Mican Posted June 8, 2011 Share Posted June 8, 2011 Can somebody offer me assistance with this problem, please? I am installing a 100v line PA system with a maximum of 10 speakers tapped at 20w. For this I have bought a 240w/5zone amp. At the moment the speaker lin lengths are between 20 and 100 metres from the amp. I have only installed 7 speakers so far in 3 zones, only 140w plus 25%. All worked when I had only 1 zone connected, (any 1 zone of the 3). However when I connected 2 or 3 zones the output power of the amp dropped considerably, probably by more than 50%. This resulted in the protection circuit cutting in when I tried to raise the output level. I have written to the retailer of the amp and await a response. Just thought somebody could throw some light on this problem and a possible fix or is it a fault in the amp. Thanks for any advice. Link to comment Share on other sites More sharing options...
RayL Posted June 8, 2011 Share Posted June 8, 2011 A possibility is ultrasonic oscillation. Your amp isn't being overloaded by the speakers. At a certain critical point an ultrasonic oscillation is being generated by your particular setup. You can't hear it but the amp is straining at full power and as a result the protection is coming in. Cures? Sometimes it is the lack of a proper earth on the system so check the mains earthing. It might be the LS cables that you are using (unshielded?) that aree interacting with input feeds. Does the amp have a power output meter and an overall treble cut control? Try turning the control down and see if the meter suddenly drops back. Ray Link to comment Share on other sites More sharing options...
Brian Posted June 8, 2011 Share Posted June 8, 2011 What amplifier and speakers are you using? Are you sure they are 100v units and not 70v units? Are you sure you've set the speaker taps correctly? On some speakers I have used the connections are not very clearly marked and I've made mistakes. Link to comment Share on other sites More sharing options...
Mican Posted June 8, 2011 Author Share Posted June 8, 2011 Thanks for the replies. I'm using 2 x 1.5mm double isolated loudspeaker cables so that should not be the problem. All earthing is correct. The amp is a HPA 240w (not sure if they're available in the UK). There is an output meter marked -20, -10, -4, 0, +6 and protect. The output level is fine at -20 above that it cuts out. There is also a 7 band graphic equaliser which I have adjusted to cut out the 4 and 8k frquencies without change. There are no separate bass or treble controls. The Rondson speakers are wired for many different taps. 50v, 70v, 100v without outputs of 5, 10 and 20w. I can only assume the labelling is correct and have followed the instructions. I have wired 100v systems internally before without problems but not using this equipment which is on the cheap side. I will try and retap the speakers at 10w and see if that helps. it just means a lot of ladder climbing! Thanks for help. http://www.blue-room.org.uk/public/style_emoticons/default/sad.gif Link to comment Share on other sites More sharing options...
paulears Posted June 8, 2011 Share Posted June 8, 2011 changing the speaker tap shouldn't make a difference - if the meters indicate it's fine until you up the volume, this too suggests some kind of output you cannot hear - so the instability mentioned above could well be the cause. For comparison, have you tried metering the feeders where they connect to the amp - nearing in mind it's a high impedance system - feeder faults or even a dodgy transformer should be pretty obvious? Failing that - it will be a case of disconnecting each speaker one at a time to see if you can cure it. Link to comment Share on other sites More sharing options...
Mican Posted June 8, 2011 Author Share Posted June 8, 2011 I have spent the last few hours trying to look at all possibilities. The amp has 5 sets of 100v outputs for zone settings. at the moment I have wired 3 of the 4 zones, one with 3 speakers and 2 with 2 speakers. I should explain these zones are 3 sides of a square here in our village. All is fine if one zone is switched on, either number 1, 2 or 3. I get good level and no problems. When I switch in a second zone or all three together the level drops and cuts out if I try to restore it to a previous level. It has to be an internal problem in the amp, (in my opinion). I don't understand where an oscillation might occur. The only input at the moment is an Mp3 player using RCA sockets of the earpiece out. As far as resistance and impedance is concerned can someone please reassure me that this is not an issue. The speakers are 8ohm, maximum 20w sound projectors as commonly used on external walls, etc. The manual gives a output impedance of 500ohms at 20w and 100v. The amplifier has 100v and 4ohm outputs and the manual says the impedance at 100v is 42ohms. I presume this is not relevant as I am not using separate transformers. It's just a little confusing why this information is included. Thanks again for any advice.http://www.blue-room.org.uk/public/style_emoticons/default/smile.gif Link to comment Share on other sites More sharing options...
Brian Posted June 8, 2011 Share Posted June 8, 2011 The speakers ... . 500ohms at 20w and 100v.That's correct. R = V x V / W = 100 x 100 / 20 = 500R The amplifier has 100v and 4ohm outputs and the manual says the impedance at 100v is 42ohms. That's also correct. W = V x V / R = 100 x 100 / 42 = 238W As zones get switched in they are placed in parallel across the output. As it seems to be any combination of more than one zone which causes the problem I think that leaves us with two possibilities... 1) Faulty amplifier2) Faulty/Mislabelled speakers Do you have access to any test equipment? At a minimum a source of audio tone, a few resistors and a multimeter with a sensitive AC volts scale. With that you could double check the impedance of your speakers. Link to comment Share on other sites More sharing options...
MarkPAman Posted June 8, 2011 Share Posted June 8, 2011 ............. the manual says the impedance at 100v is 42ohms. I presume this is not relevant as I am not using separate transformers. It's just a little confusing why this information is included. Thanks again for any advice.There are transformers in the speakers though? I must admit I'm still confused about how all this is set up. Link to comment Share on other sites More sharing options...
paulears Posted June 8, 2011 Share Posted June 8, 2011 There's no chance that although you have the volume taps working, the input is low Z rather than the 100V input? Link to comment Share on other sites More sharing options...
Mican Posted June 8, 2011 Author Share Posted June 8, 2011 There is a transformer in the speaker which is connected by a 6 way cable(bare ends). Black is common and the other 5 coloured wires are different wattages (5, 10, 20W) depending on whether you are using 50v, 70v or 100v. There are no rotary dials or switches to control input and no option for low impedance connection. The amp uses euroblocks, one for all the zones with 5 power outputs and 5 common outputs. The 4ohm output is separate. I think it has to be a fauty amp but tomorrow I'll check all the connections in the speakers. Don't have anyway of checking impedances but I did test each speaker separately before mounting. I'm going to try another amp tomorrow as well but its only rated at 100w so can't wire in all the lines at 20w. If I retap in at 5 or 10 watts I can test all speakers together. Thanks again. Link to comment Share on other sites More sharing options...
MarkPAman Posted June 8, 2011 Share Posted June 8, 2011 May be worth checking that none of the bare ended wires you're not using (in the speakers) are shorting to each other, or anything else. I think you may be right about the amp though. Link to comment Share on other sites More sharing options...
Mican Posted June 10, 2011 Author Share Posted June 10, 2011 Thanks for all the suggestions. After spending almost the whole of Friday going round and isolating all the unused wires in the loudspeakers it appears that I have succeeded in my installation. Tonight for 5 hours we had music at a good level in 3 zones on all 7 speakers installed so far without a hiccup. It looks like it was a problem with a couple of wires touching each other and shorting the system. It is a lesson to me to cover all eventualities before I do the final installation and not be to easy going. Many thanks again to all for your advice . http://www.blue-room.org.uk/public/style_emoticons/default/biggrin.gif Link to comment Share on other sites More sharing options...
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