This is topic Odd number of surrounds in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.
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Posted by Greg Mueller (Member # 17) on 10-18-2001, 06:40 PM:
What's the best way to hook up three 8 ohm surrounds (per channel) to produce a load the surround amp won't fry over? ------------------
Greg Mueller
Amateur Astronomer, Machinist, Filmnut
http://www.muellersatomics.com/
Posted by Mark Gulbrandsen (Member # 72) on 10-18-2001, 08:08 PM:
Greg,
What type of surround amp do you have? Parallel is the best if your amp can do 2 ohms. If you can't drive 2 ohms then a resistor to make up the missimg 4th would be a good way. It will consume some of the power,probably 25%. It should be an 8 ohm 25 to 50 watt resistor. Another but sort of antiquated way is to use a matching transformer to deal with the impedance mismatch. Again,the transformer will soak up some of the power and the core might saturate trying to achieve realistic digital levels. Both E-V and Crown make large matching transformers. Personally I prefer the resistor as it does not add any inductance to the circuit if you use a high quality Dale 1% metal cased resistor. They can be gotten for about 20.00 new off the shelf. I'm not a big fan of series/parallel although on occasion its the only way you can do it. With the small number of speakers you have, the all parallel will yield better results. Also consider getting an additional speaker for each side. The more,the better. You can never have TOO many.
Mark @ GTS
Posted by Greg Mueller (Member # 17) on 10-18-2001, 08:21 PM:
Mark
It's a vintage SAE A202. 100w per Ch------------------
Greg Mueller
Amateur Astronomer, Machinist, Filmnut
http://www.muellersatomics.com/
Posted by Mark Gulbrandsen (Member # 72) on 10-19-2001, 08:17 AM:
Greg,
For that particuluar amp, series parallel with the extra resistor making up the difference for the forth speaker would probably be the safest way to go. Those are good sounding amps but they don't like loads below 4 ohms either.
Mark @ GTS
Posted by Greg Mueller (Member # 17) on 10-19-2001, 10:02 AM:
Sounds like it may be time to consider upgrading the amplifier section.
The problem I'm trying to get away from is having the surrounds (I only have one on each side now) be directional (except when they're supposed to be). But, in a small screening room you run out of places to put speakers and if I were to put them to close (like right over head) it would (I think) again be directional. So I figured given my seating arrangement and wall space, 3 per side would work out.
Comments?------------------
Greg Mueller
Amateur Astronomer, Machinist, Filmnut
http://www.muellersatomics.com/
Posted by Darryl Spicer (Member # 711) on 10-19-2001, 10:16 AM:
here is a thought.....If we are talking just mono surround effects not stereo surround. You could place two on the back wall equally spaced from the corners inward. place one in each corner and place one on each side....In a sense this should spread the surround out and creat less holes in the surround causing the phantom effect to shift and sound directional....This was my best setup I could get in my home theater sound system. For stereo surrounds the back wall speakers are less effective in small settins than they are in large settings....I took and fed my stereo surrounds into a stereo reciever running in three channel mode to eliminate the phantom effect and give it it's own center surround channel.
Posted by Greg Mueller (Member # 17) on 10-19-2001, 10:32 AM:
This would be stereo surrounds. I have this experiment I wanted to try, after I get this situation handled. I have this NEC surround genereator that I used to use before I got a CP unit. What I want to do is run the stereo surround signals in where the normal RL signals go in and have it generate a center channel. In this case it would (hopefully) generate a center surround channel, instaed of the center front channel it was designed to produce. I have some speakers mounted in the center of the back wall which have their own wiring running to them, for the job. I asked Brad about this once and he thought it would work. It's all(ways) a great experiment.
------------------
Greg Mueller
Amateur Astronomer, Machinist, Filmnut
http://www.muellersatomics.com/
Posted by Darryl Spicer (Member # 711) on 10-19-2001, 05:28 PM:
if the nec surround generator was desighned to produce a center channel for the front channels this will work because all it is doing is feeding the mono signals that would appear in both left and right to the center......if it was only desighned to produce a surround channel off the left and right channels of a two channel stereo signal it will not work....it must be able to decode four channels....what I have done is take the left and right surround output cables and sent them to the prologic receiver that powers just the left center and right rear channels.
