PA question

hello there .

disconnect each speaker and test across each driver terminals for 16 ohm .

if the test reveals open circuit or a dodgy reading the driver will have blown .

as you connect speakers in parallel the impedance halves .

so 1x 16 ohm speaker presents a 16 ohm 150w load on the amp
2x 16 ohm presents an 8 ohm 300 w load
4 x 16 ohm presents a 4 ohm 600w load

amplifiers usually have more than one channel

if its split so 3 speakers per channel its about a 6 ohm load per channel

if its all 6 on one channel its 2.66 ohm load .

not good for an amp thats rated spec is for 4 ohm .

the idea is not to go below this rating .

no mention of the amps power rating

you could start by dissing the speakers and seeing if you get an ohms reading on them. should read somewhere between 3-20 ohms each. if open circuit, then its either the cable or the speaker.

cheers for the info!
I'm not to sure about the amps power rating. Didnt manage to find any markings on the amp

PA systems work on a 100v rail to allow multi speakers without effecting resistance. So be careful here..you will get a shock..!!..Each speaker will have a tapped transformer installed to allow wattage selection, usually 0.5w, 1w, 1.5w, 3w, 5w, 10w. So there is no reason to measure resistance here, as you will be measuring the primary winding of every transformer on the 100v line.
Easy way to check this, is dead testing, dissconnect the 100v speaker line, and place a cable signal tone tester, same as you use to trace broken cables.....once connected, you will hear a very low tone in each speaker, walk around the school and find the speakers with no tone and investigate, or link out to test speakers further down the line. We use a cheap tone tracer from maplins, £15 works well upto 30 speakers on one 100v line

Tazz it sounds like it's all low-impedance - he hasn't mentioned anything about a 100V line output from the amp but did say the output was stated as '4.8Ω'. I think this might be an error and should read '4 - 8Ω', i,e, 4Ω minimum. As such it is as likely to be a domestic style amp as a commercial PA amp.

Luke, some pics would be helpful, especially of the horn drivers. If they are connected directly to the low impedance (i.e. 4Ω or similar) output of the amplifier, they won't have transformers. In this case, you can take a resistance reading directly across the two terminals on the driver, which should be something below 16Ω. If the two terminals are on opposite sides of the driver body, this suggests they do not have transformers. Incidentally 150W rating is unusually high for ordinary horn speaker compression drivers and I am suspicious that they are not what they seem.

Are all speakers connected in parallel to one 2-core cable? Seven 16Ω speakers in parallel will load the amplifier with 2.3Ω which is too low for a 4Ω output, although as at least one speaker is working the amp seems to have survived this. Or are both channels of a stereo amp used, feeding via two 2-core cables? In any case, you should be able to do continuity tests from the amp to each speaker. One other remote possibility is that they are connected in series-parallel, although the odd number of speakers suggests not. However if there were once 8 speakers, they could have been connected to present an 8Ω load to the amplifier. This would mean that any open-circuit speaker would prevent its series partner working.

If the cable goes into a deep removable can on the back of the driver, there is probably a transformer inside it. In this case, measuring the resistance across the cable does not tell you whether the voice coil of the driver is intact, as you will be reading the transformer primary as Tazz mentioned. For 100V line working, the amplifier must be a proper PA amplifier with a 100V line output and the speaker feed will be connected to the 100V and common terminals.

Re polarity - loudspeakers will work with either polarity but they should all be the same. If any speakers have opposite polarity relative to others nearby, their sound waves will tend to cancel out and cause quiet spots instead of reinforcing each other.

You have all answered the questions yourselves.....if there are 6 speakers and 7 horns (14 total), connected to a amplifier.......you would be touching 0 ohms connecting speakers in parallel ....this is proved by your own calculations above. The 4.8 ohm lablel is the minimum cable inductance permitted, and is used as standard on 100v pa amps per channel to calculated with speakers the total load, so as not to exceed the total wattage permitted. This is why we used 100v pa racks on our PA/VA fire alarm systems....30-40 speakers down a balanced 100v line, powered by 250watt amplifiers on one fp200 cable. Try doing that with your hi fi.

To the OP...confirm the system by looking for amp make, or checking there are transformers in the speakers.
1. disconnect speaker cable from amplifier and connect tone generator to speaker cables.
2. check to hear low audio tone form each speaker
3 any speaker not giving tone repair or link out.

Unit you will need is available from Maplins and other places, comes in useful for finding broken cables to, so will not be a waste of money.

I would agree that Lucien seems to have a good handle on this. The length of cable to the roof may have enough resistance to protect the amp. It all seems a bit strange though. It may be one of those systems that have been cobbled together using gear that has been donated from different places and installed by willing but inexperienced engineers. Surely no one would choose to install 7 PA horns as large as these in a school (hall?)? On the face of it, it is unlikely to be satisfactory by today's expected standards without spending money that could perhaps be better spent on a more modern system. At the very least, even assuming the drivers were OK, it would require a degree of 'old school' expertise to get the best out of it.
At entry level, a couple of full range 200W speakers and a mixer-amp could cost less than £300 and, IMHO, be much more suitable.

in a school (hall?)?

Click to expand...

I inferred from 'on the roof' that it was an outdoor system covering the grounds from the roof of a building, in which case it might be just the ticket. Indoors - would be strange I agree. Hence also my suspicion about 150W rating - more suitable for horn-loaded mid-highs in full-range cabs than for tin trumpets. OP- more info needed please!

Last time..........you do not install standard amplifiers in school and public builds.....They are 100v pa/va systems to meet Rasta clarity standards and BS5839 part 8, this is not a home hifi system...this is a school.

Tazz - you're making a big assumption here that the system was installed to some sort of spec. It could just have easily been the pet project of some science teacher. I would hear Lucien out on this as I too am also inclined to think that way.

So connect 14 speakers to a 4ohm or 8 ohm amp......??????

tazz said:

So connect 14 speakers to a 4ohm or 8 ohm amp......??????

Click to expand...

luke12345 said:

I've been asked to look at a really old PA system at a school that consists of 7 horn speakers, of which only 1 is working.
The other 6 are on the roof all red from 1 cable straight from the amplifier.

Click to expand...

Tazz not understanding where you have got 14 speakers from OP says 7

Ok miss read....still too many speakers to wire in parallel.......

We're thinking about different things altogether, and the OP didn't give us much info to start with. You are picturing an alarm / paging system that is part of the building services, and I'm picturing something for playing muzak to the punters at the summer fete. It could be anywhere in between.

At school we had a lot of fun designing and building amps and speaker cabs and some of the gear we made as teenagers is still in regular use. I built a 75W valve amp to use in the assembly hall that I'm listening to now. One of my mates went in for horns in a big way - I well remember walking round the roof of the music block screwing Grampian 25W drivers onto 24" re-entrants ready for open-day and this is how I imagined the OP's system.

BS5839-8? We had the second master shouting 'Come on chaps you can see it's a real one'