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Speaker wiring/amp matching - 2x(3x8ohm)
Old 3 weeks ago
  #1
Here for the gear
Speaker wiring/amp matching - 2x(3x8ohm)

Hi,

I'm building a 3-way PA and am wondering how to match power to my mid-range speakers.

2 cabinets, each with holes for three 8" drivers.

Likely going to load them with Eminence Alpha 8As - 125watts and 8ohms apiece (250watts peak).

I know that if I wire 3 of those speakers in each cabinet in series, the total impedence will be around 24ohms per cabinet.

At that strange rating, my guess has been that I'll need a power amp that pushes 750watts/side @ 8ohms to power both cabinets in stereo.

Is that correct? If not, what's the right answer? What wattage should I consider at 4ohm/side?

Thanks for the help.
Old 3 weeks ago
  #2
Gear Head
 

Why 3 drivers? Why not 4? There is no good solution with 3 drivers, in parallel they present a 2.66ohm load which isn't great for budget amps, in series it's a 24ohm load which increases the size of the amp needed. You can't really do 1 driver in series with the other two in parallel either because the single driver will limit output of the whole set.
Old 3 weeks ago
  #3
Lives for gear
 

Amplifiers are basically designed to be voltage sources. An amplifier providing 750 watts @ 8 ohms is outputting about 77 Vrms, and (if a good design, with good low-impedance handling characteristics) will supply 1500 watts @ 4 ohms, or 375 watts at 16 ohms. More generally, the actual power output when driving an n ohm load will be about 8/n times the power output when driving an 8 ohm load. For a 24 ohm load, this works out to one-third the power output; and your 750 watt amplifier would supply 250 watts total to your three drivers.

As paul points out, there's no easy good way to effeciently interface three 8 ohm drivers to an 8 ohm or 4 ohm amplifier output.
Old 3 weeks ago
  #4
Lives for gear
 
MIKEHARRIS's Avatar
solder a penny in between the 3 drivers...but thats just my two cents.
As stated...3 is odd man out
Old 3 weeks ago
  #5
Gear Head
 

What kind of system are you planing to use this in?
I remember some folks in the 80/90's would use a mid range cabinet with one speaker pointing forward and the two others out to the sides, in order to get better dispersion.

a bit like this :

https://i.pinimg.com/originals/59/b9...e948b17d7b.jpg (just with one more in the center)


If you are connecting this three speakers i parallel to get 2.66 ohm you can use a passive crossover filter to get a little higher impedance. If you make a 6db high-pass filter at let's say 200hz, the series condenser will give you twice the impedance at the crossover point (and higher bellow 200hz) if you also add a series coil to cut of at let's say 1500 hz, that will also increase the impedance of the system.

Before you calculate the low pass condenser look at the eminence chart to get the correct value and impedance at the high pass frequency. Also make sure to use a bi-polar condenser that can handle 160 volts and up.*

https://www.eminence.com/pdf/Alpha_8A.pdf

Even with a passive filter like this, you can still use this speakers in a tri-amped system.
Old 3 weeks ago
  #6
Here for the gear
Awesome! Thanks for the help!

I'm building the system from spare stuff I have lying around my band's practice space. We have 2 old Kustom K300 tuck n roll towers from the 70s - which each have holes cut for two 12" speakers, and three 8" speakers (so, 10 speakers total between the two). I'm planning to use the 8" sections to use as the MF for the full PA, and drill a second jack to break out the 12" speakers for guitar amps.

I like the sound of a 6x10 bass cab a lot better than a 4x10. So it made sense to me to try and mimic that setup.

Subs are Peavey Black Widow 1505-5 16ohm built into cabinets each with a JBL 2445J HF driver attached to a selenium horn.

Plan is to have a separate power amp for HF, MF and LF, and then run guitar amps directly into the 12" speakers. Bass/keyboard/vox would be run through the 3-way system.


So, for the MF section, do I understand correctly that I would need 1125watts/side @ 8 ohms to drive it? Would something like an iNuke 6000 work for that (1500watts/side @ 8ohms)? Eminence says the Alpha 8A handles up to 250watts, so it seems like if I'm maxing out that amp, I still wouldn't be blowing anything up.
Old 3 weeks ago
  #7
Gear Head
 

Holy hell... I don't know where to start.
You say the subs have a PV 1505 with a big ole JBl compression driver? Is that a single 15 or double 15 plus horn? Those are NOT subs but it could be a kick ass FOH PA cabinet so why don't you biamp them and use it that way. With a 500w 2ch amp and some DSP processing you would be surprised what can be done with these components.
As for the Kustom boxes I don't know what to tell you, all the drivers in those things would have originally been wired together in some configuration and driven with a fullrange signal, and pretty much the only thing they did well is get loud with not much power... that was the technology of the 70's. Trying to make the '8's and 12's do different things in the same box is not going to work very well for several reasons and even if you do figure something out you just have more midrange output, what you reallly need are subs.
Old 1 week ago
  #8
Here for the gear
I agree with Paul, given those Black Widows and the horns, you are ready to go. In my view none of these power and impedance questions make much sense before talking of balancing dBs and that also means crossover bandwidths. By the way, 2 drivers will only sound marginally different from 3.

I suggest the best way to design a rig is start with the sound you want and work backwards to the kit. So you want 132 dB flat from 40Hz to 20kHz. Measure what the big beasts do, then use the 8 inchers to fill in any gaps around the middle (if there are any). Or you could use the in independent cabs as fills or even stage monitors.

If you really want to wire three together, I think best use parallel and an amp that is known to be good for 2 ohms (not just claimed). Like Drew says, it's not nice and I think if a X-over has enough Z to make up the difference then it will cook.
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