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-   -   wiring 3, 8 OHM speakers into one amp? (http://www.diyaudio.com/forums/pa-systems/232429-wiring-3-8-ohm-speakers-into-one-amp.html)

SillySounds 20th March 2013 10:24 AM

wiring 3, 8 OHM speakers into one amp?
 
Hello there folks

Sorry this might be a bit of a newbie question but im planning on building a sound system for outdoor use with a fairly limited budget (500 - 700 ish). The system i was hoping to build would comprise of a bass bin (MBP-46) and two, two way pa speakers (midtops).

My question is; can i wire 3 8 Ohm drivers into one amp?

can i maybe wire the midtops in parallel in one channel and the 8ohm bass driver into another or would that present a mismatch in what load the amp see's? (as i said im not very clued up)

or would there be a way of wiring the 3 drivers together so they would be on the same circuit or would that just be impossible?

Or am i just going to have to bite the bullit and buy 2 8 ohm bass drivers (2 bins) and buy an amp for them and then have another amp for the midtops (bi-amping). my problem is that the budget has to also go on; shelter, lighting, transport, tools, ect.

Any advice or ideas would be greatfully welcomed as im at a loss to keep costs down and make this work.

AndrewT 20th March 2013 11:21 AM

and water proof against the english weather !

SillySounds 20th March 2013 11:42 AM

As i said some of the budget has to go on shelter, I'm buying a 20 x 30 m tarpaulin sheet that is to be pulled over a frame i made last year.

But then again that isn't any guarantee considering the skitzo weather we seem to have.

JMFahey 20th March 2013 12:04 PM

If your amp can drive 4 ohms per channel, yes.
2 midtops to one, sub to the other.

AndrewT 20th March 2013 12:09 PM

Quote:

Originally Posted by JMFahey (Post 3419006)
If your amp can drive 4 ohms per channel, yes.
2 midtops to one, sub to the other.

and capability of driving 4ohms is NOT the same as stable into a 4r0 test load.

A 4ohms capable amplifier should be able to drive a 2r0 test load with only a small drop in output voltage (small drop <1dBV and better if the drop is < 0.6dBV) and preferably also able to drive a 1r3 test load for a few seconds.

SillySounds 20th March 2013 12:11 PM

Thank you JMF, so as long as the amp can run minimum 4 ohms per channel it doesn't matter if its say; 4 ohm one channel and 8 ohm on the other. Its because i can source a 15 inch driver in 8 ohm quite cheaply in my locality, i just needed it confirming so i wouldn't be blowing anything up :). Thanks again for the reply!

SillySounds 20th March 2013 02:06 PM

Quote:

and capability of driving 4ohms is NOT the same as stable into a 4r0 test load
Andrew - what do you mean when you say its not the same as stable into a 4ro test load? does this affect my loading the amp in different impedances on different channels. would this make the amp unstable?
Sorry about all the questions but also what do you mean with "test load"?

Jsixis 21st March 2013 01:53 AM

if you build a crossover your amp will only see 8 ohms

AndrewT 21st March 2013 10:01 AM

Quote:

Originally Posted by SillySounds (Post 3419128)
Andrew - what do you mean when you say its not the same as stable into a 4ro test load? does this affect my loading the amp in different impedances on different channels. would this make the amp unstable?
Sorry about all the questions but also what do you mean with "test load"?

Some amplifier retailers/manufacturers tell the truth and do not need to exaggerate power claims to sell their product.

Other retailers/manufacturers put profit above all else and sell rubbish that if truthfully specified would never sell to a knowledgable customer.
These types of sellers will use many ways to describe their Power Amplifiers to make them look as good as or better than those available from competitors.

An example is an 8ohms capable amplifier that is properly specified as a 100W into 8r0 @ 1kHz with distortion <0.1% when the mains supply is @ rated voltage.
This amplifier will be able to drive a reactive speaker because the manufacturer has specified it as suitable for 8ohms.
A consequence of the ability to drive a reactive load is that the same amplifier can also drive a test load that has a much lower resistance than the 8ohms of reactive speaker.
A maximum target for a resistive "test load" would be half the 8ohms, i.e. 4r0. Any half decent properly designed 8ohms amplifier must be able to drive a 4r0 until the heatsink temperature reaches some manufacturer determined limit. A typical power into this half load would be at least 150W into 4r0.
A very good amplifier that can drive very reactive 8ohms speaker will easily be able to drive a 3r0 test load. and achieve at least 170W into 4r0.
An excellent 8ohms amplifier will will drive a 2r0 test load for at least a few seconds before reaching that temperature limit. and acheive at least 180W into 4r0.

If you buy a genuine 4ohms capable power amplifier then it MUST be able to drive a 2r0 test load to at least +50% power, for much more than many seconds and preferably drive 1r5 and 1r0 test loads for a few seconds.

Does that make some sense?

chrispenycate 27th March 2013 11:24 PM

I agree that an eight ohm nominal speaker is likely to drop below eight ohms at some frequencies, so the amplifier has to have a little reserve in the power supply and output stages, but several amp protection circuits current limit, not just temperature limit. In which case, the amp will not drive lower impedances at full drive, but will quite cheerfully deliver rated power into half impedance, and a bit less as the impedance drops further.

Don't forget that to get the best out of the system you'll need a crossover to remove the high frequencies from the sub, and lows from the two ways. Not complicated, and greatly improves the performance of both the amp and the speakers.

Have you already got the amp? It would be possible to use two cheaper two channel amps, one (in bridge) for the low end and the other driving the high end in stereo. You'd need to mono up the low frequency signals, but that's no problem, even passively (no, for PA the stereo is not that important, but a fair amount of recorded music was not prepared to sound good in mono, with phase cancellation and odd delays.


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