Film-Tech Forum ARCHIVE: Bi-Amp Stage Speakers

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Posted by Ian Bailey (Member # 1752) on 07-21-2006, 08:41 AM:

Hi
What is the advantages achieved by Bi-Amping?
Is it done by having say the Centre Channel output from the Processor going to two separate power amplifiers.One amp drives the base speakers and one amp drives the horn?
I suppose you would do the same for the Left and right speakers?
Cheers Ian

Posted by Darryl Spicer (Member # 711) on 07-21-2006, 11:02 AM:

Bi-Amping helps take the strain off of the amps. You must have either an amp that can do the crossover work or have the crossovers installed inline before the signal goes to the amps. That was one of the big requirements with THX because they had the crossover design integrated in their system before going to the amps. I always felt the sound sounded cleaner by bi-amping instead of having the crossover work done at the speaker.

Posted by Gordon McLeod (Member # 33) on 07-21-2006, 12:54 PM:

One has better head room and usually lower distortion in a biamped system as well one can time align the drivers of the speakers

Posted by Steve Guttag (Member # 268) on 07-21-2006, 05:14 PM:

The reasons for bi-amping (or tri-amping or quad-amping...etc) are many fold.

First is the crossover itself. A passive crossover can be an excellent piece if it is properly designed and is designed to work with the drivers/cabinets. It is certainly possile to design a passive crossover for a speaker system that will do as well and even outperform an electronic form of the crossover (remember, any active component will introduce some form of distortion and noise). However, today, more often than not, the passive crossovers just do the basics of the job to keep the drivers from blowing up, and that is about it. They have the fundimentals of the L, C and R circuit to hit the crossover points. This type of passive crossover is almost always a bad choice...the speaker will play but it could play better if a proper crossover (active or passive) were used. One thing that seems to be lost on modern passive crossovers are the multi-tap chokes that allow for properly balancing the two systems that are being crossover because you don't want to do this with just EQ or you will introduce time-domain issues. Part of the passive crossover design should take into account the speaker design as well to have everything properly aligned at the crossover point too.

In short, don't expect the passive crossovers in the typical cinema speaker of today to be anything more than to allow the speaker to work and maybe they added some resistors and such to get the levels a closer match on some models of speakers.

A passive crossover also consumes power. In the vernacular of the biz, the passive crossover introduces an "insertion loss" wich is the power that is consumed in the crossover...there is no "free" with power...everything takes a piece of it. Power that goes into the crossover, is power that does not come out of the speaker so that comes off of power you feed to the amp.

Speaking of the amp...if you under power your amp and "clip" the amp sometimes on those really loud scenes...guess what...the amplifier is putting out a distorted signal...that if you looked closely at with an O'scope is full of HF harmonics...guess where that goes? Yup, into the crossover that dutifully sends it up to the "HF" driver which typically damages or destroys it.

So what does bi-amping get you? First off, a heck of a lot of control. You may precisely taylor the crossover points and even make them asymetrical if the speaker will perform better. Add time-delays to offset physical delays in the speaker design (if the various drivers do not have their cones/diaphrams in physical alignment with each other, the sound at crossover will not add in a maximum manner).

There is no insertion loss with an active crossover since it is before amplification. Since each amplifier output may be controlled independantly (or with the crossover itself) one can precisely balance the HF/LF levels for a perfect match at crossover without using any EQ. Thus, typically a bi-amped system will have less EQ applied to the same speaker that would otherwise be passive crossovered and will almost always sound cleaner and clearer.

If you clip the amplifier that feeds the bass speakers, this distortion will not be sent to the HF diaphram though it may still harm the LF cone, it is more durable.

Some advise (and I have mixed views on it) of using a non-polar capacitor on the HF section...the reason is to ensure that no bass can feed it. When a diaphragm is connected straight to the amplifier, you run the risk of hum and other items going to the HF speaker too...a non-polar capacitor could protect the diaphragm from trying to reproduce this audio that is not desired and is potentially harmful to the diaphragm. One must remember that the capacitor will introduce a time-shift though and it must be sized to provide the protection and also handle the current going through it.

With active crossovers you now have both analog and digital varieties. With the digital ones, one can often completely tune the system with a few parametric filters and completely negate the need of 1/3-octave EQ.

Many of the new modern speakers seemed to be desiged such that they depend on these DSP crossover will offset frequency response issues with the designs. I have found that the best speakers need the least amount of tuning of any sort. In other words...the best fix is the one you don't need.

