Bridging questions

[MADNESS]

I have bridged 2 speakers off an amp together before. I was asked this question and did not know the answer. A friend of mine has a large amount of sound equipment for a band.
He wants to know for the monitors if you can daisy chain 4 off of 1 side X 2 or 8 bridged mono.
The speakers are 250 RMS, 500 peak and he has a QSC PLX3402 amp. I know how to run the system I have but this is more than I have done in the past. I know I can get some idea here!!

 

[Balanced Line]

The important factors in this question are:
1) What is the minimum load the amplifier can accept?
2) What is the impedance of the speakers?
3) Are they connected in parallel?
4) If they are not identical, do they have nearly the same sensitivity?

Bridging uses both halves of one stereo amp to provide a single output. One half of the stereo amplifier pushes the positive half of the waveform and the other pushes the negative half of the waveform. When a stereo amplifier is used in bridge mode it now has only one output, it is effectively a mono amp. Additionally it can only drive a load with twice the resistance it could formerly. In other words if the minimum load in stereo (2 channel mode) was 4 ohms, when bridged, the minimum load will be 8 ohms.

Unless you use some kind of series - parallel connection it is unlikely eight speakers can be connected to one bridged output.

To find the total resistance of set of speakers in parallel divide the impedance by the number of speakers. Eight speakers, each having an eight ohm impedance will present a one ohm load to the amplifier. This assumes they are all the same impedance. If the impedance varies of the speakers varies, the formula is a little more complicated.

Remember too if there is a difference in the sensitivity of the speakers the more sensitive speakers will be louder.

The 3402 has a minimum output impedance of 2 ohms per side in stereo (2 channel mode). When bridged, it can drive a minimum load of 4 ohms or two 8 ohm speakers in parallel. In stereo mode it is technically possible to drive four 8 ohm speakers per side connected in parallel.

By any chance are you the type to make your poor granny run the high hurdles? The amp you cite is a very nice piece of equipment but your intended configuration pushes it to the max.

 

[Fred Wright]

Bob addressed the techical aspect. Now I've got some thoughts to add if I may.

Why the heck would any soundman run four stage wedges from a single amp channel?:wtf:

Reason I say this, wedges (or monitors) are inevitably gonna have to be adjusted to the individual performer's preference. You can't run the full PA mix through the wedges. You'll have half-deaf lead guitars screaming that they want more ME in this monitor. If all your wedges are one amp channel you're gonna have a stage sound war. This is why many bands are going to in-ear monitoring systems.

Each wedge needs its own feed from the assigned performer's mics and/or instruments. That means dedicated amp channels for each wedge.

Quad channel amps are available for just this reason.

 

[Jeff Romard]

I could be wrong on this but running the QSC on those speakers in Bridged Mono probably isn't a good idea. If my numbers are right you are dropping them to 4 Ohms (assuming they are 8 Ohms) and putting out 3400/side in to them

Fred?

 

[JoeChartreuse]

Bob, beautifully explained. Jeff, if there are 8 8 ohm speakers in parallel, you have 1 ohm impedance. Not a good move.

 

[Precisionpower]

In order to power 4 monitors with that 3402, the monitors would need to be
16 OHM.


16----16----16----16
-8- -8-
--4--ohm load the amp "sees"


Not very often do you see 16ohm stage monitors. Most i've seen are 4ohm

Chaining (4) 4 ohm monitors to the Qsc will FRY that poor thing..
I wouldn't do it

 

[Jeff Romard]

Bob, beautifully explained. Jeff, if there are 8 8 ohm speakers in parallel, you have 1 ohm impedance. Not a good move.

I'm real lousy with Ohms law :sqembarrassed:

The first time I read the post I had it down to one ohm then I reread and had second thoughts...

Either way not a good idea

 

[Fred Wright]

I could be wrong on this but running the QSC on those speakers in Bridged Mono probably isn't a good idea. If my numbers are right you are dropping them to 4 Ohms (assuming they are 8 Ohms) and putting out 3400/side in to them

Fred?

According to QSC's specs on this model, yes. Presuming each wedge is rated at 8 ohms impedance and they parallel all four, the impedance will be much lower than 4 ohms. Some wedges are 4 ohm cabinets. That's far too much power dissipation to send to stage wedges.