Posted by Mark Gulbrandsen (Member # 72) on 10-19-2001, 05:56 PM:
Greg,
There are very few surround speakers available that are smallish. The JBL 3310 is small, 17 by 19 by 10, but you might want to look at something like the JBL Control series of speakers. The Control 1 is only 10 by 7 by 6 inches. These small speakers are in use in smaller screening rooms and work very well when there are enough of them. The main thing is to create as even a sound field as possible. This will eliminate the localization effect that you have now, and probably will still have to some extent from only three speakers per side. Even in a smaller room you really have to have rear speakers as well for this soundfield to work properly. The small size of the Control series will allow you to get in enough speakers, 6 to 8 per side, without them becomming an eyesore, or getting in the way. You would still need lots of power and a new amp might very well be in order for this too.
Mark @ GTS
Posted by Greg Mueller (Member # 17) on 10-19-2001, 06:18 PM:
The NEC unit is really kind of trick and sometimes I like it better than the CP unit (but only sometimes). The way it is supposed to work is you put in the LR stereo signals in and generates Center (front) and stereo surrounds. Even if you feed in mono it sounds pretty neat and will give you surrounds. The part I really like was when I ran nonsync in. You got output from your full system. It has it's own pink noise generator, and a remote control so you could go sit in the audience and balance things the way you like. But that was yesterday. So now I'd like to hook it to the stereo surrounds and generate the center surround. I wonder if I have it in mono surrounds like in SR or A, if it will still generate a center surround------------------
Greg Mueller
Amateur Astronomer, Machinist, Filmnut http://www.muellersatomics.com/
Posted by Brad Miller (Member # 2) on 10-19-2001, 07:07 PM:
If you are trying to use the NEC unit as an EX processor, when you play analog material only the back wall "EX channel" speakers will play. The left and right surrounds will be dead.
Posted by Stefan Scholz (Member # 147) on 10-20-2001, 07:44 AM:
Seriously, I have never liked to serial wire speakers,or use series resistors with the circuit. Reason, it interferes with the power amp's damping factor. The damping is neccessairy to prevent woofers from nonlinear traveling, degrading the sound quality. It is sometime not economical to use extra power amps, but with 10 Ohm surrounds (also around), 5 can be paralleled to a single power amp, and normally this gives ample drive to each.
If you're using 4 Ohm amps, a matching Xformer is the better vway. To avoid those saturation and inductivity effects a high quality, large type is recommended to use, but can be as expensive as another power amp.
What to recommend to Greg, ... I do not know.Stefan
Posted by Mark Gulbrandsen (Member # 72) on 10-20-2001, 12:07 PM:
Stefan,
I really disagree with you about using matching transformers for surround speakers. There are several reasons why not to use them. First,
1, You say you want to maintain the usable damping factor of an amplifier....Using the transformer would lessen any benefits from a high damping factor just by inserting it in the circuit as you are now going through the transformer windings. Even with a very high quality transformer you could never maintain the original high damping factor. I might consider using an autoformer here but core saturation is still an issue at high levels and damping factor, which is not as critical an issue as some think is still lessened.
2, The transformer would have to be really large to be able to handle all the power required for digital levels, and not run into core saturation, especially at low frequencys. Adding a resistor is much preferable and won't lower the damping factor by any more than if you inserted another speaker. Another benefit of the resistor is that you can get them non-inductive. So the resistor will have less effect on the sound overall than a transformer would ever. The transformer would cost at least 20 times the price of the highest quality US made non-inductive resistor. What Greg should really do is to buy more of a smaller type of surround speaker and an amp that can handle two ohm loads. This will give three benefits...
1. Improved coverage and less localization effect.
2. The surrounds can then be wired for either 4 ohms or 2 ohms, all in parallel which is the preferred way to wire them. There are way less phase problems that will occur at different frequencys than if the sound for the next speaker in line had already passed through another speaker. Also, good used power amps are a dime a dozen these days. Just check E-Bay.
