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This topic comprises 2 pages: 1 2
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Topic: Bi-Amp Stage Speakers
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Steve Guttag
We forgot the crackers Gromit!!!
Posts: 12814
From: Annapolis, MD
Registered: Dec 1999
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posted 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
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Steve Guttag
We forgot the crackers Gromit!!!
Posts: 12814
From: Annapolis, MD
Registered: Dec 1999
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posted 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.
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