For all practical purposes, a wedge needs no more than 300 WRMS dissipation fed to it. Some are rated for 400 or more @ 4 ohms but you've got to bear in mind what you're dealing with here. This isn't your mains or FOH you're driving. Wedges are directional by design and do not need a boatload of amp headroom.

Thoughts?

 

[Balanced Line]

Mine is not to question why...

Fred, just use the less efficient speakers in front of the performers that need less volume. Yuk yuk.

I was thinking of just this point (varying the volume) as I was answering the question but thought is was a "just make it work" kind of question. Coupled with the thought is was a newbie kind of question, you just want to cut to the chase.

The QSC amp in question has spectacular specifications but points out a basic difference in philosophy. Talk to the people that been there and done that, you'll find folks that want more amps run more conservatively. Bi-amping too adds a layer of complexity but provides intrinsic redundancy.

Some old expressions are still most apt - don't put all your eggs in one basket.

Jeff, if I'm reading the question correctly Madness stated: "if you can daisy chain 4 off of 1 side X 2 or 8 bridged mono". The amp will allow a four ohm load in bridged mode but that means the amp only has one output because they are bridged. It seems Madness wants to hookup eight speakers. If eight 8 ohm speakers are connected in parallel that represents a 1 ohm load. No can do on a bridged output. Four 8 ohm speakers in parallel represents a 2 ohm load which the QSC amp in question can do in 2 channel mode but as Fred stated presents practical operational problems. Moreover the words daisy-chaining are used which suggests series connection but let's not make this any more complicated.

The reason I'm repeating this is there seems to be a lot of confusion over bridging. Bridging does not provide any magic way of getting more out of the same gear. When you bridge an amp you:
1) Have half the number of amplifier channels ( a 2 channel amp is now mono)
2) Each of those channels can drive less load (the minimum impedance driven by the amp doubles)

Now having read Precision's assertion RE: 4 ohms being typical, if that is the case cut my numbers in half and yes it isn't going to work. This problem has insufficient information to answer with any certainty. Beyond that, just because you can do something doesn't make it a good idea. And Jeff, sorry, we were both writing at the same time so I didn't see your reply in time.

 

[sparkieg]

According to QSC's specs on this model, yes. Presuming each wedge is rated at 8 ohms impedance and they parallel all four, the impedance will be much lower than 4 ohms. Some wedges are 4 ohm cabinets. That's far too much power dissipation to send to stage wedges.

For all practical purposes, a wedge needs no more than 300 WRMS dissipation fed to it. Some are rated for 400 or more @ 4 ohms but you've got to bear in mind what you're dealing with here. This isn't your mains or FOH you're driving. Wedges are directional by design and do not need a boatload of amp headroom.

Thoughts?

Agreed.

Dependant on how many stage front men are in any given band, each in my experience ALWAYS wants their own monitor. If you are daisy chaining the monitors, you won't be able to give each band member their own "individual sweet" mix that they want. And as Fred said above, there would be a monitor war.

My advice would be if you absolutely have to daisy chain any of the monitors would be to do no more than two (2) of the stage monitors, and make those two either one on each side of the drummer, or two of the stage front men that stand close to each other that are needing to hear basically the same mix.

In my experience, the drummer and bassist mix needs to be very similar,
the lead guitarist needs his own mix, the rythym guitarist needs his own mix, and any keyboardist needs their own mix.

Remember that in most cases, the drummer is on a riser on the back of the stage and his mix HAS to be louder than the others and that it is imperative that he be able to hear each vocalist track, the bassist track and the rythym guitarist track. This is why I say that if you are going to daisy chain any of the monitors, do it for the drummer.

Just my thoughts.

As for your original question of running in bridge mono mode to increase the power to the monitors, I don't see the need to do so. Save that bridged mode for the house mains if you are going to bridge any of the amps.

 

[Guest]

A leason in Levon OK maybe not!

Now I am going to throw some confusion into the mix everybody is right and everybody is wrong!

Each stage monitor needs to be run from a single side of an amp. Except that the Keyboard and horns can use use the same mix so the monitors could be daisy chained, All vocals need the same mix so lead and backup vocals can be daisy chained. Bass guitar generally will run a bass cabinet so he will not need a monitor as each of the guitars (unless of course they are running through pods or processors and directly into FOH without their own cabinets in which case each would need their own Monitors)

Many stage monitors are 16 ohms (at least back in the old days)

A bridged amp which is 4 ohm capable when bridged can run three 8 ohm speakers and the amp will still see a 4 ohm load (however one speaker will be running 6 DB louder)

4 8ohm speakers per side is a 2 ohm load and many amps are 2 ohm capable. 8 16ohm speakers per side will also be a 2 ohm load.

