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#1 danger mouse

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Posted 11 July 2006 - 08:31 AM

hi .ive got an amp that puts out 1200 rms at 8 ohm in bridge mode , is it possible to comnnect 2 speakers to that by daisy chaining them . the speakers are 8 ohm each and are 500 rms each. i know i wont be able to turn it up to the max but thought if i go to 75% of the volume on the amp id get 450 rms into them which would be ok .

#2 dangerman

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Posted 11 July 2006 - 09:27 AM

HUH!!! another dangermouse!!! fear.gif

#3 Gary

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Posted 11 July 2006 - 09:47 AM

QUOTE (dangermouse @ Jul 11 2006, 10:27 AM)
HUH!!! another dangermouse!!!  fear.gif

ah, but he's Danger(space)mouse.

Which doesnt mean he's got a rocketship. biggrin.gif


When you plug the two 8 ohm speakers daisy-chained (eg: Speaker 1 plugs into the amp, then speaker 2 plugs into speaker 1) then the speaker load changes from 8ohms to 4ohms.

Normally, its fair(ish) to say that whatever watts an amp can chuck out into an 8ohm load, is double the figures when presented with a 4ohm load. However, I've got a dull, smokey recollection about Bridge mode calculations not always staying that clean and precise.

But lets say for a moment, they did.

1200 rms @ 8ohms, could offer 2400 rms @ 4ohms

The 2 x 500 watt (assumed RMS) @ 8 ohm speakers daisy-chained would be capable of 1000 watts RMS @ 4 Ohms.

Thats 2400 watts output going into 1000 watt speakers. Its risky.

Either turn the amp attenuators down on the amp to about half way, and/or use a compressor/limiter to ensure that what leaves the mixer never ventures over a certain level.

But...find out what the amp rating is in bride mode at 4 ohms.

Edited by Gary, 11 July 2006 - 09:58 AM.

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#4 Welsh Audio Man 21

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Posted 11 July 2006 - 11:11 AM

Its never recommended to use bridge mode all the time....

#5 spinner

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Posted 11 July 2006 - 11:55 AM

For a while I used an amp in bridged mode that produced 1200 watts at 8 ohms.

The Speakon plug from the bridged outlet had 2 leads attached to it each going to a cabinet containing a Fane Collossus 18" 600 watt bass driver.

I'm not sure what impedance that produced but I never had a problem with the amp and I'm still using it 6 years later ( albeit not in bridged mode ).

#6 norty303

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Posted 11 July 2006 - 01:05 PM

ALL amps are capable of running bridged, even those without bridge switches (actually, some newer digital amps can't but we're not in that high end market so we'll discount them, ok ;) )

When you bridge an amp you're effectively using the 2 sides as a 'whole' which is why you have to change your wiring so that you're using the 2 positive terminals (if binding posts) or using 1+ 2+ (if on a 4 pole speakon). If your amp has only 2 pole speakons then you'll need to make up a lead that takes 1+ form output 1 into 1+ of a speakon, and 1+ from output 2 into 1- of the speakon.

When you flip a 'bridge mode' switch all you're doing is reversing the phase of the second channel so you now have a +ve and a -ve terminal. You can do this on an amp without a bridge switch by making up a V lead to go into the amp 1 and 2 inputs from a single XLR, but reverse the poles on one of the ends, so you have an in-phase and out of phase signal at each input.

As to power ratings and capabilities.....

This is a simple way to remember it:

If you add together the per-channel and impedance ratings of your amp you'll get what it will put out at that impedance.

e.g. A Behringer EP2500 puts out 2 x 700w at 4ohms. this means it'll do 1 x 1400w at 8ohms (2 x 4ohms = 8 2 x 700w = 1400w)

Whilst its putting out that power at 8ohms, each side of the amp will be 'seeing' a 4ohm load. This is why they run hotter bridged.

Where people come unstuck is when they try to run a 4ohm bridged load off an amp that isn't capable of running at 2ohms per channel. If you're amp is only rated for 4ohms per channel the MAXIMUM load you can run bridged is 8ohms.

So for your amp is rated for 2ohms per channel operation then you CAN run a 4ohm bridged load.

If an amp is rated for bridged mode operation then there should be no problem with doing it within those rules, however the lower the impedance the harder the amp works and the lower the MTBF (Mean Time Before Failure) will be.



The comment about turning the gains down on the amp to restrict power....

This will not in any way limit the amount of power the amp is capable of producing. Increasing the input will still result in the amp giving maximum power regardless of gain position (obviously you'll have to go a long way if its only on 1!!)

I usually always run with gains on full to stop the 'ooh, i'll just turn this up a bit' people with wandering hands. restrict output further up the chain if need be.
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#7 danger mouse

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Posted 11 July 2006 - 03:57 PM

the amp is a c-mark MR2450, it dosent say its stable at 2 ohm in the manual just 80hm stereo rms-450wx2 , 4ohm stereo rms- 700xw 2 and 8ohm bridged rms- 1200w. so would it only take one 8 ohm speaker in bridged mode and that would be well dodgy unless it was a 1200w rms speaker! so you cant blow it , wow bit confussed,do you think its best to leave alone and run it like i do. 450rms from amp with 500 rms speakers .

#8 norty303

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Posted 11 July 2006 - 05:39 PM

Basically you don't have much choice (other than getting a bigger amp)

If you parallel your speakers together it'll be a 4R load. Your amp can't cope with a 2R load per channel, therefore you can't use it bridged into the pair.

Its worth saying that if these are mid/top cabs then there's probably little to be gained by getting a bigger amp. If they're subs then it may be worth trying to get more power into them.