So to sum it up,

Bi-amping will normally but not always result in a cleaner, more high-powered system that better and more precisely matches the speaker's response to the processor and the room environment.

Steve

Posted by Ian Bailey (Member # 1752) on 07-22-2006, 05:44 AM:

Gordon-I thought you Time align the speakers simply by positioning the speakers correctly in the vertical plane.But now that I think of it I suppose you can time align by introducing a certain amount of delay to the relative speakers.
Steve-thanks for all the information,that's excellent.
Just a few questions:
What brand of Active crossovers are normally used.
Where are the crossovers positioned in the sound path-before or after the power amp.I suppose if they were before the power amp then the amp wouldn't have to drive as hard as it is only amplifying the frequency range required instead of the whole spectrum.I couldn't see that it would be too much of a problem if the cross-overs where after the amp.
Cheers Ian

Posted by Gordon McLeod (Member # 33) on 07-22-2006, 08:36 AM:

If you have the QSC 1400 serries amps I like the USL plug in octal modules
otherwise I often use the Smart 3 channel crossover or the Rane twin units

Posted by Steve Guttag (Member # 268) on 07-22-2006, 10:41 AM:

It makes absolutely zero sense to have an "active" crossover after an amplifier...so they come before the amplifier.

As to what brands, that is like asking what is the best projection system...everyone has an opinion on them.

There are those that have modules for their amplifiers (QSC, Crown and Ashley immediately come to mind). QSC has cinema specific crossovers for their amps (XC-1, XC-3). Crown has some speaker specific for their CE and CL lines (remember, Crown and JBL are now both part of Harmon). As Gordon mentioned, USL has octal (tube type sockets) that plug directly into a QSC series 1 amp (1200, 1400, 1700). USL also makes their frame for 3 or 5 octals (the XTA series...which despite its cheapness has a way cool bypass crossover feature that works really well). The USL XTA stuff are also speaker specific for traditional speakers of the '90s (JBL 4675 and JBL 4670) though it is possible to modify them a bit and USL has always been helpful.

Then there are the crossover monitors...the popular ones now are the QSC DCM line. However, USL has their CM series that offer analog or digital options. THX has been doing the crossover/monitor thing for some time though they are going to break away from that in the future and merely certify suitable crossovers instead.

Then you get into the generic stuff. Gord mentioned Rane and SMART, though the SMART is more cinema specific. There are many choices out there; my personal favorites are RANE...the AC22B for analog and the RPM26Z for digital (somewhere on this site you can search for examples of these in pictures I posted...I recall it drifting from crossover examples into rack tidiness discussions). But there are a lot of manufacturers out there that are both cinema in origin and sound reinforcement in origin. When I use a Rane RPM26Z, I generally do not need any 1/3 octave EQ from the cinema processor....and when I set up a multiplex...presumming the rooms are similar, one setting of that unit will have all rooms with identical equipment in similar rooms using the same settings and achieving a response within SMPTE specs. I'm a bit of a minimalist on the EQ department...just enough to do the job and nothing is more pleasing than needing none and having it sound good and have the full and proper response.

As for time alignment...some speakers you can not physically time-align (at least not safely. Take the JBL 4675...the Horn is so deep as compared to the bass cabinet...the HF driver is going to be notably further away. In order to get good transmission through the screen, you want everything as close as possible to the screen, especially the higher frequencies. Thus it is better to get everything up as close as possible as the sound exits the horns and other various components and time align in the crossover. Then again, if you have an Altec A1, A2, A4, A5 or A7, you can have your cake and eat it too...they drivers are aligned inherently and have the whole speaker close to the screen. Those speakers really much better than people give them credit for...the modern 3-ways try to mimick them in a notably poorer fashion until you get into the over $3K price range.

Posted by Monte L Fullmer (Member # 2797) on 07-22-2006, 01:59 PM:

Steve,

what kind of crossover are used that is mounted on those old Altec/Lansings - "Voice of the Theatre" speaker cabinets?

Could one take those old cabinets and bi-amp them by disconnecting that crossover completely?

-thx Monte

Posted by Steve Guttag (Member # 268) on 07-22-2006, 03:06 PM:

The Larger systems used crossovers from the N-500 series. Cinema speakers were always specified with 500Hz crossovers to improve the dialog by keeping it out of the crossover region. Sound reinforcement typcially would cross the same speaker over at 800Hz to improve the power handling by not having the HF diaphragm play down to 500Hz.

