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This topic comprises 2 pages: 1 2
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Topic: AMC Theaters sound system
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Steve Guttag
We forgot the crackers Gromit!!!
Posts: 12814
From: Annapolis, MD
Registered: Dec 1999
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posted 10-22-2018 07:09 AM
Yes, in tri-amp mode, the HF/VHF use a passive crossover. However, as making crossovers go, it is FAR easier to match a coaxial compression driver (similar wattage, zero time difference) than the tremendous power differences between an LF section and either a Mid or HF section.
I'm not saying it can't be done. There are some really impressive crossover designs out there and the fact that there are multiple designs for various frequencies of crossover goes to show that there is a market for different designs. The best are going to be purpose built where all of the mechanical/electrical parameters are known and likely the design was finalized on the actual speaker to ensure correct phase/power response. With passive you have the impedance curve of the driver(s) to consider as well as their physical geometry and the actual crossover points. To do right is a bit of an art form. That is a level of detail that us unlikely to go into a typical cinema crossover that has about $25-$100 worth of parts. It will function as a crossover and protect the HF...it may even have a simple first order boost for CD horns but it isnt going to be too fancy. Think about it...the person that is thinking passive is also thinking "cheap" so you can put the design effort into a high-end passive crossover that would cost something that approaches just bi/tri/quad amping the speaker. Again, the lower the frequency, the trickier it is to make a good passive (and not waste too much energy just driving the components of the crossover itself...as the power goes up, the component size grows with it). Now, Home Hi-Fi...that is another story...people will spend silly money on that sort of stuff. JBL just issued a new L100 speaker called the legacy to look like the 1970s model...starting price $4K/pair!!! They are backordered for months on them! And those aren't even on the high-end of home hifi. JBL's Everest speakers cost more than many homes! https://jblsynthesis.com/productdetail/everest-dd67000.html
But that's not cinema. People shopping passive crossovers are shopping cost too. And, to be honest, the little crossover in the JBL 4722N does okay for the value conscious cinema sound system.
If you download the MHV-1090 user manual (not the spec sheet), they show the various responses of the speaker in tri-amp versus quad-amp mode. Depending on the particular installation, if I needed to say bridge a pair of amp channels to give more to the LF section, I could see tri-amping the speaker using a 4-channel amp. One nice thing about the DPA-Q amps is that there is a bit of "gain sharing" between the channels so the LF section can take more than its 25% of the available power while the HF/VHF will never use their allotment.
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Jay Palmer
Film Handler
Posts: 2
From: Hollywood, California, USA
Registered: Oct 2018
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posted 11-02-2018 06:09 PM
Interesting topic here. Sorry for the long-winded first post!
10 years ago at Fox Studios we retrofitted the JBL 2-way system with BMS Neodymium Co-axial 2” compression drivers. The resultant was a 3-way discreet system that sounded much better than the old ferrite JBL driver. It was common knowledge that the ElectroVoice EVHP9040 horn was a better sounding horn (They all have their characteristic sound) than the 2360A JBL horn normally used. It was just as the Stag Theater at Skywalker that Mark G referred to.
So what we wound up with was a 4675 twin 15” JBL bottom with 2226 woofers driven by a Bryston Class AB amp, an ElectroVoice EVH 9040 horn with a BMS 4592ND Neo Co-ax 2” driver driven by a sweet-sounding Parasound amp. We chose to Tri-amp the whole works rather than use the passive x-over. I cannot remember which crossover/processor we used. Dolby Lake maybe?
Note: Using a BMS passive crossover results in perfect time alignment between HF and VHF. Using an active crossover and bi-amping the coax driver, you must delay one of the components x microseconds. We chose active because it usually sounds cleaner.
The system sparkled and sounded very smooth while easily going to 20 Khz.
We then took lots of HF boost out of the EQ for those screen channels.
Subsequently, other studios followed suit with some adding very low freq components like the Bag End ELF system extending down to10 Hz!
Fast forward to today
We want to retrofit our existing 5000 series JBL system.
The 5674 with the Diamond-Quad-15” bottom
I have on my desk 2 new BMS Drivers
1. BMS 4595HE 1.5” (High End) Co-ax Neodymium
2. BMS 4599HE 2" Neo Dual Diaphragm Driver, 2x 3.5" VC, 400 W AES, 123 dB
It is the loudest audio transducer ever made (and very low distortion)
The midrange frequency range is optimized for human voice
These would be used to replace the HF and MF drivers in our JBL 5674 system
But the JBL 2490H midrange driver is a 3” throat and the BMS 4599 is a 2” so I built a throat adapter…. I’m not sure if this will work better than the JBL mid??
I do know that that the BMS 4595HE HF will be better than the JBL 2451 HF driver.
Way back when the first 3-way JBL system went into the Academy, the HF horns needed to be pushed as close as possible to the screen to get usable high frequency response.
Now we’ll be able to move the HF horn back, which usually results in a less-distorted/more-natural sound. And we’ll still get significantly better and smoother high frequency response.
To note, the QSC 4-way 424, 434, 444 Cinema speakers with the asymmetrical long/short throw horns also use the BMS Co-axial driver for their very top end. And yes it should sound very good!
I've been down to their Costa Mesa factory and they really build some good sounding speakers even their teardrop hang-from-the-ceiling speakers were excellent
Some Scandinavian Theater operators tell me that they also replace JBL woofers with BMS woofers. I met the owner of his BMS component company and his “Coda” finished speaker company. They showed at InfoComm and their line arrays I will test in March.
A very innovative company!
bmspro.com
If you guys are interested I'll post my findings and detail the retrofit?
My next….shorter post will be regarding TAD Cinema speaker systems
Take care all,
Jay
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