3. More surrounds equals more level which equals less overall distortion level.
Mark @ GTS
Posted by Ray Derrick (Member # 1001) on 10-21-2001, 04:16 AM:
I agree with Mark.Stay clear of transformers, they will just degrade performance. You should also be cautious when running speaker systems below 4 ohms unless you are familiar with the characteristics of the speakers and amplifier you are using. The rated impedance of a speaker is a "nominal" impedance - the actual impedance may vary considerably over the frequency range. Some speakers are worse in this respect than others and it is not unusual for a speaker rated at 4 ohms to have impedance dips well below 2 ohms at some frequencies. Unless the amplifer can handle this you can end up with amplifier overload in certain parts of the frequency spectrum.
That's not to say don't use 2 ohms, just be judicious in your choice of equipment.
Another tip, never use less than two pair of surround speakers, even in your home system. And if you can only manage two pair, make sure that one pair is mounted just in front of, and the other pair just behind, the main listening area. This will give the best dispersion of the surround sound field.
------------------
Ray Derrick
President
Panalogic Corporation Pty Limited
44 Carrington Road
Castle Hill NSW 2154
Australia
Phone: 61 (0)2 9894 6655
Fax: 61 (0)2 9894 6935
Posted by Steve Guttag (Member # 268) on 10-21-2001, 01:03 PM:
Ray (and others),Many of the amplifiers out there can now safely handle 2-Ohm nominal impedance loads (per/channel).
QSC has a marvelous amplifier the DCA-1222 that can run with 1.6-Ohm loads all day...in fact that was one of it's design criteria.
QSC also has a 4-channel amplifier that will handle down to 2-Ohms a channel. This not only has the obvious EX potential but allows one to break up the surround array so that one can run all speakers in parallel up to a 16-surround array.
As to the imedance curves...this really isn't too much of an issue in movie theatres and particularly on the surrounds. Most of the low-impedance part of the curve of most speakers is in the very low frequency. Most of the surround speakers in use today have an LF cutoff below 80Hz or 50Hz (depending on size and quality). Most cinema processors (and some amps, like QSC) offer the ability to high-pass the surround channel to either 50Hz and above. As such, one never will have the low part of the impedance curve come into play.
As a side note for the DTS users out there. DTS places their subwoofer track onto the Ls and Rs surround channels. It is REAL important to high-pass your surrounds at a frequency at or above where your surrounds can safely play. If you have ever used the emperical test disc and played the sweep section, you will hear when the subwoofer section comes...a distinct "flapping" sound at first when the sweep first starts....that isn't your subwoofer but the poor hapless surround cones flapping about.
Another reason to high-pass is it simply wastes amplifier power and potentially will damage the speaker to have a surround speaker play below it's cutoff frequency.
Steve
------------------
"Old projectionists never die, they just changeover!"
Posted by Ray Derrick (Member # 1001) on 10-21-2001, 09:27 PM:
I agree QSC make some quite superb amps. We tested some of the MX series in our lab a few years back and the performance was just excellent, despite the fact that the power transformers are not toroidal and look like something out of an army surplus store.Regarding speaker impedance, I was not making this out to be a big issue, just something to be aware of.
We have done extensive testing of a range of speaker sytems for a client including the EV TS940D cinema system. In that particular case we found that the impedance (nominal 8 ohm) varied from a minimum of 3 ohms at 160Hz to a maximum of 24 ohms at 3.15kHz The impedance was less than 4 ohms between 100Hz and 300Hz. Their surround speakers showed a simlar trend, so the impedance dips are not necesarily at quite as low a frequency as you may surmise Steve.
But as you say, with well designed amps this is not a problem.
------------------
Ray Derrick
President
Panalogic Corporation Pty Limited
44 Carrington Road
Castle Hill NSW 2154
Australia
Phone: 61 (0)2 9894 6655
Fax: 61 (0)2 9894 6935
Posted by Steve Guttag (Member # 268) on 10-21-2001, 10:01 PM:
Ray,When you did your tests on the EV...did you use their crossover or in bi-amped fashion. The bass section of the TS940D is a 4-Ohm cabinet (dual 8-Ohm drivers). I am a bit surprised by the low impedance in the 160Hz region. The driver used in the TS940D is the DL-15W...I shall pull EV's specs on it. Thanks for the enlightenment.