To answer your question, yes, you can and should bi-amp the Altec speakers too. I've used RANE, QSC and Ultra crossovers with success. In fact, Ultra Stereo can provide the information needed to bypass the delay section of their XTA modules.

Posted by Gordon McLeod (Member # 33) on 07-22-2006, 04:56 PM:

Almost all the old altecs I install or service are now biamped as a matter of course

Posted by Louis Bornwasser (Member # 3063) on 07-22-2006, 07:29 PM:

If you use Dolby (CP-45 and CP-650) crossovers are built in. If not then we use the XR-1 Ashly crossover with the FTX-2001 Ashly amp; still made, but only for us.

I, too, am a minimalist with almost no use of third octave controls. (Some say that to turn more than 6 is an admission of incompetence.) I use the crossover controls and bass and treble and can get spec 85% of the time. Louis

Posted by Ian Bailey (Member # 1752) on 07-22-2006, 08:00 PM:

Thanks for all the help guys.
That's interesting that the CP650 has inbuilt crossovers.
Are the crossovers of suitable quality to the other brand names that have been mentioned?
Ian

Posted by Steve Guttag (Member # 268) on 07-22-2006, 08:35 PM:

The CP650 can have built in crossovers. It is the CP650XO and requires a particular option board running with later firmware.

The CP500 also had a bi-amped built in option...there are others too, including the Panastereo CSP1200.

Personally, I don't favor built in crossovers in the cinema processor. They rarely have the features I like and always result in more work to install (you now have more cables going between the CP and the monitor). Having the crossover at the amplifier results in little or no extra wiring...having it at the monitor can result in the least amount of wiring with the newer monitors with the HD15 connectors/cables though I don't advocate that sort of thing but it does make for faster wiring and does not require any real skill.

Posted by Monte L Fullmer (Member # 2797) on 07-23-2006, 02:15 AM:

quote: Steve Guttag
by not having the HF diaphragm play down to 500Hz.

..and have been there and done that...with wrong installs from circuit techs.

And they ordered DTS-6AD units with fullband settings without the 500hz chips installed, or QSC's amps switched to allow fullband through both drivers..

....busting diaphrams left and right in the HF drivers...must had to replace a good 8 drivers do to that...

Lotsa replacing drivers, installing those chips and getting the QSC's to go to biamp conditions.

-Monte
-Monte

Posted by Mark Gulbrandsen (Member # 72) on 07-23-2006, 09:32 AM:

quote: Steve Guttag
The CP500 also had a bi-amped built in option...there are others too, including the Panastereo CSP1200.

I did a number of CP-500's with the internal x-over and found that they worked extremely well with 4675's in the situations I put them into. Never used them with any other speaker though. With any of the internal x-overs there is not that much extra wiring to do and to be be honest it is hardly what I'd consider to be anything difficult, nor does it make for any big disarray of cabling that can't be kept tidy. The wiring on an external stand alone x-over is going to be quite similar if you're monitoring the whole shebang anyway. Having the x-over internal to either the processor of booth monitor certainly eliminates one point of twiddling in a rack that could be tampered with by booth mongers. Keep in mind that there are no professional booth engineer types employed anyplace I do service.

I wouldn't call the majority of whats being used the HD variety what you mainly will find are the DB series of connectors on todays equipment... DB Connectors are not what I would consider to be High Density connectors. I too for a very long time would not accept them until just very recently... but now having done a a number of installs this way I have no problem at all with them. What it really boils down to is how good of cable one purchases for these installs. There are the lousy 2 buck cables and then there are the more expensive shielded/twisted paired variety which I am sticking with, and you CAN make up your own where needed. I've done actual crosstalk measurements on racks and the quick cabling system installed with high quality cables specs out AS GOOD as a hard wired rack does... so what is there to gain with with hard wiring any longer...? For equipment that does not allow direct use of DB type cabling there are a multitude of backplane adaptors available from at least a couple of different sources to allow its use. Quick cabling also allows practically instant equipment substutution and upgrades. It saves your customer alot of labor, and frees up $$ that can be spent on better equipment overall.

