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Author Topic: 3 speakers/ 1 amp  (Read 1521 times)

Robert Lunceford

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3 speakers/ 1 amp
« on: July 22, 2006, 02:52:39 pm »

I would like to power 3 Bose MA12 line array speakers with a single QSC PLX2402 power amp. The MA12(8ohm) speakers are stacked 3 high creating a 3 meter high line array. I would like for all 3 speakers to receive equal amounts of power. I DO NOT want to run all 3 speakers from one channel (2.6 ohm load).
If I run 2 speakers from one channel (4 ohm load) they will each receive 350 watts. The 3rd speaker on the 2nd channel (8 ohm load) would then receive 425 watts.
I can lower the gain control on the 2nd channel, but how do I know how far to lower it? Is there a way to measure the outputs with a DC meter and if so, what is the best method?
Would it be better to add an 8ohm dummy load to the 2nd channel. If so, what would I use?

Robert Lunceford
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Greg Hertfelder

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Re: 3 speakers/ 1 amp
« Reply #1 on: July 22, 2006, 09:41:25 pm »

No, don't bother with a dummy resistor, but I like the depth of your thinking. Here are two alternate methods:

Alternate Method 1: Series/Bridge Mode: Wire all three speakers in series (not parallel), resulting in a 24-ohm load and equal wattage distribution. Configure the power amp to bridge mode for additional power. (Read the power amp manual for specifics on configuring for bridge mode, which usually requires flipping a switch and using a designated set of speaker terminals.)

Alternate Method 2: Measuring SPL
You could also measure the sound pressure level (SPL) of each speaker. You will need a pink noise generator (or CD/MP3 file), hearing protection, SPL meter (or separate wired mic & mixer), empty room, and preferably an assistant.

IMPORTANT: Exposure to loud sound can cause permanent hearing damage. Wear industrial hearing protection available at a hardware or sporting goods store. Start with power amps powered off and volume knobs off. DO NOT position anyone without hearing protection near speakers when energizing with pink noise.

Compare the acoustical outputs of each speaker with the SPL meter first by measuring (only) the output of power amp channel 1, and then (only) power amp channel 2, and finally by all power amp channels up. SPL measurements should be made in the center of the relevant speaker's throw pattern. Common SPL measuring levels vary, but are in the vicinity of 95dB. Make all changes slowly and methodically.

After applying pink noise to the mixing console (0dB on the VU meter), power on the power amp, and slowly raise the gain on channel 1. Measure the SPL. Mark the channel 1 power amp knob position and lower the knob all the way. Slowly raise the gain on power amp channel 2 until the measured SPL equals the SPL of power amp channel 1. Mark the knob position. Raise the channel 1 power amp knob to the marked position.

Listen to conventional programming (mic, CD, etc.) through the system.

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Ivan Beaver

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Re: 3 speakers/ 1 amp-No problem
« Reply #2 on: July 23, 2006, 08:42:15 pm »

No problem, just parallel the inputs(switch) to the amp and then hook 2 speakers to 1 side and 1 to the other.  You will not have any difference in level.

The reason for this is that the max output levels are just that-MAX output.  Below that the levels will be the same.  If you are running your system all the way into clipping, then there will be a difference of capability- though only .8dB.  Nobody will notice any difference either way.

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Can I have some more talent in the monitors--PLEASE?

Ivan Beaver
dB Audio & Video Inc.

Greg Hertfelder

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Re: 3 speakers/ 1 amp-No problem
« Reply #3 on: July 25, 2006, 11:41:27 pm »

Ivan
Are you sure about that? The QSC web site says that the PLX2402 will output 425 watts/channel into 8 ohms and 700 watts/channel at 4 ohms. Wouldn't sound pressure level in the 4 ohm channel be greater than the 8 ohm channel?
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Mac Kerr

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Re: 3 speakers/ 1 amp-No problem
« Reply #4 on: July 25, 2006, 11:57:02 pm »

Greg Hertfelder wrote on Tue, 25 July 2006 23:41

Ivan
Are you sure about that? The QSC web site says that the PLX2402 will output 425 watts/channel into 8 ohms and 700 watts/channel at 4 ohms. Wouldn't sound pressure level in the 4 ohm channel be greater than the 8 ohm channel?
Yes and no. As was explained over at the LAB in this same topic, the amplifier has a certain amount of gain, and it will be the same in both channels. The 4 ohm channel will only be delivering 350 watts per speaker at clipping, which will occur slightly before the 8 ohm channel clips while delivering 425 watts into the single speaker. So, yes the 4 ohm channel will be louder because it has twice as many speakers, each getting the same power as the single speaker, as long as the system is below clip, but in answer to the OP's question, no, all the speakers will be the same level.

Mac
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Greg Hertfelder

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Re: 3 speakers/ 1 amp-No problem
« Reply #5 on: July 26, 2006, 10:55:41 pm »

I see your point, Mac.

Of course, the damping factor takes a bit of a 4 ohms, but I guess we've learned to overlook it for the sake of convenience.

Thanks for contributing.
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Mac Kerr

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Re: Damping factor
« Reply #6 on: July 27, 2006, 11:13:46 am »

Greg Hertfelder wrote on Wed, 26 July 2006 22:55

Of course, the damping factor takes a bit of a 4 ohms, but I guess we've learned to overlook it for the sake of convenience.
While it is true that damping factor, the ratio of amp load impedance to output impedance, will decrease at 4 ohms, it has no real impact on the 2.2" drivers that roll off at 120Hz in the Bose MA12 being discussed. Damping factor helps control the excursion of large low frequency drivers. There are none in an MA12.

Mac
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Re: Damping factor
« Reply #6 on: July 27, 2006, 11:13:46 am »


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