I seem to be saying this a lot recently, but a Behringer EP1500 would do the trick just nicely, bridged into the 2 cabs. (750w per cab) Thats 1.5 times the rms rating and just about fits perfectly with what is the best power to put into a speaker.

A friend has finally just killed an EP2500 but it was 2 years of relentless bridged 4R work in a DnB club running some hefty scoop bins. Paid for itself a loooong time ago.


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#9 Welsh Audio Man 21

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Posted 11 July 2006 - 05:44 PM

ok norty, after a few posts and threads, i think i understand my speakers now

600w per channel 4ohms from my amp... 4 speakers, 300w rms at 8 ohms each, parrallelled in pairs, that makes the amp run at a 4 ohm load?

So does that mean each speaker gets 600w, or is it halved... 300w per speaker?

#10 norty303

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Posted 11 July 2006 - 06:09 PM

QUOTE
600w per channel 4ohms from my amp... 4 speakers, 300w rms at 8 ohms each, parrallelled in pairs, that makes the amp run at a 4 ohm load?

So does that mean each speaker gets 600w, or is it halved... 300w per speaker?


Ok, first lets get away from 'making an amp run at a certain load'. This makes it sound as if it has definite modes of operation which it doesn't.

It has a certain load placed on it, and delivers a given amount of power at that load. They choose 4 and 8R to quote in the specs as this is traditionally how they are run but they can equally do 6, 12 or 16 and will have a relative output into those loads.

Anyway..... biggrin.gif

Your amp gives 600w per channel into 4ohms, therefore if you place a 4ohm load on it, the speakers will receive 600w. That is 600w total for the whole load (2 speakers, the amp has no idea how many speakers you've connected tho, just the load)
So, 300w per speaker. It doesn't matter what the rating of the speakers are (even if mismatched), they will still always receive 300w (assuming 2 speakers or cabs of an 8R load each in parellel)
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#11 Welsh Audio Man 21

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Posted 11 July 2006 - 06:34 PM

Thats just the answer i was looking for, cheers! thumbup.gif

#12 superstardeejay

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Posted 11 July 2006 - 06:47 PM

Uggh I wouldnt say that just any old amp is capable of being bridged!!

It is true that the bridge switch reverses the phase of one channel and then connects them in mono and yes you use the +ve of the left channel for one speaker wire and the +ve of the right for the other. But you may also need to link the two -ve terminals together as they arent always internally connected to 0V or earth. And if you have a grounded bridge amplifier, eg certain Studiomasters or some others that don't even mention this, manually connecting them in bridge may make them go bang!

Bridgeable amps are also calibrated or designed to be calibrated more accurately/ run more stably in such a way that you don't get large circulating currents running between both sides, they usually have built-in automatic DC offset adjustment so they dont run out of symmetry when they warm up in bridge mode. Some non-bridgeable amps may not have this taken into consideration in their design.

One to watch BTW




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#13 danger mouse

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Posted 11 July 2006 - 07:58 PM

is it correct to say that i can only run two 8 ohm speakers daisy chained in bridge mode of something like a behringer then , why would it make a 2 ohm load on the amp , when the speakers are a 4 ohm load ( 2x8ohm=4ohm) .let me know , ta.

#14 danger mouse

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Posted 11 July 2006 - 08:39 PM

saw this and it seems to say that you can connect a pair of speakers 8 ohm in bridge mode . still confused !

"Bridge Mode" means combining both the two internal amplifier boards together to double the rated output. By doing this, your amplifier ceases to be a 2 channel or stereo amplifier, and becomes a Mono amplifier with the combined power output of BOTH the amplifiers into half the load.

For example - An Amplifier usually provides it's full rated power into 4 Ohms of load, which means connecting 2 8 ohm speakers per channel (A total of 4 speakers) to get this.

However if you switched a compatible amplifier into "Bridge Mode", this essentially means that both the left and right channels are combined and so you would get the combined power output of both channels into 4 Ohms (one pair of speakers). However the output would be MONO.

#15 norty303

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Posted 11 July 2006 - 09:56 PM

QUOTE
It is true that the bridge switch reverses the phase of one channel and then connects them in mono and yes you use the +ve of the left channel for one speaker wire and the +ve of the right for the other. But you may also need to link the two -ve terminals together as they arent always internally connected to 0V or earth. And if you have a grounded bridge amplifier, eg certain Studiomasters or some others that don't even mention this, manually connecting them in bridge may make them go bang!

Bridgeable amps are also calibrated or designed to be calibrated more accurately/ run more stably in such a way that you don't get large circulating currents running between both sides, they usually have built-in automatic DC offset adjustment so they dont run out of symmetry when they warm up in bridge mode. Some non-bridgeable amps may not have this taken into consideration in their design.


You're right of course and I was being far to general in generalisations!! biggrin.gif


QUOTE
is it correct to say that i can only run two 8 ohm speakers daisy chained in bridge mode of something like a behringer then , why would it make a 2 ohm load on the amp , when the speakers are a 4 ohm load ( 2x8ohm=4ohm) .let me know , ta.


Two 8R speakers 'daisy chained' (wired in parallel) combine to make a 4R load. If you run a Behringer bridged into a 4R load each side of the amp is 'seeing' a 2R load.

Go back and re-read what i wrote about calculating loads...

8R bridge mono = 4R stereo
4R bridge mono = 2R stereo

QUOTE
However if you switched a compatible amplifier into "Bridge Mode", this essentially means that both the left and right channels are combined and so you would get the combined power output of both channels into 4 Ohms (one pair of speakers). However the output would be MONO.


What this bit fails to mention is that running bridged into 4R may well result in it going 'pop' if its not 2R stable.
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