my profile | my password | search | faq & rules | forum home

» Film-Tech Forum ARCHIVE   » Operations   » Film Handlers' Forum   » Info wanted on RCA Drivers MI 9448 & MI9458

Author Topic: Info wanted on RCA Drivers MI 9448 & MI9458 Lindsay Morris Expert Film Handler Posts: 233 From: Darlington, WA, Australia Registered: Sep 2002 posted 01-28-2012 09:08 PM                     Looking for some information on the above HF drivers such as freq range covered (do they use 400Hz or 2Khz crossovers) Power handling and impedance. Have browsed the Manuals section but these seem to be fairly recent models as compared to what is listed in the Manuals which are electrodynamic type drivers..these are permag drivers and quite large. Looking to see IF I can adapt one of these to replace a blown HF driver type Westrex 555 (electrodynamic type) which has been belted by lightning and now has an o/c voice coil. I have another 555 which could go up there (11m above the ground) but it sounds rather raspy as compared to the RCA pair I have been given. The Westrex 555 simply screws on but the RCA bolts onto the rear of the horn similar to how Altec mount theirs. So I will need to do some messing about to adapt the RCA unit onto the existing horn as I do NOT have an RCA type horn...whatever that may look like. In the past I had lost 3 other drivers to lightning and I thought I had cured all that by installing a lightning spike sticking up well above the HF unit on top of the screen connected to a whacking big thick earth cable straight down the back of the screen to earth but a recent quite nasty storm took out the HF voice coil yet again and I have run out of easy fit drivers. All work has to be done via a hired cherry picker which complicates things a bit as most operators want to get in and get out ASAP. We are limping along with a Vitavox HF unit sitting on the to of the bass bin under the screen and coverage across the auditorium area is very patchy as compared to what I normally get from the unit on top of the screen pointing straight into the audience well above their heads.  |  IP: Logged Cameron Glendinning Jedi Master Film Handler Posts: 845 From: West Ryde, Sydney, NSW Australia Registered: Dec 2005 posted 01-28-2012 11:23 PM                     Lindsay, I have never seen the RCA in question but a quick google that I did has suggested that there are several who are passionate about them. From what I've read I suspect that its closer to 400hz for the crossover. As these are now becoming rare due to their age perhaps you should look into buying a modern 2" P.Audio compression driver for $199 and see if a collector wants yours? web page  |  IP: Logged Cameron Glendinning Jedi Master Film Handler Posts: 845 From: West Ryde, Sydney, NSW Australia Registered: Dec 2005 posted 01-29-2012 04:04 AM                     P.audio also offer a 1.4 inch version of that driver, I suspect thats what your horn matches now that I have found photo's. Hard to think that the MI 9448 would now be over 60 years old, The MI 9458 was produced until the 1970's.  |  IP: Logged Robert Koch Film Handler Posts: 93 From: Williams Ca USA Registered: Apr 2006 posted 01-29-2012 04:32 PM                     both these drivers used 400 cycle crossovers utilizing the RCA passive crossover in the booth, then the output of the crossover impedense both lf and hf was 250 ohms and was fed to the speaker array on the deck where you had matching xformers to math the driver impedance of 15 ohms. These were RCA`s last and best attempt at pm drivers; earlier stuff all had field coils. Prior to the drivers mentioned their earlier attempts were pitiful at best. The drivers you have sounded very good. The only bad side, although I never experienced it, was that the magnets would loose their strength. I dont mean to imply that they ever approached the quality of the WE594or the later Altec 288`s.  |  IP: Logged Lindsay Morris Expert Film Handler Posts: 233 From: Darlington, WA, Australia Registered: Sep 2002 posted 01-30-2012 01:56 AM                     Thanks Cameron and Robert for replies. A 1.4" version would suit better as the throat on the horn (Raycophone) is only JUST that big and would be better suited I feel to a 1.4" diam driver as existing are around 1" types. Possibly some mismatch may occur but one can only try and see how it sounds handling a range of signal inputs. Have fired up both the RCA units on the bench attached..sort of.... to a weenie Westrex horn that allowed the beefy RCA's to bolt on and both sound rather nice but that of course is with only a few milliwatts driving them. So I guess next step is to get the horn down and mock up some attachment and give them a good work out in the open air and see how they scrub up and if no good have a look at the driver Cameron has suggested. Must check the VC impedance of the RCA's as both seem quite happy being feed with a bipolar 20uF cap off my home cinema sound system and I would have thought they were around 15 ohm but RCA used to LOVE using transformers everywhere and even had transformer coupled PE cells on many old mono setups. Always sounded good with plenty of HF component in the early days of carbon arcs and mono sound.  |  IP: Logged Lindsay Morris Expert Film Handler Posts: 233 From: Darlington, WA, Australia Registered: Sep 2002 posted 01-31-2012 02:54 AM                     An update.. The RCA units check out DC wise at 10 ohm so most likely are 12 ohm units which were common around the time these units were used. (I have a couple of quite old handmade/handwound X/overs that are massive in size that state Z = 12 ohm). The 10 ohm DC value is similar on the Westrex 555 unit so I guess RCA must have had a matching transf on the stage unit to bring the line back down from 250 ohm to 12 ohm...as I said they did seem to like using transformers a lot including interstage coupling and they must have good design way back then as the top end was always quite good. If I cannot get an adapter made up I think I will run with Cameron's suggestion of a modern 1.4" HF compression driver as the recommended X/over is 1khz which is pretty close to these original units at 400hz and I will not need to change the current x/over arrangement which is good as it will be a real pain to get at the x/over as I have got it buried deep inside a huge W bin that is waterproofed for outdoor use. SO a simple change of the driver attach point on the horn should see it all sorted. Thanks guys. Now just gotta get the cherry picker guys on site and get the horn and fried driver down from on top of the screen.  |  IP: Logged Lindsay Morris Expert Film Handler Posts: 233 From: Darlington, WA, Australia Registered: Sep 2002 posted 03-08-2012 06:27 AM                     We have got our voice back After much messing around I finally found a local cherry picker operator who was prepared to come and take me up in the bucket so I could retrieve the dead Westrex 555. Then off to my machinist mate to have an adapter unit made that would allow the RCA to bolt onto a flange with a screwed extension that would fit the original horn. He struck trouble as well as his screw cutting lathe had been blown up in the same storm that took my driver out so I had to wait a while while he found a sparkie that could work out the contactors in his lathe control box. Got the adapter last week and finally teed up the cherry picker to get back up there and fit it all back together. All went well and sounded OK just from a non sync source and today finally took a test reel up with some music and dialogue and it sounds great all on its own ( had the L & R as well as the surrounds OFF to test). It needed just a light tweak on the 2K pot in the Eq to liven the sound up a bit and we are off and going again. The temporary HF setup was struggling a lot to feed adequate HF to the audiences as it was more or less shooting straight over their heads and although I had heard about the effect called "lobing" with multicellular horns I had never actually experienced it myself UNTIL I set the temp horn up and boy was it crook. There were patches all over the auditorium that had quite good sound and then a few feet either way it was shite. I had gotten a few grizzles over the past weeks on particularly busy nights when we were frequently VERY full as I suspect that IF a patron was behind a "boof head" in front it would soak up much of the HF component and make the sound muffled. Was particularly bad for The Help and NOT helped by the southern accent of many of the actors But now it should be all OK...we have our voice back and it sounds great. All I have to do now is to source a 1:1 transformer to isolate the HF driver winding from the earth side back at the sound rack so that future lightning zapps do not try and take that path via the frame to VC again. Might have to wind one myself as have found nothing so far and in the interim will be unplugging the damn thing each night...just in case. The old RCA sounds quite warm and seems to be able to handle all that I can throw at in terms of Watts without any distress whereas the temp driver had a few raspy spots over the past weeks IF the sound level got up a bit. For the record the M9448/58 are 15 ohm drivers 400 Hz and up and good for around 30W. Thanks for pointers to some of the other forums I did a lot of research on them and like Cameron says it may well be better to go with a new modern driver as these old girls are going for $800 on EBay. The 2" driver would suit as the response is 500HZ and up and at 400Hz it is only a dB or so down and mechanically would match up well with the horn throat which is 58mm in diam so a threaded adapter made by my machinist mate is the next project and I will try a modern one up there for next season. But it will also have an isolating transformer inserted to break that pesky earth connection.  |  IP: Logged Kenneth Wuepper Phenomenal Film Handler Posts: 1026 From: Saginaw, MI, USA Registered: Feb 2002 posted 03-08-2012 07:22 AM                     Lindsay, I hate to be negative about your idea of the isolation transformer but: Proper lightning protection is your answer. If there is enough voltage to jump a few miles from ground to cloud, how will a simple isolation transformer prevent the attraction of the lightning? There are experts in the field of lightning protection and you might want to consult with one of them to help make your area safe from future static electrical discharges.  |  IP: Logged Tony Bandiera Jr Film God Posts: 3067 From: Moreland Idaho Registered: Apr 2004 posted 03-08-2012 02:39 PM                     Kenneth is spot-on, only proper grounding/bonding will protect against future lightning hits. An iso transformer will not do any good at all and will most likely degrade the sound quality. One point I am not clear on: are we talking about an indoor hard top cinema or an outdoor cinema/drive in?  |  IP: Logged Lindsay Morris Expert Film Handler Posts: 233 From: Darlington, WA, Australia Registered: Sep 2002 posted 03-08-2012 07:10 PM                     Kenneth & Tony, Thanks for the comments BUT I am reasonably certain that an isolating trannie will work which is based upon my past experiences within the big Telco of Aust working in areas that had high lightning exposure. Used to drive us nuts at times. We are talking about a steel framed and sheeted outdoor screen that is 10m wide and to the top is almost 11m above the ground. I fitted a lightning rod to the screen some time back and that is connected to the screen and is then connected to a substantial earth stake immediately below via a hefty earth cable. Because that rod is part of the screen as is the frame of the horn unit for the very millisecond that the lightning discharge takes place the many hundreds of amps flowing create a voltage potential that looks for other paths to go to ground. The MEN power line earthing system offers another VG pathway and as that is also part of the sound system earth as well it does not take many volts to jump that tiny annular gap within the driver and seek out that path as well. In doing so it vaporises the VC winding. Another option is to mount the horn up on insulators and keep it electrically clear of the screen BUT if Mother Nature decides to zapp the horn I am back to square one again. Ages ago whilst working for the Telco in the NW of West Aust we had issues with TV distribution amps taking the TV signal from the Telco equipment hut across to the ammenities buildings of the railroad line camps for an iron ore company. The alternative lightning discharge path was taking out output transistors in the Video Distn Amps and at one stage we had exhausted the entire stock of those transistors from the supplier in Aust. We discovered a company in USA supplied what was termed a Video Isolation Transformer primarily meant to break the hum loop problem on long cable runs (which we also had) but also to give some lightning protection. We bought one and tried it out...BINGO it worked and when we tried to buy 3 more we struck trouble as the powers that be in the office sent our request to an engineer who decreed that what we were attempting to do would degrade the TV signal and HE would design a suitable lightning protection module. He did and we installed it to his specs and it did ZILCH...we went back to killing transistors faster than we could replace them so in desperation we put the trannie back in line whilst he thought through his design..in the end the only thing that worked was the transformer. He gave up, we bought 3 more TV transformers and installed them and no further issues arose despite some rather nasty lightning strikes later on. I did try 2 x 100V/16 ohm line trannies back to back and that worked OK but I, like Tony says, thought that that arrangement degraded the audio a bit..sort of sounded harsh. So I am now pulling apart one of those trannies to count the turns on the 16 ohm side and see IF I can replicate 2 such similar windings on a toroid and see how that goes. One final option is to insert a relay in the audio line at the screen and break BOTH legs of the HF driver line when it releases. There is a 12V supply at the screen for other uses that can drive the relay but having seen inside the VC area of the destroyed drivers I think that the relay contact gaps may NOT be enough to stop the discharge path. Tis but one of the joys of outdoor movies but probably will all be over as digital comes along as I simply cannot justify the cost of conversion. So as film supply stops so will my problems with blown speakers as I will not be there anymore.  |  IP: Logged James Westbrook Phenomenal Film Handler Posts: 1133 From: Lubbock, Texas, Usa Registered: Mar 2006 posted 03-08-2012 10:05 PM                     This would explain the need for the cherry picker, or as we call them, a "bucket truck."  |  IP: Logged Gordon McLeod Film God Posts: 9532 From: Toronto Ontario Canada Registered: Jun 99 posted 03-09-2012 10:40 AM                     Also a good TVSS system on the entrance service is a good precaution  |  IP: Logged Robert Koch Film Handler Posts: 93 From: Williams Ca USA Registered: Apr 2006 posted 03-10-2012 02:16 PM                     RCA drivers are 15 ohm NOT 12.  |  IP: Logged Louis Bornwasser Film God Posts: 4441 From: prospect ky usa Registered: Mar 2005 posted 03-10-2012 03:32 PM                        Sounds like the drive ins here that (attempted) to use solid state amplifiers. Lightning strikes even during off hours killed the amps, and unlike tubes, could not be quickly repaired. Solution: the knife switch from the old carbon lamphouse was repurposed to open the output to the field during off hours. Louis  |  IP: Logged Lindsay Morris Expert Film Handler Posts: 233 From: Darlington, WA, Australia Registered: Sep 2002 posted 03-10-2012 09:48 PM                     A further update, Well ummm the toroid transformer winding is not exactly easy to get a nice snug wind and first attempt at a reasonable unit has ended up in the bin. Lousy response!!! more to audio trannies than I first thought. One of my ex Telco mates suggested that I look at using a pair of MOV's rated at 275V wired from either side of the VC to the frame of the horn. As it is going to take around 10Kv to jump the VC gap inside these start to conduct at 250V and by 275 are fully conducting and can handle a peak current level of around 6500 amps...might just do the job at least in the interim. Now where is that cherry picker/bucket truck again??? and revisit the scene of the crime. The power feed to the cinema is well earthed at the distribution pole, again at my main switchboard plus other random earths via the metal framing of the bio box and so on. The street feed is not a simple open wire 3 phase 4 wire arrangement but is a what they term over here as an ABC feed system (Aerial Bundled Cables) so that the local trees cannot whack a couple of wires together and start another bushfire. So with all 4 wires twisted around each other lightning has a devil of a job getting in that way. My screen earth may not be of a low value & I no longer have access to an Earth Megger which would show me the actual resistance within a few minutes of use. Possibly if that is high then the mains earth may be offering a better path so that is the way the strike attempts to dissipate. So maybe the MOV's will afford some degree of protection. Robert, I had found a neat little label inside one of the other drivers I was given showing the impedance as being 15 ohm which I had noted in a previous posting...makes one wonder why the makers sort of almost hide those small bits of critical info Louis, DI solid state amps certainly did not like lightning at all and a couple of country DI's in Aust had a fair degree of difficulty in maintaining their newly installed solid state amps. I can recall them having a couple of shots at replacing the solid state ones a couple of times and then went back to valves in the end until the DI closed. One got around it OK by ensuring that the ONLY earth was at the amp rack much like standard audio screening methods to reduce hum. The ground current can be thousands of amps for a few micro seconds so if a cable has an earth at each end and that sort of current is floating around a metallic path is better than an "earth" or soil path and immediately you have troubles with huge spikes doing nasty things to sensitive bits of gear.  |  IP: Logged

All times are Central (GMT -6:00)

Printer-friendly view of this topic

Hop To: Select a Forum: Category: Operations -------------------- Digital Cinema Forum Film Handlers' Forum Large Format Forum Ground Level Feature Info, Trailer Attachments & REAL Credit Offsets Category: Community -------------------- Film-Yak Film Handlers' Movie Reviews The Afterlife Bob Maar's Joke-A-Thon Category: Classified -------------------- Equipment and Supplies Wanted/For Sale Help Wanted/Seeking Employment Category: System -------------------- Software Beta Testing Forum

The Film-Tech Forums are designed for various members related to the cinema industry to express their opinions, viewpoints and testimonials on various products, services and events based upon speculation, personal knowledge and factual information through use, therefore all views represented here allow no liability upon the publishers of this web site and the owners of said views assume no liability for any ill will resulting from these postings. The posts made here are for educational as well as entertainment purposes and as such anyone viewing this portion of the website must accept these views as statements of the author of that opinion and agrees to release the authors from any and all liability.