... I've had some issues at a different cinema with staffing and have been locked in a multiplex booth for some time now. Hopefully we can get more information up to tackle this problem correctly sooner than later. It would be nice to make sure everything is correct and is 100% the way it is suppose to be in this situation. Thanks everyone for your input on this issue.  |  IP: Logged Louis Bornwasser Film God Posts: 4441 From: prospect ky usa Registered: Mar 2005 posted 09-11-2009 12:21 PM                        I always heard that J.B.L. himself was a lefty which would account for the "backward" polarity. (ccw, not c.wise.) Obviously, as Steve stated, impedance doesn't matter much if you are bi-amped; you can just reset the gain. If you are not bi-amped, you must reset the "sensitivity" strapping or control to add or lose gain. A byproduct is that the crossover frequency will shift enough to measure, but it has always been inaudible. We stock vintage, but new Altec diaphragms as well. Louis  |  IP: Logged Jarret Chessell Master Film Handler Posts: 288 From: London, Ontario, Canada Registered: Jan 2009 posted 09-12-2009 12:25 AM                     Okay, some pictures... hopefully we can sort this all out now. Photographer was Victor today Let me know if you would like additional information, thanks again.  |  IP: Logged Tony Bandiera Jr Film God Posts: 3067 From: Moreland Idaho Registered: Apr 2004 posted 09-12-2009 12:40 AM                     Sam and Steve: I knew after I posted this quote: ME If so , you could add an 8 ohm, 50 watt resistor in series to get close to the original 24ohms (crudely). (Bold added) ..That you guys would say something. Yes it is NOT the right way to fix the problem, BUT in a very rough sense it should help--a bit. Is it good enough to compensate for the mismatch? Maybe, maybe not. Obviously impedance varies with elements of frequency, inductance (and capacitance?) as well as simple resistance. And as Steve's post with the curves showed, it will vary with the acoustic load on the diaphragm as well. If I still had my old Altec horn and crossover I would love to set up a test and see what effect, if any, adding the series resistance would have. Would it be good or bad?  |  IP: Logged Sam D. Chavez Film God Posts: 2153 From: Martinez, CA USA Registered: Aug 2003 posted 09-12-2009 01:50 AM                       The crossover will like it better but you end up with less energy getting to the diaphragm so there would be a shelf in the response starting at 500 Hz as 1/3 of the energy intended for the speaker gets dropped across the resistor. You may be able to set the attenuator bar on the N500 crossover to make up some of it but it is the wrong way.  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 09-12-2009 05:23 AM                     You definitely want the 20221 diaphragm with that set up (N500C). If you flip the diaphragm over and look on the silver ring, there will be numbers rubber stamped onto the ring...one is the ring's number and the other, if it is there should be the diaphragm number (ie 20221 or possibly a different impedance if it had been changed). There is also often the QC inspector's number stamped there. You will need a diaphragm kit for a modern diaphragm for the shorter screws and the right angle push ons. In a pinch, you can always cut down the existing screws and solder the wires directly to the diaphragm but it is often just easier (particularly behind a screen) to just put the kit in. Steve  |  IP: Logged Bob Koch Expert Film Handler Posts: 183 From: williams ca Registered: Nov 2001 posted 09-12-2009 01:58 PM                     Note the electrical out of phase hook-up on the N500c diagram. Some systems used it; others did`nt. As with most everything else,I`ve forgotten.  |  IP: Logged Victor Liorentas Jedi Master Film Handler Posts: 800 From: london ontario canada Registered: May 2009 posted 09-12-2009 02:29 PM                     Yes i found the #20221 on the diaphragm.  |  IP: Logged System Notices Forum Watchdog / Soup Nazi Posts: 215 Registered: Apr 2004 posted 04-05-2011 01:35 PM                 It has been 569 days since the last post.  |  IP: Logged Victor Liorentas Jedi Master Film Handler Posts: 800 From: london ontario canada Registered: May 2009 posted 04-05-2011 01:35 PM                     I used a phase checker device to check phase on all my speakers and all was in phase but i decided to redo the connections on the 288c horn and swap the phase. I wanted to hear the difference in sound on this center voice of the theater speaker. At first i thought it sounded clearer but now i suspect it sounds wrong. Since i made 2 changes at the same time its hard to know for sure.I think the new clean connection helped but not swapping phase. My silly question is should the horn of this speaker be in phase by design or out of phase?  |  IP: Logged Louis Bornwasser Film God Posts: 4441 From: prospect ky usa Registered: Mar 2005 posted 04-05-2011 07:09 PM                        In former times, you deliberately introduced an out-of-phase, played a "warble tone" (500 Hz with frequency shift) and tuned the horn location for a NULL. Now, reverse the hf and you are "in phase." louis  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 04-05-2011 08:37 PM                     Don't confuse the Altec diagram for wiring out of phase...just the opposite. Altec used a convention that had the L1, L2 terminals reference to motion relative to the magnetic structure. As such, Compression drivers push INTO the magnet on forward motion and Cone drivers push OUT of the gap on forward motion. Think of the compression driver as one that is physically mounted opposite from the cone driver. As such, they are IN PHASE to each other if they are wired opposite (L1 on the HF driver is "-" but "+" on a cone driver. -Steve  |  IP: Logged

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