..linking FOH speakers and monitors

E

earworm

New member
hi there,
got a little question here,
i'm the PA man of a small pop band, they got their own (junk) equipment and my job is to make them sound better,
the incredible BAD mixing desc only has speakon outs,
those two cables go the the FOH speakers,
then we got two monitors onstage, they are connected to the FOH speakers (with jack or xlr)
now , i'm not 200% sure but i think the stage monitors are just passive,
so they take the 'power' from the FOH speakers,

can i put a graphic EQ between the FOH speakers and those monitors?
or is there any current going from FOH to monitors? i guess not...
just want to make sure i don't blow anything up ...
 
nothing goes betwwen an amp and the speakers. NOTHING....EVER!!

You need another amp to run monitors. There should be a monitor output on the board (or you can use an aux send) Use that to feed an amp. Connect that amp to the monitors. You would put the EQ between the monitor send and the amplifier.
 
earworm said:
now , i'm not 200% sure but i think the stage monitors are just passive, so they take the 'power' from the FOH speakers,

can i put a graphic EQ between the FOH speakers and those monitors?
or is there any current going from FOH to monitors? i guess not...
just want to make sure i don't blow anything up ...
Click to expand...

farview is right. However I thought I'd address a couple of points.

The monitors don't 'take' power from the FOH speakers. It's kind of like plugging in another light bulb in a wall socket that already has one lamp in the other jack. It's a parallel connection, so the overall setup will draw more current from the board. The question is can the board handle that? It probably can, but you need to learn how to calculate ohms to be sure.

For example, the speakers & monitors are probably 8 ohms, so each side with a main & monitor will be 4 ohms. If you board outs are labeled 4 ohms or less minimum, you won't break anything.

Not breaking things is not the only issue. With the mains and monitors wired together, you can't control EQ and volume independently. Thus farview's advice to get another amp for the monitors.

If you are on a very tight budget, you can run the mains off the left channel and monitors off the right. I did that in the bad old days :o but it's a pain in the butt to mix, and not intuitive at all if somebody else needs to take over the mix. And you would still not have monitor EQ.
 
ok thanks for the input,
i know that the best thing to do is use an AUX for the monitors, togethjer with another amp,
but we don't have that budget yet, thats my plan, to buy an amp and eq so i can control the monitors
but i'll have to wait untill newyear cause now we're saving up money to buy 20 meter speakon cables, i already got my stagebox, but i can't stand in the back of the club yet since the speaker cables aren't long enough

we're upgrading step by step, next year we'll sound good i hope :rolleyes:

now of course the question:
what amp would you suggest?
when i rent PA equipment i work with nexo, crest, i've had crown in my setup, but thats over our budget,
i just mean, what brands should i avoid? like behringer and stuff,
i see there are lots of cheap PA amps on ebay, what are the "dangerous" things i should look out for?
i guess the cheap ones have a low signal to noise value?
we got FOH speakers of about 400watt each (only for vox,bass,keys,gut)
 
I don't know what is available where you are, but if you can find a peavey cs800, you will be in good shape. For the money, these things are indestructable. They have made them for almost 30 years, if I were putting a PA together now, I would have several.
 
thanks,
well seems like peavy does make some nice PA stuff then,
i remember that a while ago i was looking up info about small PA speakers and the peavy ps15 (or something similar) received some decent comments

and when i check out ebay.de (germany) i see ALOT of
"Hollywood" amps, they seem to be the new hype of cheap pa-amps,
anyone heard those machines work?
 
I would highly recomend either QSC PLX, or RMX series amps. these amps are very fairly priced, and are rock solid. I've worked with the peavey CS series often, they are extremely heavy, and their transient response is lacking.

i don't think anyone's made this point yet, but with 20m speaker cables, you'll be losing probably about 20-30% of the amplifier's output in the wires! it's very common practice to place the amplifiers at the stage, where your speaker cable runs will be as short as possible. line and mic level signals are much more efficient when it comes to traveling long distances.
 
Cave Dweller said:
i don't think anyone's made this point yet, but with 20m speaker cables, you'll be losing probably about 20-30% of the amplifier's output in the wires! it's very common practice to place the amplifiers at the stage, where your speaker cable runs will be as short as possible. line and mic level signals are much more efficient when it comes to traveling long distances.
Click to expand...

Excellent point. Shorter is better. More resistance eats up headroom, and makes you push your amps harder.

A couple things about this, the gauge of the wire makes a difference, and so does the load impedance. Lower gauge wire loses less power, and higher impedance loads cause less power loss into the cable as well. So 12 gauge loses less than 16 gauge, and you will lose more power into the cable if you have a 2ohm load than if you have an 8ohm load.

80 feet of 12 gauge loses 3% into an 8ohm load, but 12% into a 2ohm load.

80 feet of 16 gauge loses 8% and 26% into the same loads.

