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12 spkrs on 4-8 ohms amp
12 spkrs on 4-8 ohms amp
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Old 31st October 2017, 02:26 PM   #1
darrensound is offline darrensound  India
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Location: India
Post 12 spkrs on 4-8 ohms amp

In my church I have a Power AMP - StudioMaster DJA 800 which has 2 channels with the following specs :
8 Ohms(2 X 165WRMS)
4Ohms(2 X 250WRMS)

Click the image to open in full size.

We have 12 speakers - Quest MS801W whose power handling is between 120 to 240W @8Ohms...These also have a Line Matching Transformer tapping setting on it(100V, 60W / 40W / 35W / 20W +8O Direct)..

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The problem is :- Of these 12, 10 speakers are already connected currently to the same AMP by someone in sets of 5 each per channel - I assume they are in parallel, can I use a multi meter to measure the Ohms where the wires terminate at the rear side of the Amp..... because for some reason we feel the output is pressed, it lacks clarity/loudness when we speak into the mics and play the Keyboard....during prayers.

So I have to connect 6 speakers to one channel and then the other 6 to the other channel....
So If I do a series/parallel connection(first set of 3 spkrs in parallel then series and next set of 3 spkrs in parallel again replicating the same for the other side) on both side I will get 5.33 Ohms in total on each side which is between 4-8 ohms. This is the wiring diagram I drew for my understanding

Click the image to open in full size.

Is is a good connection and also how much power will each speaker drive, I assume the power for each speaker will be somewhere between 30-40w, but isn't this less that the min requirement of each speaker as per the specs......or will it just work fine.

Last edited by darrensound; 31st October 2017 at 02:31 PM.
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Old 31st October 2017, 03:29 PM   #2
W0J0 is offline W0J0  United States
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Join Date: Dec 2014
First thing to do is figure out if these speakers are already set up in a 70v distributed configuration--they'd almost have to be if they're currently all wired in parallel.
After you determine the configuration, you'll want to then purchase an amp capable of properly driving the speakers.
6*20(min tap)=120 watts per channel; 6*60(max tap)= 360 watts per channel and you still have no headroom built in.

8ohm direct:
2 series groups of 3 in parallel = ~6 ohm
Your amp outputs ~180 watts per channel @6 ohms. So 180/6 =30 watts per speaker

Sensitivity: 1W=91dB @1m therefore: 2w=94dB, 4w=97dB... 32watts=106dB@1m + 7.5dB for the array gain = 113.5dB. Sounds ok, right? But when you apply the inverse square law, 2 meters reduces SPL to 107.5dB, 4 meters is down to 101.5dB and that is max amp output before distortion. So it's easy to see why you're getting undesirable results.
The spec sheet calls for 120 watts program per speaker and 240 watts max so you could easily connect an amp capable of 720-1440 watts per channel into 6 ohms and not be in danger of over driving your speaker arrays. It would give you plenty of headroom to stay out of the distortion range of the amp.
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Old 31st October 2017, 04:28 PM   #3
conanski is offline conanski  Canada
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What setting are the speakers on now? Looking at the available options I can only see bad mismatches which will result in either something close to a dead short at the amp or only about 16w to each speaker which certainly isn't going to be enough.

There is no minimum required power input level for a speaker system, but you may need more power than you have now. More on that later.

Yes you can use a multimeter to measure the DC resistance of the speaker cluster, that number will be a little lower than the AC impedance but we can deduce that from the meter reading.

What you are dealing with there is a distributed sound system, this allows connection of multiple speaker systems in parallel via matching transformers which presents a safe load to the amplifier and produces consistent output from the speakers.
If you revert to a standard series/parallel connection format you have to take into account the added impedance(resistance) of the various lengths of speaker wire between each speaker and the amp as that is likely undersized for this format and will contribute significantly to the load, and that will result in uneven output from the speakers and less power delivered than the calculations show.
So what I'm going to suggest is that you use the distributed format with all 12 speakers in parallel connected to the amp in bridged mono, and set the speakers on the 40w tap. That presents an 8ohm(7.58) load to the amp and provides roughly 40w(43.7w) to each speaker which is about the max available from this gear... I don't suggest running amplifiers at 4 ohms bridged for long term reliability.
If that produces enough clean sound for you then you're good, if not then you need a more powerful amp.

Last edited by conanski; 31st October 2017 at 04:35 PM.
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Old 2nd November 2017, 08:26 AM   #4
darrensound is offline darrensound  India
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Hi Conanski,

The tapping is set to Direct 8 Ohms on all speaker, So it looks like I will need to get a more powerful AMP....

Then I can connect the first 6 speakers in parallel to channel 1 followed by the other set of 6 speakers to channel 2.....so If i connect these 6 speakers in parallel, how many ohms will it come to and will the following amp work....

DJA 2500
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Old 2nd November 2017, 03:45 PM   #5
conanski is offline conanski  Canada
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No you cannot connect six 8-ohm speakers in parallel on any amp, that would be a 1-ohm load. The only way this will work is utilizing the transformers in the speaker. Reset all the speakers to the 40w tap and connect them all to the bridged mono amp, if you are unsure about setting up bridged mono or how to connect the speakers just ask.
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Old 4th November 2017, 07:06 AM   #6
JMFahey is online now JMFahey  Argentina
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Bridged out int 8 ohms means 64V RMS ... meaning 70V nominal by any realistic account.

So since you have a 70V MONO 500W system, setting all speakers to 70V in 40W power they will collectively pull 40x12=480W RMS ... perfectly matching the speaker output, power, voltage, impedance.

So I am adding nothing different to what conansky suggested, just adding the Math confirming it.

What about the current system?
Looks poorly setup and matched; if following the above suggestions it should be quite competent.
Design/make/service musical stuff in Buenos Aires, Argentina, since 1969.
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