Multiple Amp/speakers For Live

[Gnash5150]

I've gotten to a point where I want to hook up more power to my live rig. I have the two tops and the two sub cabs for a stereo rig. I have a xover and amps for the tops and a xover and amps for the bottoms. I want to duplicate this exact setup for double the power. I'm lost. haha I didn't even stop to think. I only have, ya know, 1 Main output from my mixer to my existing rig. How do the big rigs at concerts have all those identical amps and enclosures. Do they just run all those amps from the busses. My mixer has 8 busses, but I'm using most for monitors. My crossovers only have 1 output. I'm just stuck, but I'm thinking I need Y cables from my xovers to their respective amps.

[vicenzajay]

Originally Posted by Gnash5150

I've gotten to a point where I want to hook up more power to my live rig. I have the two tops and the two sub cabs for a stereo rig. I have a xover and amps for the tops and a xover and amps for the bottoms. I want to duplicate this exact setup for double the power. I'm lost. haha I didn't even stop to think. I only have, ya know, 1 Main output from my mixer to my existing rig. How do the big rigs at concerts have all those identical amps and enclosures. Do they just run all those amps from the busses. My mixer has 8 busses, but I'm using most for monitors. My crossovers only have 1 output. I'm just stuck, but I'm thinking I need Y cables from my xovers to their respective amps.

Y-cables will do just fine for you...and keep your live PA system consistent from speaker to speaker (assuming the same type of speakers/amps for each freq region).

Jay

[xstatic]

You are better off using a passthrough or some sort of active balanced split than just slapping a "Y" cable out there. It will work though.

[Gnash5150]

Could you link me to a "Pass Through" or similar splitter option from the Musician's Friend site. Thanks, man.

[RawDepth]

They use amps that can pass the signal through like this one. The input signal not only goes into the amp channel but also jumps up to the output jack so you can run it to another amp. You can sort of daisy chain as many as you want together.

You only need one crossover, by the way, to feed all of your amps. You only need a second one if you exceed the number of outputs.

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[ocnor]

To do it correctly you need a rack mount 2/3 way crossover and at least 3 power amps. http://www.zzounds.com/item--DBX223

The L/R mixer mains > crossover inputs > crossover high out L > bridged power amp > L top cab.
crossover high out R > bridged power amp > R top cab.
crossover low out summed > bridged power amp > L/R bottom cabs daisy chained.

That gives you stereo tops running full range and mono bottoms.
You can do it with one amp in stereo for the L/R tops but it doesn't give you the headroom that bridged amps will.

[xstatic]

Personally, before you start blindly bridging amps and such, I would take a look at how much power your amps have at which impedence, how much your speakers need etc... Also, make sure your subs are not 4 ohm subs or anything before you link two of them together with a brdiged amp so you don't fry your stuff. Personally, I do not like bridging stuff very often. I prefer to just get the right amps from the get go, and I hate having to mono with Y cables. With all the DSP based crossovers available nowadays, you may as well get one that will actively mono things for you.

[ocnor]

Originally Posted by xstatic

Personally, before you start blindly bridging amps and such, I would take a look at how much power your amps have at which impedence, how much your speakers need etc... Also, make sure your subs are not 4 ohm subs or anything before you link two of them together with a brdiged amp so you don't fry your stuff.

Of course you have to know the speaker impedances and the amp specs when bridged. I thought that was a given.

Originally Posted by xstatic

Personally, I do not like bridging stuff very often. I prefer to just get the right amps from the get go, and I hate having to mono with Y cables. With all the DSP based crossovers available nowadays, you may as well get one that will actively mono things for you.

Most power amps are meant to be bridged. That's the only way to get full power from them. You are cheating yourself out of wattage/headroom by not bridging. An analog crossover can run mono or stereo or a combination of the two without having to convert the signal to digital and back the way a DSP crossover does.

[vicenzajay]

Originally Posted by ocnor

Most power amps are meant to be bridged. That's the only way to get full power from them. You are cheating yourself out of wattage/headroom by not bridging. An analog crossover can run mono or stereo or a combination of the two without having to convert the signal to digital and back the way a DSP crossover does.