Now where everyone is correct is that it doesn't make sense to daisy chain a bunch of monitors together because it simply defeats the purpose of monitors. If your friend wants to put a monitor system on stage that is basically the whole mix in one or two systems there is a much easier way to go about it.

Take two monitors or main speakers put them on poles on each side of the stage at the front edge, aim them across the stage (much the same as you would for a small dance floor) creating an X on the stage with your sound. On your monitor mix (stage mixer) put the instruments that the band members need on the right side of the stage to the right and for those to the left put the stronger mix to the left. Center the vocals in the mix. Everyboby can hear the full mix and have an increased output to the side it is needed the most on. It is called cross stage monitoring!

Or get a Bose L1 stick for each band member! That's what they were made for!

 

[Balanced Line]

Ditto!

A leason in Levon OK maybe not!

Now I am going to throw some confusion into the mix everybody is right and everybody is wrong!...

A bridged amp which is 4 ohm capable when bridged can run three 8 ohm speakers and the amp will still see a 4 ohm load (however one speaker will be running 6 DB louder)

Well there's a good deal of that going around, you might want to check your numbers

There is a good deal of practical advice in this post that cuts to the chase. Audio can be made much too complicated too quickly without purpose. Often simpler solutions gained through experience are better.

Inasmuch as this is a basic question and started as “can you hook up this to that” it was answered in kind. A great number of the people on these pages simply plug a neutrik connector to their amplifier and another to their speaker without regard to how the connections are made. Adding the complication of a series-parallel connection does a disservice to those looking for a simple answer most especially when it isn’t explained. Why on the one hand provide simple straight forward advice and then complicate it with information we both know will confuse the original poster? Isn't the purpose to help the person that asked the question?

In any case, connecting speakers in series is another example of could do but probably shouldn’t so why add a fly to ointment and further confuse the issue?

 

[Guest]

OK now you have me confused porbably with someone elses post.

I never mentioned anything about series hook-ups and didn't suggest any!

4 8 OHM speakers are a 2 OHM load and most amps can handle that without a problem in stereo mode!

For anyone that has any knowledge of amps and how bridging works knows how to setup "stacks" on the amp for three 8 ohm speakers.

Apparently you are a lot more confused than anyone else.

If you don't understand and want to know the hows and whys, I will explain it for you!

 

[Balanced Line]

Share or don't share your choice

Thunder, you are accusing me of the very things of which you yourself are guilty. I already laid out some of the same points you are now making. Read my post. Yeah 8/4 = 2.

I can assure you that if you have lost me, you have lost many if not most of the others too but you've been around here long enough to know that. So I ask again, is that your purpose?

If you had any interest in imparting an understanding here you would simply do so and not blow smoke up everybody's ass. If you have information to share, share it. In any case don't be so arrogant as to expect people to beg to sit at the feet of the master. Share it or don't share, your choice.

 

[Guest]

Thunder, you are accusing me of the very things of which you yourself are guilty. I already laid out some of the same points you are now making. Read my post. Yeah 8/4 = 2.

Then why would you say that I need to check my math?

If you had any interest in imparting an understanding here you would simply do so and not blow smoke up everybody's ass. If you have information to share, share it. In any case don't be so arrogant as to expect people to beg to sit at the feet of the master. Share it or don't share, your choice.

I believe I have shared this information several times with those who didn't understand it, on this and other forums! For the vast majority DJs using two speakers this information would be of little use for people running large rigs it is general knowledge anyway. If someone is interested in learning how I am more than willing to share it but there is no reason to repost it for those who are not interested in knowing the hows and whys of stacking on an amp.

I wasn't and didn't contradict you on your post and I didn't take you to task over any preceived Inaccuracies that you may or may not have posted!
Why are you trying to do so with my post! When there are no Inaccuracies in it?

 

[Balanced Line]

No Concern

...If you don't understand and want to know the hows and whys, I will explain it for you!

As I've said before, share, don't share, do as you choose.

It is no longer of any concern to me.