PA ohm question

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Nigel
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PA ohm question

Post by Nigel » 06 Oct 2018, 14:59

Hi guys,
I recently did a mission trip to the Solomon Islands. Part of the trip was working with a community who own a PA, I suspect it was donated at some point and they only got very basic instructions on how to use it. . I've convinced them of the need to use DI boxes and am bringing 4 over there with some other bits and pieces that will help them out.

They are running 2 speakers from a Peavey PA. ( https://assets.peavey.com/literature/manuals/xr800f.pdf ) but have no foldback speakers. The musicians can't hear much of what is going on. One speaker goes from speaker left and the other from speaker right on top of page 4 of the manual.

The speakers are Peavey branded as well. However, there is nothing on them saying the ohms.

I'm heading back in a month or so. How do I tell what ohmage the speakers are if its not written? There are 2 other PA speakers not being used (old ones put in the corner when the PA attached to them blew up). Assuming the current peavey speakers are 8ohm and the amp can run down to 4 ohm. Can I run the other 2 speakers from the same outputs (assuming they are 8ohm?)

I'm just looking for some options for them and I suspect they have everything they need to create their own foldback. I only know enough about ohms to keep my bass amps out of trouble and this is just above what I understand. Any advice from people who know would be appreciated.

cheers,
Nigel
Mailloux Custom "Lucky 13", EA Doubler, Barefaced Midget 110, Mojo-Sonic 1x10.

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petersemple
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Re: PA ohm question

Post by petersemple » 06 Oct 2018, 15:47

If you have a multimeter you can measure the DC resistance of the speaker. From memory, most 8 ohm speakers will measure about 6.5 ohms on DC resistance. That is not perfect, but not a bad test. Best way to find out for sure is to get the model number of the bin and look up the specs.

If the speakers are all 8 ohm, and the amp will handle 4, then you can run 2 speakers off one amp channel. If you don't need to run in stereo, then you could also use one channel for front of house and one for foldback, which would give you the option to have a different foldback level and maybe even mix (use the pan controls on each channel to control the relative amount of each channel in FOH and foldback).
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Petebass
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Re: PA ohm question

Post by Petebass » 06 Oct 2018, 15:50

You've got a couple of options:-

1) There is usually a sticker on the magnet of the speaker which tells you the ohms. All you have to do is remove the front grill, then unscrew the speaker, take it out and have a look. You should be able to do this without having to do anything to the wiring.

2) Do you have access to a multimeter? If so, set it to measure ohms. Plug in a speaker cable that has either bare ends or a quarter inch jack. An 8 ohm speaker will measure at about 5.something ohms, and a 4 ohm speaker will measure at 2.something ohms.
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Nigel
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Re: PA ohm question

Post by Nigel » 06 Oct 2018, 17:03

So, from what you both said. I need to look at the speakers. If they are 8ohm I am fine. Like Pete no 1 said I can run front of house from the 2 left side outputs and foldback from the 2 right side outputs, which seems to be what the illustration suggests on page 14 of the manual.

I think the maintenance guy at work, who also plays guitar, would have a multimetre I could borrow and he could show me how to use it. I'm looking for 5ohms or more and it is an 8 ohm cab.

Thank you, Petes.
Mailloux Custom "Lucky 13", EA Doubler, Barefaced Midget 110, Mojo-Sonic 1x10.

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Nigel
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Re: PA ohm question

Post by Nigel » 06 Oct 2018, 17:16

If he looks at the sticker on the back of the 15 inch speaker, will the ohm of the horn impact anything at all?
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david1234
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Re: PA ohm question

Post by david1234 » 06 Oct 2018, 17:39

I'd get a $20 multimeter and test with that. The impedance of an 8 ohm speaker isn't going to read 8 ohms (think 4 to 6) as you're only measuring the DC resistance part, but if you get 2 speakers of about 5 ohms DC you're good to go.

Alternatively, buy them a small active monitor for foldback.
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Nigel
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Re: PA ohm question

Post by Nigel » 10 Oct 2018, 10:04

Thanks, I went to jaycar the guy was good he showed me how to measure the ohm resistance with a speaker in the shop and then on my bass cab. Seems somewhere between 5.5 to 6.5 is normal.
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Petebass
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Re: PA ohm question

Post by Petebass » 10 Oct 2018, 12:07

Nigel wrote:
06 Oct 2018, 17:16
If he looks at the sticker on the back of the 15 inch speaker, will the ohm of the horn impact anything at all?
You can safely ignore the the horn. I can bore you with the science if asked, but I'll spare you other than to say that the horn will never actually operate at anything remotely close to it's rated impedance. The crossover will prevent that from happening and of course, there will always be a crossover or some sort because no crossover means blown horn.
* 2 Stingray 5's (one heavily modded), Ibanez SR805, Ashbory, + more...
* Carvin B1500, Epifani PS1000, Ashdown Superfly amps.
* Lots of DIY neo/lightweight cabs including one made from composites http://www.ozbassforum.com/viewtopic.php?f=5&t=2546

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noplanb
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Re: PA ohm question

Post by noplanb » 10 Oct 2018, 19:31

To confirm: - from p4 of the maual
"Two cabinets can be connected to each amplifier, as long as the combined impedance of the cabinets is not less than 4 ohms. (i.e., two 8 ohms cabinets in parallel = 4 ohms, four 16 ohms speakers in parallel = 4 ohms, etc.) Do not use the Bridge Output (5) while using these outputs "
I'd suggest never use bridge mode at all! - too easy to make a mistake - tape over the middle bridge output socket.
Great idea to use one amp for FoH and the other for 2 foldback speakers - the manual seems to show this too.
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Nigel
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Re: PA ohm question

Post by Nigel » 11 Oct 2018, 09:09

Thanks guys. A little under a month before I go, starting to feel real and a bit scary. This aspect of my trip is a sideline issue. I’m mostly teaching teachers how to teach. But the little church will have 3 or 4 DI boxes and using their existing equipment will probably have foldback for the band.
Mailloux Custom "Lucky 13", EA Doubler, Barefaced Midget 110, Mojo-Sonic 1x10.

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