Here is a table from Tracertek that lays it out pretty well.

cable loss chart


The other thing to note is that the losses for the most part are pretty insignificant in terms of volume. If you lose 50% of your power into the cable, that really only translates to 3db less volume from your system. Any loss under 25% knocks less than 1 db off your volume.
 
interesting replies,
but i know that cable wires should be as short as possible,
BUT, here's the BUT,
we work with a mixing desk with a built in amp,
there are only speakon outputs,
so i can't run 2 XLRs to the stage, to an amp... the mixer is the amp :'(

now, another question about those impedances, i'm not good in that...

i thought that:
to our mixer i can connect a pair of 8 ohm speakers and it will work fine...
but now we link our 2 monitors directly to the FOH speakers,
does that mean that all 4 speakers should be 4 ohm, so in total they're 8 ohm?

today i took another look at the FOH speakers and saw that they're 8 ohm,
what happens if i connect 4 ohm speakers to that? can the amp handle this?

i think its easy to see that i'm confused, am i right to say that
"the more speakers you connect to an amp, the lower the impedance has to be", and all speakers must have the same impedance?

when you take a look at specs of amps you read things like:
400watt at 4 ohm
200watt at 8 ohm (you get 8 ohm when u use 4-4ohm speakers,no?)
and are 8 ohm speakers louder than 4 ohm speakers?

:o :o
 
earworm said:
interesting replies,
but i know that cable wires should be as short as possible,
BUT, here's the BUT,
we work with a mixing desk with a built in amp,
there are only speakon outputs,
so i can't run 2 XLRs to the stage, to an amp... the mixer is the amp :'(

now, another question about those impedances, i'm not good in that...

i thought that:
to our mixer i can connect a pair of 8 ohm speakers and it will work fine...
but now we link our 2 monitors directly to the FOH speakers,
does that mean that all 4 speakers should be 4 ohm, so in total they're 8 ohm?

today i took another look at the FOH speakers and saw that they're 8 ohm,
what happens if i connect 4 ohm speakers to that? can the amp handle this?

i think its easy to see that i'm confused, am i right to say that
"the more speakers you connect to an amp, the lower the impedance has to be", and all speakers must have the same impedance?

when you take a look at specs of amps you read things like:
400watt at 4 ohm
200watt at 8 ohm (you get 8 ohm when u use 4-4ohm speakers,no?)
and are 8 ohm speakers louder than 4 ohm speakers?

:o :o
Click to expand...


The upshot is, you can run a hundred feet of twelve gauge and be fine. Happens all the time. Ever been to a big show, with speakers 40 feet off the floor? Sometimes they lift the amps, sometimes they are self-powered, but a lot of times they run gobs of speaker wire from under or behind the stage up to those things.

Impedance, short version:

Speakers together in series makes impedance go up.
Speakers together in parallel makes impedance go down.

When you hook PA speaker or monitor boxes together by their in/out jacks, or if you hook two boxes the same channel with different cables, they will be in parallel, because that is how they are wired. It's just how it is, accept it.:p

Z=impedance

Series speaker connection:
2+2=4
4+4=8
8+4=12

Ztotal= Z1+Z2+Z3........

Parallel speaker connection:
8+8=4
4+4=2
8+4= 2.6

Ztotal= 1/(1/Z1+1/Z2+1/Z3....)

This reads "One divided by one over Z1 plus 1 over Z2, etc."

Sounds hard, but isn't. It means you are going to flip the numbers after you add them.

8 ohms paralleled with 4 ohms.

1/Z1= 1/8
1/Z2= 1/4

Basic math, 1/8+ 1/4= 1/8+2/8= 3/8

Flip 3/8 (also called the reciprocal)= 8/3
8/3= 2.6 ohms. 8ohms paralleled with 4ohms gives 2.6ohms.

Two of the same impedance in parallel has a shortcut, just divide by two. The math works the same if you do it out like the above. So 8+8=4ohms.8ohms paralleled with 8ohms gives 4ohms.

1/8+1/8=2/8
The reciprocal of 2/8 is 8/2, which is 4.


Unlike impedance or more than two means you need to do the math.

Now to your questions, which you will be able to answer yourself after awhile thinking and some experience.

Does your amp say 4ohms total per side? If it does, you are all set, if all your boxes are 8ohms. Two 8ohm paralleled per side will give 4ohms per side, following the rules.

Your mixer should have some kind of spec like that on it. If it is a mono mixer, it will say something like "4ohm total load". If it has two amps, it will say something like "4ohms per side or channel". If it just says 4ohms total, you might be pushing it, as your 4 8ohm boxes together make 2ohms.

Whichever it is, those totals will be the numbers you don't want to go below.

So, and 8ohm and a 4ohm make 2.6ohms. If the amp isn't rated for 2ohms, it will get very hot and maybe burn up, literally. :D


Amp specs:
Amps use current and voltage to make power. Impedance restricts current. The higher the impedance, the less current flows.
The lower the impedance, more current flows, the amp can make more power.
Hence, the power ratings go up as the impedance drops.

At some point, the amp can't safely let more current flow, and burns up. The speakers act as a safety valve on the current flowing from the amp. This is why too low of an impedance burns amps up. Never go below the lowest impedance rating on your amp.


Are 4ohm speakers louder than 8ohm speakers?

It depends. Speaker loudness is related to two specs. Power rating, and sensitivity.

Power rating is pretty obvious. It is how much power the box can handle.

Sensitivity is a bit more tricky. It is how much loudness per watt the speaker can make. It is usually referenced to one watt measured at one meter away. So you will see a spec like:

Sensitivity 98db, 1W/1M.

So the speaker will give 98db SPL at one meter when fed one watt.

Another speaker at 88db, 1W/1M, will only be half as loud with one watt.

This can be totally independent (mostly) from impedance. The 98db speaker could be 8ohm, the 88db speaker 4ohm.

Different impedance boxes allows you to get as much power from the amp as possible, in different circumstances: They let you get the most power out of the amp using a single box, if that is what you need, or to use multiple boxes for better coverage, if that is what you need.
 
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