Steve
------------------
"Old projectionists never die, they just changeover!"
Posted by Steve Guttag (Member # 268) on 10-21-2001, 11:24 PM:
Ray:I've done a bit of downloading from the EV web site...
The impedance curves on the DL18W and the TL606 (the dual 15" bass section of the TS940D) concure with your findings on the low point of the impedance curve within the pass band (above 40Hz) but, according to EV the MINIMUM box impedance of the TL606 is 4.0-Ohms...and one would find that in the range from 100-200Hz. Now the fact you find it to be an Ohm less isn't surprising to me...driver construction is a bit wild and from unit to unit people would be surprised at how differently they respond. Professional drivers are quite a bit hand made and not as exacting as one would hope in this day and age....but I digress
One thing I did note about EV's impedance ratings...they tend to make their "nominal" their minimum. Their current surround speaker the SL12-2V follows this trend but seems to err on the side of higher impedance. The Older TS8-2 does have a low minium impedance of 7.2 with a nominal 8-Ohms.
I have generally found that EV publishes accurate spec sheets. In fact they tend to tell it like it is more than other manufacturers. I'll stand by my statement earlier...I wouldn't be too worried about the minimum impedance being significantly below the nominal impedance in Cinema applications. I have even run into applications where the impedance was significantly higher. So much so, I have had 6 "8-Ohm" speakers all paralleled and still didn't drop below 2-Ohms in the pass band.
I do appreciate your point though and agree one should check their speakers with an impedance bridge if they haven't worked with that type before.
Steve
------------------
"Old projectionists never die, they just changeover!"
Posted by Ray Derrick (Member # 1001) on 10-22-2001, 05:15 AM:
The figures that I quoted were for the complete system including the crossover. We also tested the bass cabinet and the HF driver on their own and the results for the bass cabinet agree with your figures i.e a minimum of 4 ohms.However the overall system is rated by the manufacturer at 8 ohms.
------------------
Ray Derrick
President
Panalogic Corporation Pty Limited
44 Carrington Road
Castle Hill NSW 2154
Australia
Phone: 61 (0)2 9894 6655
Fax: 61 (0)2 9894 6935
Posted by Greg Mueller (Member # 17) on 10-22-2001, 08:57 AM:
"...one should check their speakers with an impedance bridge if they haven't worked with that type before."What's an impedance bridge and how's it done?
By the way after reading your post, Steve, I got out my QSC manual and checked the specks on the DCA 1222 and I think I'll be mustering out the good old SAE and replacing it with the QSC. Thanks for the tips
------------------
Greg Mueller
Amateur Astronomer, Machinist, Filmnut
http://www.muellersatomics.com/
Posted by John Pytlak (Member # 331) on 10-22-2001, 01:27 PM:
Greg asked: "What's an impedance bridge and how's it done?"Some links:
http://www.mitedu.freeserve.co.uk/Theory/inzoz.htm
http://www.purebits.com/appnote16.html
http://www.purebits.com/scplugins.html#anc#6
------------------
John P. Pytlak, Senior Technical Specialist
Worldwide Technical Services, Entertainment Imaging
Research Labs, Building 69, Room 7525A
Rochester, New York, 14650-1922 USA
Tel: 716-477-5325 Cell: 716-781-4036 Fax: 716-722-7243
E-Mail: john.pytlak@kodak.com
Web site: http://www.kodak.com/go/motion
Posted by Mark Gulbrandsen (Member # 72) on 10-22-2001, 05:28 PM:
I absolutely agree with Steve about rolling off the low end on surround channels. It is an absolute wast of power and not to mention the increase in distortion you'd end up with generated by the speaker attempting to do something it was not designed for. I ahve installed alot of surround systems that run calculate to be around 2 ohms lowest impedance with absolutley no problems at all. Done this with QSC, Crown, and BGW amps, and yet to see a single failure of eiother speaker or amp.
Mark @ GTS
Posted by Steve Guttag (Member # 268) on 10-22-2001, 09:35 PM:
Jeez John...where DO you come up with all the web link stuff?!?!? It is truely amazing.Greg an impedance brigde is kinda like an Ohm meter but measures the impedance at various frequencies. (an Ohm meter only measure the DC component and will often be lower than the impedance but not always).