Mark

Posted by Gordon McLeod (Member # 33) on 07-23-2006, 10:30 AM:

Mark odd you liked the internal crossover in the cp500 as I don't think it had CD horn correction in it (I may be mistaken)
I don't like the use of premade cables as I find racks tend to be messy with them

Posted by Mark Gulbrandsen (Member # 72) on 07-23-2006, 11:10 AM:

Yea, had great luck with the 500's internal 683 card in the installs I used it in. Although its more primitave than todays x-overs it does offer all the necessary adjustments one needs to correctly align things. It definately has CD compensation and LF delay. Both delay and x-over freq make use of several factory presets or are also user configurable. For what it is and when it was designed its actually quite a versitle card. Of course todays stuff if far more flexible. The toher nice feature if the 683 is that the card also sounds good. I look foreword to trying the 650's interanl card since it offers for the most part the full gamut of adjustments needed.

DB cables come in many differnt lengths and can be easily made up to a specific length if needed. Neatness is easily possible with some pre-planning and use of the typical cable trays. Also the flexibility and durability that these cables afford is also very adaptable to slide out equipment.

Mark

Posted by Steve Guttag (Member # 268) on 07-23-2006, 11:19 AM:

If you are referring to QSC's method of connecting, then they are indeed HD connectors...they put 15 pins where a typical 9 pin DB connector would fit. There are standard DB15 connectors but those are not typcially used in Cinema. I found the CP500 internal crossover to be wanting and it did not incorporate any screen loss or CD horn compensation.

As for HD15 cables...have you done any capacitance tests...you have high-level and low-level cables in the same jacket...very bad idea.

As for using discrete crossovers...yes they do introduce extra cabling but I feel I get substantially better control and performance with them to justify the extra cabling.

With crossovers at the amps, then the cabling is very much mimimalized. The XC series of crossovers for QSC are flexible enough for many situations. We always make our own cables, regardless of connector on the ends. For this sort of rack, we would use a 6-snake cable like Belden's 1409R to come off of the CP650 and feed the amps. The cables can be premade out of the rack and installed quickly with user removable connectors (XLR or DB25) so that goes quick too.

Posted by Mark Gulbrandsen (Member # 72) on 07-23-2006, 11:56 AM:

Steve,

Actually way back when I worked in television we were concerned alot with excessive capacitance in the cabling at the station.... especially when we constructed the stations new location. But in analog television we were dealing with some pretty high frequencies over some hundreds of foot long runs and at those higher frequenbcies any rolloff did matter and capacitance also affected chroma system phase. Fortunately the line amps (op-amps generally operating in class a ) in todays cinema audio equipment are on the norm extremely well designed and capable of driving extremely low impedances with abnormally large amounts of capacitance. They generally don't suffer any audio range roll off of any sort until get into large amounts of capacitance in long cabling as might happen with back stage located power amps. Indeed todays buffers and line amps are pretty much blind to any normal amounts of it. Unless your using 6 to 8 racks for sound systems and piling in the cable the amount of capacitance in any of the cabling that any of us are using in typical sound racks is so low as to not even matter...the amount we deal with might cause some roll off in freqs above 100 to 250 khz, and thats not necessarily a bad thing. Even Belden 8451, a popular cable, is pretty low... in the mid 30's per foot if I remember correcty and some of the better cables in production today are even less than that. Back about 1977 it was partly a capacitance issue that caused many major broadcasters to switch to Gepco Wire away from Belden when they began upgrading or building new studio facilities. NBC, ABC, and PBS were three of the biggies that switched. There are some Belden cables that are VERY old designs and were fine in the old B&W tv and mono sound days but that today are very unstutable for much of anything. All in all cable capacitance in the normal single rack sound system is irrelevant.

BTW, I just checked and 1409R measures at 39 pf per foot... thats a tad bit more than 8451 at 34 pf per foot. If you are really worried about capacitance as you claim you are then you might want to take a look at Gepco's equivelent of 8451 since it is a mere 28pf per foot... Their snakes are also 28pf between conductors, way better than 1409R checks. I've been using their wire for the most part for hard wired racks since I've been in this buisness. For general wire types Gepco's products are almost always far superior to Beldens products.

And this from the spec sheet of the LF155 an older obsolete op amp design......
"New output stage allows use of large capacitive loads (5,000 pF) without stability problems"

This National Semiconductor Buffer Amp spec sheet is also iteresting, In particuluar page 12 paragraph 1. Figure 24 on page 13 is quite revealing as to just how much capacitance today's op-amp designs are capable of tolerating.

