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Old 2nd November 2011, 06:14 PM   #1
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Default Daisy chaining PA speakers

Hi new to here little techy experience and have a question hopefully somebody can help me with?

I have an Alto D4 4 channel amp (specs below)
I have 4 x 500w speakers @ 8 ohms each
If each speaker is 8ohm and each channel is 750w @ 4 ohms and connected as stereo then I only get about 350 w per channel!

Question can I run two speakers off each channel which is bridged?
so the 4 stereo channels are now bridged to make two then run two speakers one connected to amp then one connected from " through" speakon connection @ 8ohm on each bridged channel?
If i'm right then 2 x 8ohms will become 4 ohms ???

Appreciate any help advice

SPECS
Power Rate @0.5% THD. Both channels driven:
4 x 540W @ 4Ω (RMS); 4 x 340W @ 8Ω (RMS)
4 x 750W @ 4Ω (EIAJ); 4 x 370W @ 8Ω (EIAJ)
Bridge Mode: 2 x 1500W @ 8Ω (EIAJ); 2 x 740W @ 16Ω (EIAJ)
Dimensions (WDH): 483 376 88.8mm (19" 14.8" 3.5")
http://legacy.altoproaudio.com/admin...8720513739.pdf
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Old 2nd November 2011, 06:25 PM   #2
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If you place two 8 ohm speakers in parallel that is four ohms and in bridged mode the amplifier is not rated for that and will burn up.

If your speaker is rated for 500 watts at 8 ohms and the amplifier is rated at 340 watts into 8 ohms, it will still be able to blow up your speakers! That is because amplifier power is easily measured and is real, while loudspeaker wattage is hard to measure, although a few do it, and is mostly advertising.

If you have loudspeakers really rated at 500 watts and drive them with 500 watts after two hours half of them should fail! Is that really what you want?
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Old 2nd November 2011, 06:31 PM   #3
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This is why I'm asking. with this amp what is my best option?
Run all the speakers seperatley on the four channels?
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Old 2nd November 2011, 06:32 PM   #4
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The speakers concerened are 4 x Kam soundforce 15
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Old 2nd November 2011, 06:35 PM   #5
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Basically, it wouldn't be a very good idea at all to run the amplifier bridged with speakers in parallel. As you stated correctly, the impedance of the two speakers in parallel is 4 ohms (as they are both 8 ohm speakers). This presents a 4 ohm load to the amplifier which it will not have any trouble dealing with.

But, when you put the amplifier into bridged mode, the load each amplifier sees is half of the total load. So your 8 ohm paralleled speakers (producing a 4 ohm load), now present a 2 ohm load to each amplifier. This is waaay too low for almost any amplifier to handle without overheating or going into protection (because of too low a load or over-current), an that is even without factoring in how the that nominal 2 ohms may drop to at certain frequencies.

Put simply, unless you want to overload your amplifier and cause damage to it and more than likely the speakers, it is a bad idea. An alternative method would be to wire the speakers in series (making a total load of 16 ohms) so that each amplifier sees an 8 ohm load. It will like that much more than a constant 2 ohm load.

Edit: Reading the posts that were made whilst I was writing this reply, I honestly don't think that you will have a power problem unless your speaker placement is terrible.
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Old 2nd November 2011, 06:37 PM   #6
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Why would you do anything but run one speaker off each channel?
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Old 2nd November 2011, 07:26 PM   #7
AndrewT is offline AndrewT  Scotland
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I agree with the posters above.
Dedicate an amplifier channel to each speaker.
When a fault develops you only lose output from one speaker. The other three keep going for a loss of <2dB. The audience might not even notice the loss and give you time to fault find and swap components.
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Old 2nd November 2011, 07:28 PM   #8
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Quote:
Originally Posted by Cal Weldon View Post
Why would you do anything but run one speaker off each channel?
Well, there was the system I sold to a local college with one 800 Watt amp channel per woofer box (double 18). They looked at the amplifier ratings and realized they could bridge the amps and get almost four times the power out.

The result was four blown boxes and two dead amplifiers, but they did last almost a semester!
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Old 2nd November 2011, 07:53 PM   #9
AndrewT is offline AndrewT  Scotland
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I am guessing but part of the problem is the way the specifications are written.
300W into 8ohms
600W into 4ohms
1200W into 2ohms

All seems OK. The retailer/manufacturer is not hiding anything and surely they couldn't be telling us lies.

But what are the specifications actually telling us?
I bet they are not saying this amplifier is suitable for PA duty driving a reactive 2ohm speaker.

I guess that those specifications actually apply to resistive test dummy loads of 8r0, 4r0 and 2r0.
Further, they are implying that by using the exact doubling of maximum power into the halved loads they are quoting short term transient power output based on peak output voltage before the capacitance has had a chance to discharge.

Had they actually tested the medium term maximum output power at a known maximum distortion the Power specification would look very different.
eg.
290W into 8r0 @ 1kHz 0.1% distortion + noise.
520W into 4r0 @ 1kHz 0.1% distortion + noise.
800W into 2r0 @ 1kHz 0.1% distortion + noise.

When we read specifications, we have to read between the lines and extract from the numbers a feel for, what they are actually revealing and actually guaranteeing.

This is not easy because many of the manufacturers have jumped onto the numbers bandwagon. They recognise that unrealistic specifications although not actually tellings lies dupe many customers into believing they are buying good value for money product.
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Old 2nd November 2011, 08:31 PM   #10
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Thanks for all your replys! I'll go with looks like the safer option of one on each channel. Any reason why speakers all seem to be 8ohm but the amps all rated as 4ohms? Sorry if this sounds a stupid question!!
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