I have found (4 years of running PA for live shows) that often better sound quality is achieved by not bridging the power amps. Many power amps are designed to function optimally at a certain impedance load...that optimal level of performance is usually not the (Edit: 4-ohm bridged load - 2-ohm's per channel) that might certainly be within operating specs but perhaps not optimum for the amp. Of course, this means that you will have to ensure that your amps are "beefy" enough to run their associated application...for sub amps, I use QSC 2402's or higher in order to have plenty of headroom for each channel. However, I find that the phase/sound quality relationship between is better by running each sub on its own channel - perhaps as a function of cable length differences between the speakers and the amps if you daisy chain the speakers instead of run the cords directly from the amplifier...the electrical engineers can weigh in at this point.

YMMV of course.

Fully agree on the analog crossover...why mess with your signal at the crossover point unless you have very thorny venue issues that require a full PA processor (like the DBX, LA Audio, or other units) to take care of all your duties. Those units can, however, be nice all-in-one solutions with a decent reference mic to get you in the EQ ballpark quickly if you're changing venues nightly. I am "old school" in my mind, and I prefer to EQ with my ears and use analog crossovers post the board and master EQ.

On the Y-cable/active split issue - I have found little, if any, signal degradation using Y cables to split the signal from a FOH point of view as long as the signal run for the Y-cable is relatively short. Obviously impedance/capacitance are all affected by cable length, so YMMV...but if all you are doing is running a couple of feet or inches of extra cable to separate amp inputs to run several speaker stacks, Y-cables will provide a reliable solution.

Of course, if your amps allow a "pass through option", that will work just as well.

Jay

[ocnor]

Originally Posted by vicenzajay

Many power amps are designed to function optimally at a certain impedance load...that optimal level of performance is usually not the 2-ohm bridged load that might certainly be within operating specs but perhaps not optimum for the amp.

How often have you seen an full range top cab rated at 2 ohms? Most of the time they are 8 ohms. That said I never run an amp bridged below 4 ohms. I bought my first PA around 1982 so I've been doing this slightly longer than your 4 years.

[vicenzajay]

Originally Posted by ocnor

How often have you seen an full range top cab rated at 2 ohms? Most of the time they are 8 ohms. That said I never run an amp bridged below 4 ohms. I bought my first PA around 1982 so I've been doing this slightly longer than your 4 years.

You've got me on experience...mistype on my part for 2-ohm bridged. What I meant was 4-ohm bridged which is essentially both channels working at 2-ohms....usually not the optimum design for the amp involved. Just check the THD ratings at that setting and it's fairly obvious that the sound quality is measurably less stellar. More importantly - to me - I can noticeably hear better sound quality and phase relationship (especially for the subs) when I run the sub amps in stereo and each speaker has it's own channel. Thus my advice and point stands for my own experience.

Like I said - YMMV.

Oh, that 4 year lot is getting "paid" for doing it - so I suppose I'm not doing a shabby job of it. I've been doing it for over 20 years myself...21 to be exact.

Jay

[vicenzajay]

Originally Posted by ocnor

How often have you seen an full range top cab rated at 2 ohms? Most of the time they are 8 ohms. That said I never run an amp bridged below 4 ohms. I bought my first PA around 1982 so I've been doing this slightly longer than your 4 years.

One more thing - for my full range tops, I use a QSC RMX 800 to power them for my main PA...even running both sides at one speaker per channel (at 8 ohms), pinking the channels tops out at about 30-35% of the gain per channel (feeding it with an Allen and Heath board through a 31 band EQ and a BBE crossover). I can get stupidly loud on the full range at that gain setting and still have massive headroom available per channel should I need it. Bridging actually does me no good in this situation as I'd much rather add a speaker in parallel to each side (for large venues) and still have separate gains for L/R FOH instead of bridging to a single gain. At this point I would still be running the amp at 4-ohms a side...still optimal THD.

Obviously the same discussion can be developed for 3 or 4 way setups.

Just another point in favor of using separate channels. Again, YMMV, and I'm certainly not saying there aren't other ways to skin the cat.

Jay

[Lt. Bob]

I'm also not a big fan of bridged mode. I have had amps get wierd when run bridged even though I'm always careful to make sure the correct load is on them.
I bought my first PA in 1969 so I've been doing it a while but we didn't really have bridgable power amps in general use in those days.

[boingoman]

Originally Posted by ocnor

Most power amps are meant to be bridged.

Say what?

Power amps are meant to be run in a way that best matches them to their load, and the way that best matches the system design.

If that means bridging, so be it. If it means not bridging, then that's the way to go.

[vicenzajay]

Originally Posted by boingoman

Say what?

Power amps are meant to be run in a way that best matches them to their load, and the way that best matches the system design.