Mark

Posted by Gordon McLeod (Member # 33) on 07-24-2006, 05:24 PM:

regarding the cp500 crossover card Mark G said "It definately has CD compensation"
I checked with Lonny and he says it doesn't have CD correction

Posted by Mark Gulbrandsen (Member # 72) on 07-24-2006, 06:43 PM:

You're right! I only installed three of them... musta been thinking about some other Dolby board. I thought there was a suitcase link on the board for screen loss in/out. As I said its a pretty basic but good sounding board. It does have LF delay though. I still have the raw and eq'd files in my R-2 folders... will have to dig em out.

Hi Lonny!

Mark

Posted by Steve Guttag (Member # 268) on 07-24-2006, 06:45 PM:

Mark, I have nothing against Gepco...it is not WAY better than Belden but typically on par with it. Belden is merely my standard cable but not my only source. FWIW...I don't use 8451...for single cable runs, I use 9451. I can't imagine why anyone would use 8451 when they know of 9451...which wires substantially faster (shield is bonded to the jacket so stripping is very clean), is slightly smaller in OD, and is available in all of the colors.

As to the capacitance...I was not referring to the conductor-conductor but with the HD systems of the DCA amps you now have high and low levels of the same signal within the same jacket...that is just plain bad design. Additionally, the HD15 terminals on the DCA amps do NOT have the same gain structure as the XLR or phoenix connector inputs...it is quite easy to clip out a DCM monitor but not clip out the amplifier, despite having the amplifier set to WFO.

Sorry, no sale. The CP500 did NOT have CD horn compensation unless you called raising the treble compensating for the CD horn.

The best CD horn compensation or screen loss is going to come from a proper filter that is matched to the horn...DSP crossovers will typically do this with ease.

However, the QSC XC-1 for the MXa amps did a VERY good job on the JBL 4675...the USL XTA-1 also do well with the JBL 4675...it is what happens when you get a product designed specifically to work with another product.

BTW...it is the Cat 517 for the CP50 and the CP200 that has the screen loss compensation switches on the output card.

Steve

Posted by Mark Gulbrandsen (Member # 72) on 07-24-2006, 06:52 PM:

quote: Steve Guttag
Additionally, the HD15 terminals on the DCA amps do NOT have the same gain structure as the XLR or phoenix connector inputs...it is quite easy to clip out a DCM monitor but not clip out the amplifier, despite having the amplifier set to WFO.

Agreed on the gain structure of the two. Capacitance is going to be so low as to not even matter. It would be inter-cable coupling that I would worry about and with twisted pair cabling in the better quality cables there just has not been any problems since there is a cancelation effect between the pairs. I admit that with cheapo cables it could be a problem. Have not had the clip problem you talk about... If that is the case then QSC needs to take a look at the problem. Perhaps pad down the amp inputs a little more or install a limiter before the monitors power amp.

While you might think these are HD connectors and they may be classified as such by the manufacturers I wouldn't consider anything less than 50 pins any sort of HD connection.

I think the major broadcasters would disagree with you that Gepco is on a par with Belden. They switched to it after exhaustive tests that indicated that it was vastly superoir.

Mark

Posted by Steve Guttag (Member # 268) on 07-24-2006, 07:09 PM:

Believe it or not...the HD15 is the same type of pinout as the standard DB50. That is, a triple row. The HD15 designation is to distinguish it from the standard DB15 which is a double row connector that is wider. The HD15 is the same profile as the DP9...hence it is higher density. I don't name them but I'm not going to buck the system...if you prefer...call it the VGA connector...not as technically on the mark but it congers up the right picture in most peoples mind.

Anyway...I do work with broadcast types a lot...and I tell you...there is a pretty even split on the Gepco versus Belden...like me...while most have a preference mainly for tooling and availability/pricing reasons...either are equally acceptable. I see 1694A and 1695A from Belden more around hear than Gepco's equivalent for HDSDI. Kings crimps seem to be the standard too for BNC though I'm a rebel in that I use AMP...I've just had such success with those Pro-Crimper IIs in so many applications.

Posted by Mark Gulbrandsen (Member # 72) on 07-24-2006, 11:06 PM:

All I'm saying is that what they call an HD-15 isn't very high density. When you get to 50 pins... yes.

I agree with you on the Amp BNC's... I've literally installed thousands of them and never one bad one. We used to get them in boxes of 1000 at the station.

Mark