Required power

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Hi guys,

Pretty new to PA systems - have always used systems provided by venues so far, so not really had to think about it, but I've just been given some speakers. I've read info about the amp power required for speaker outputs, but this has thrown me a bit.

I've been given 2 x 500W, 4ohm tops and 2 x 250W 8ohm subs. One of the bass bins has a faulty crossover, so the previous owner was running separate cables to each cab, rather than to the sub and then to the top. I will look at replacing the crossover at some point.

Using the "amp with twice the power of the speakers" theory, how would this work with running the speakers in parallel, and the difference in impedance?

Also, would I need an amp with 4ohm or 8ohm output?

It might be a stupid question, but trying to Google this has not been successful.

Thanks in advance.

Si
 
Rule of thumb in PA systems;
Sub power is four times treble power, that is how our ears work so;
250W Sub means 125W bass/mid and 60W treble.
Impedances are evened out when using passive crossovers. See Crossover Detail | Eminence Speaker
for a simple crossover. 8R is quoted so halve the impedance means double the crossover point. 250hZ becomes 500hZ.
Two 4R speakers in parallel will give the amplifier a 2R load but using a passive crossover the 4R load is achieved by effectively disconnecting one half of the system when the frequency goes above or below the crossover point. Roughly speaking.
 
Thanks Jon.

So if I was to run a 4ohm and 8ohm in parallel, I would get 6? Would I then need to use an amp with an output impedance of 6 or less, or would it need to be 4, due to that being the lowest impedance speaker?

Sorry for the basic questions - as I said, new to this, and I've not had enough coffee yet :)

And how about the amp power? With a 500W and 250W on each side, would that equate to 750W, or is there more to it than that?

Si
 
Parallel is (r1*r2)/(r1+r2) so 32/12=2.67 ohms
Usually more amplifiers is less expensive than one 2 ohm capable amplifier of higher wattage. Lower power amp to one speaker can use cheaper speaker wire, often 14 ga, wheras all on one circuit requires 12 or 10 ga speaker wire. Use Y cables to drive more than one amp with a single analog source.
Us diy types on a budget often buy well used PA amps, and repair them. Peavey equipment comes with generally available schematic diagrams. There is a part # crossover table to industry numbers for 80's 90's amps. Behringer copywrites schematics and sues those that distribute them. Crown QSC are repairable but schematics are harder to find.
 
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I've been given 2 x 500W, 4ohm tops and 2 x 250W 8ohm subs. One of the bass bins has a faulty crossover, so the previous owner was running separate cables to each cab, rather than to the sub and then to the top. I will look at replacing the crossover at some point.


Do not replace the passive crossover in the subs.. remove them both as they generally perform very poorly. The proper way to deploy a system like this is with an active crossover, this device goes before the amplifiers or is included inside the amplifier so you will need at least 2 amplifier channels but preferably 3-4 depending upon specific speaker models.



What models are your speakers?



For any kind of modern music a PA system needs 2-10 times more subwoofer output than mid/high output, actual power needed for that is very much system specific but in general you can see it is not common for subs to have a lower power rating than mains.

Speaker power handling is also very much dependent upon program material, live bands have the highest dynamic range and therefore require the biggest amplifiers, popular recorded music has the lowest dynamic range so you would be well advised to limit amplifier power to the speakers long term average rating(RMS) or less.
 
Conanski


I think he means a Zobel network instead of the crossover.
I don't know passive subs with crossovers.


If the drivers inside have less than 95 dB efficiency like Chinese crap You sure will need more amplifiers and speakers. I call them silent speakers.
My drivers have 98dB (18'' subs ) all others are in the 100-105 dB range. Compression drivers are often padded because they have higher efficiency 110-120dB. Horns will help a lot on the subs
 
Hi guys,

Pretty new to PA systems - have always used systems provided by venues so far, so not really had to think about it, but I've just been given some speakers. I've read info about the amp power required for speaker outputs, but this has thrown me a bit.

I've been given 2 x 500W, 4ohm tops and 2 x 250W 8ohm subs. One of the bass bins has a faulty crossover, so the previous owner was running separate cables to each cab, rather than to the sub and then to the top. I will look at replacing the crossover at some point.

Using the "amp with twice the power of the speakers" theory, how would this work with running the speakers in parallel, and the difference in impedance?

Also, would I need an amp with 4ohm or 8ohm output?

It might be a stupid question, but trying to Google this has not been successful.

Thanks in advance.

Si

Since your mains are 4 ohms you need 1 stereo amp for that.
Subs using another stereo amp you can;
A; run each sub off each side of the amp
b; run both subs off one side of amp 4 ohms, use the other side for a monitor or two.
Power wise, 250 watts is considered small today but realistically anything above that power will be wasted as heat. Your EV speakers probably start out at 100db at 1 watt and they may max out around 130db max.
There is math to prove it but I never learned it.

That said, I run a samson 2800 amp 900 watts each side @ 4 ohms one side to a pair of mains and one side to a pair of subs. It is stupid power I bought before I learned. Samson SX-2800, it idles most of time.

"One of the bass bins has a faulty crossover, so the previous owner was running separate cables to each cab, rather than to the sub and then to the top. I will look at replacing the crossover at some point."

should be separate cables to each cab, very few commercial PA systems have subs that can drive tops, there are some out there but usually the speakers are passive (so you need a crossover) or self powered no cross over needed. If you need a crossover, look at the DSP units because you will get a crossover, plus other features like limiters, gates and compression.
 
I would leave the crossovers out of the subs and wire the speakers directly to the speakons and use some external processing unit like the Behringer CX3400 or a DBX Driverack.


I have a wedding party DJ friend who has two 15 inch cheap Peavey subs and two noname 15 inch tops and he wires the Amp outputs to the subs and from there to the tops and the subs don't have crossovers. All runs from a noname amp, and of course there is frequency overlapping but he uses the mixer EQ to tweak it a bit. Sound is not so bad. Try !
 
500W+250W is indeed 750W, but perceived loudness is another thing...You don't perceive more "half power"


Say You have one friend shouting, another friend comes in and shouts together. Do You hear twice the loudness ? No off course !!

You need 10 times the power to double the volume to the ear.
So 10 watts is twice as loud as 1 watt.
 
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