Steve
------------------
"Old projectionists never die, they just changeover!"
Posted by Ray Derrick (Member # 1001) on 10-28-2001, 01:23 AM:
Mark, this is not something that will normally cause amps or speakers to fail. The effects are usually more subtle than that. I remember one instance where we encountered a surround system that would make a "cracking" sound every time there was a loud passage with lots of low mids. It was traced to the amp's protection circuits operating because the impedance of the surround system was about 1 ohm at around 200Hz and the protection ciruits thought there was a short whenerever there was a lot of energy at that frequency. We re-wired the system in series parallel and the problem went away.You are obviously very experienced and skilled in your craft and would avoid using such amplifiers under low impedance conditions but many techs don't have that experience. So in my opinion unless you really know the gear, it is much safer to stick with series parallel than work a system at 2 ohms.
------------------
Ray Derrick
President
Panalogic Corporation Pty Limited
44 Carrington Road
Castle Hill NSW 2154
Australia
Phone: 61 (0)2 9894 6655
Fax: 61 (0)2 9894 6935
Posted by Mark Gulbrandsen (Member # 72) on 10-28-2001, 09:25 AM:
Ray,
I absolutely agree with you but only to a point. Having all speakers in series eliminates alot of phase shift from the signal not going through one speaker to get to the next and it helps lower phase shift and delay quite a bit. If I get into a case where impedance dips too low I split up the surrounds onto more amplifiers to be safe and not sorry. I have used both BGW and QSC amps in the past to do all parallel runs with no bad side effects. In fact BGW does not use any current limiting at all in their amps. I don't think QSC does in most of theirs either. I will agree though that some amps such as Crown which do use simpler forms of current limiting protection can cause the problem that you mentioned. In an amp that has well designed current limiting all that will happen is a limiting of max power output at the given settings for the current limiting circuit and in theory thats all. Amps like the old Phase Linears were famous for snapping when the limiters turned on. They used germainum transistors in the current limit circuit too and they also took out alot of tweeters, but that was eons ago...
I certainly have installed my share of series/parallel systems though. Sometimes its the only way it will work out, especially when dictated by budget constraints..
Mark @ GTS
Posted by Richard Fowler (Member # 893) on 10-28-2001, 12:41 PM:
Phase Linear...what a time trip...the company I was working for many years ago used them for background music and live sound systems for clubs and special venue.....we always had a ton of them on our service benches...my boss's response, they are easy to fix.
Our installs are pretty standard using a speaker junction box to sum the lines prior to entering the rack; using as neccessary series and parallel and 50 watt resistors if required......checking ohms / impedence with proper tools as posted above. Four channel amplifiers have been a God send ( but service reliability has gone down ) as we run out of rack space, epecially for addtional surround zones for stadium rooms and EX format. Bass in the surrounds are not a big a problem on DTS since it is band limited to supply the subwoofer circuit....more critical for Dolby Digital which we design with power loads and decent speakers to keep us out of trouble.
Richard Fowler
TVP-Theatre & Video Products Inc. www.tvpmiami.com
Posted by Mark Gulbrandsen (Member # 72) on 10-28-2001, 04:04 PM:
I prefer to keep multi channel amps out of a system. If necessary I mount the amps in a seperate wall or floor rack. Amps are cheap and so are racks for that matter. You're right when you say the servicability goes down on the multichannel stuff. They can be a pain in the butt to service.
I worked for a Phase Linear repair center back in the mid 70's and you're right. They are very easy to fix. Mostly it was the 400's that blew up, not as many 700's bit the dust very often. Crown DC300's and D-150's ran a close second in numbers that came in for repair. Gee the DC-300 was only out a few years when I was repaining this stuff. Man that was long ago......and these amps are dinosaurs now! We even repaired Crown tape decks...those 700 and 800 series decks were really good for their time, much better than their amps thats for sure.
The junction box idea is a good one...similar to the one that Kelmar makes. I always do home runs from each speaker to a large junction/pull box in the booth. I can go either parallel or series parallel if I want to and depending on the speakers used and number of amps available. this also allows easy changes or additions in the future.
Mark @ GTS