If that means bridging, so be it. If it means not bridging, then that's the way to go.

Exactly...something many people forget. Just because an amp CAN bridge to 4-ohms doesn't mean that's the best way to run it.

[ocnor]

What I meant was that it doesn't hurt the amp to run bridged because they are designed to run that way.

[RawDepth]

I am drawing from vague memory here but, this is the way it was explained to me...

An easier way to understand bridging might be to consider the AC power going into the amp. Typically if a stereo amp is fed by 120 volts (US) then that gets split in half and each channel only gets 60 volts to do its work. In an ideal world the output voltage from each channel going to the speaker can never be more than 60 volts at max power. When you bridge the amp, this voltage is internally doubled or put back together. The two channels work together as a team and can supply a max of 120 volts to the speaker. (minus thermal loss, of course.) That is why the max wattage tends to be doubled.

(Or something like that)

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[xstatic]

I agree with you Ocnor that some of this stuff is fairly obvious and should be understood, to people that have the experience. However, the nature of the original posters question led me to believe that some of these may not be known, which is why I chimed in.

As far as bridging goes, very few amps are as efficient or run as well in bridged mode as they do when operating independently. In addition to that, the more affordable amps (especially the $2k and under ones) are typically more prone to these types of issues. It is important to remember that overdiving an amp stage is a quick and easy way to blow speakers. Combine to that the bridging MIGHT be way more power than this user needs, the amp may be a cheaper one with poor output conversion, and bridging all of a sudden may be a damaging and expensive trap. In my post what I reccomended was checking these factors. I never recommended to NOT do what you suggested, but just to make sure of a few things first.

As far as speaker impedences, I can't remember an amp off hand that runs bridged at 2 ohms. 4 ohms seems to always be the spec. Now with the tops, I agree that you probably aren't very likely at all to run into a 2 ohm cabinet. However, with subs, 4 ohms is certainly not all that uncommon. You reccomended linking two subs together on a bridged amp. If these subs happen to 4 ohms a peice, you would be putting an extraordinarily heavy load on a quite possibly cheap underbuilt amp that may be running to a cheap sub with a driver not capable of dealing with mismatches and overs very well. End result here could be both a toasted amp, blown drivers, and especially with a shoddily designed amp, you may even manage to light a sub driver on fire if the amp opens to DC on overload.

In the end, I was not saying that your advice was bad, just that without having all of the proper information that your reccomendation may cause more problems than it would solve.

As a point of reference, I have only been doing live sound for 12 years now. However, I run a live sound comapny, work for a couple of larger ones, privately contract with bands and have done at least 1000 shows now and worked with well over 3000 bands in that time. Basically, this is what I do for a living, so all of what I have said here has come from repeated experience.

[ocnor]

Originally Posted by xstatic

In the end, I was not saying that your advice was bad, just that without having all of the proper information that your reccomendation may cause more problems than it would solve.

I agree. Without knowing the exact gear being used it's hard to give good advice. My advice was given with an average generic bar setup in mind :2 tops @ 8 ohms each, and 2 bottoms @ 8 ohms each, and power amps that only produce 500 watts or less per channel @ 8 ohms.

[Gnash5150]

Yeah this turned into a good read.haha I've used Y cables coming out of the xovers, and it's working perfect! I recommend it to anyone wanting to get louder! I wasn't aware that the amps used in large concert apps have a pass through option, before I researched. My QSC RMX 2450's and 1850's don't seem to have this option, so I'm going with the Y cables.

[dementedchord]

Originally Posted by RawDepth

I am drawing from vague memory here but, this is the way it was explained to me...

An easier way to understand bridging might be to consider the AC power going into the amp. Typically if a stereo amp is fed by 120 volts (US) then that gets split in half and each channel only gets 60 volts to do its work. In an ideal world the output voltage from each channel going to the speaker can never be more than 60 volts at max power. When you bridge the amp, this voltage is internally doubled or put back together. The two channels work together as a team and can supply a max of 120 volts to the speaker. (minus thermal loss, of course.) That is why the max wattage tends to be doubled.

(Or something like that)

_______________________________

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that's just so fkn wrong.... the best way to think of bridged output is think of it as similar to a balanced signal... only in this case we're driving opposite sides of a single load (the speaker) so relative to a single channel amp it's like it's getting twice the voltage as it sees the difference between the 2 amps... makes sense????

and there's no reason not to bridge an amp as long as you watch the load for impedence mismatch and the power supply is stable...