Members Keith2 Posted September 29, 2004 Members Share Posted September 29, 2004 I'm scratching my head on this one guys. Played Sat. night w/o a sound check (at all!) and 1st set sound was great. Had nice strong monitors the whole set. 20 min. break, start 2nd set, monitors were hardly putting out at all. We run 4 mon. total. 2 each (8ohm) per channel on a RMX850. I know, first response, not a big enough amp. The 850 gain is up to about 3/4 and we get no clipping and no distortion. Our guitar players amp is mic'd on stage and is hardly audable (when monitors farted out, I couldn't even hear the guitar. Bass players amp ran direct and is turned down, so stage volume is not very loud w/ exception of drums. Haven't checked sys since teardown, but this happened once before and at next practice everything worked fine. I know this is kind of vague, but I thought I'd post it first before pulling my hair out. I've read, I think it was here, that if I'm pushing the 850 hard for monitors that I'll get less gain before feedback than if I were to use a more powerfull amp, how true, if at all, is this. Our monitor main volume usually dosen't exceed 2/3 on the board and the 850's gain sits on 3/4. Any help with where to start would be appreciated. Keith Link to comment Share on other sites More sharing options...
Members tlbonehead Posted September 29, 2004 Members Share Posted September 29, 2004 If you aren't clipping the amp,you aren't pushing it too hard. And even if you were pushing it hard,it shouldn't quit at the beginning of a set. It should be cooling down during the break. Do you have the limiter engaged? I would in this situation. Did the monitors ever come back? Did you try a quick switch by running the cable that feeds the monitor chain to the main out on the board to see if you got sound that way? My guess is that either the jack (aux out?)feeding your monitor chain is bad,or the cable going in that direction possibly. Are you sure the protect light didn't come on at the power amp? It could be a bad speaker cable or speaker jack but your protect light should have come on in that case. Link to comment Share on other sites More sharing options...
Members outtahand4now Posted September 29, 2004 Members Share Posted September 29, 2004 It's more difficult to troubleshoot after-the-fact...but I'd start at the source. 1. Your mics & mic cables must be OK because the house stayed up. 2. Is your board showing output from the mon/aux you use? 3. Is the proc. after this receiving and outputing signal? 3. Is the signal getting to the stage from the snake? 4. Is the amp getting signal? 5. Does the amp have output? 6. Are the monitors getting signal? 7. Do the monitors have output? It does sound suspiciously like a cable or jack or even a damn button on the board. I'd just start at the beginning and trace, verifying the integrity of each component. Link to comment Share on other sites More sharing options...
Members InACanProductions Posted September 29, 2004 Members Share Posted September 29, 2004 I've seen something simular happen to a friend of mine. The problem ended up being the variable speed fan wasn't variabling anymore. It wasn't cooling off the amp fast enough and it would shut down, then next day it was back up and running like nothing ever happened. I used to have a girlfriend who'd do that..we'd get in to a fight for something and the next day while I was still pissed she'd act like nothing happened. Her problem was that she had a screw loose, so as a precaution tighten down all the screws on the 850 and see if it helps. Good luck...if your 850 is anything like my ex-girlfriend you'll need it. Link to comment Share on other sites More sharing options...
Members Keith2 Posted September 30, 2004 Author Members Share Posted September 30, 2004 Allright, went out to the garage, hooked up just the 850 & monitors. I don't remember whitch cable I used the night of the gig (this might it) but....Monitors played well while I made up some more cables (30 min.) then went quiet. (Like I hit the mute button) I Switched the spkr cable and all was good. Here's the odd part, I then remembered, I have 1 spkr cable wired for bridged mode. {censored}, that's it. but why would it play ok for 1/2 hr. then stop. If my amp isn't switched to Bridge Mode, then a bridged speak-on cable shouldn't play at all, right? I'll try to re-create this again tomorrow. Would a slightly blown woofer have anything to do with this? The monitor in question is a Peavey TLM115, and I get a slight amount of scratch sound from the woofer. I'll replace it one of these days (any sugg.?) It can get a little annoying after a while, but not so bad that I have got around to replacing it yet. Thanks,Still scratching head Link to comment Share on other sites More sharing options...
Members tlbonehead Posted September 30, 2004 Members Share Posted September 30, 2004 Originally posted by Keith2 Allright, went out to the garage, hooked up just the 850 & monitors. I don't remember whitch cable I used the night of the gig (this might it) but....Monitors played well while I made up some more cables (30 min.) then went quiet. (Like I hit the mute button) I Switched the spkr cable and all was good. Here's the odd part, I then remembered, I have 1 spkr cable wired for bridged mode. {censored}, that's it. but why would it play ok for 1/2 hr. then stop. If my amp isn't switched to Bridge Mode, then a bridged speak-on cable shouldn't play at all, right? I'll try to re-create this again tomorrow.Would a slightly blown woofer have anything to do with this? The monitor in question is a Peavey TLM115, and I get a slight amount of scratch sound from the woofer. I'll replace it one of these days (any sugg.?) It can get a little annoying after a while, but not so bad that I have got around to replacing it yet.Thanks,Still scratching head Does the protect light come on when you lose power? As for a replacement woofer,what is in it? Scorpion? If it is a Scorpion,I'd just get a basket for it and throw it on. Otherwise,there are lots of choices depending on how high you want to aim. the Eminence Delta is a great budget replacement. Otherwise,Black Widows would keep you in the Peavey family. Link to comment Share on other sites More sharing options...
CMS Author Craig Vecchione Posted September 30, 2004 CMS Author Share Posted September 30, 2004 Originally posted by SpreadTheHate I've seen something simular happen to a friend of mine. The problem ended up being the variable speed fan wasn't variabling anymore. It wasn't cooling off the amp fast enough and it would shut down, then next day it was back up and running like nothing ever happened. I used to have a girlfriend who'd do that..we'd get in to a fight for something and the next day while I was still pissed she'd act like nothing happened. Her problem was that she had a screw loose, so as a precaution tighten down all the screws on the 850 and see if it helps.Good luck...if your 850 is anything like my ex-girlfriend you'll need it. I was curious where she wound up after I dumped her. Thanks. Link to comment Share on other sites More sharing options...
Members Angry Jonny Posted September 30, 2004 Members Share Posted September 30, 2004 Originally posted by Keith2 Here's the odd part, I then remembered, I have 1 spkr cable wired for bridged mode. {censored}, that's it. but why would it play ok for 1/2 hr. then stop. If my amp isn't switched to Bridge Mode, then a bridged speak-on cable shouldn't play at all, right? I'll try to re-create this again tomorrow. I think I may have an explanation for this. On the RMX amplifiers, the channel 1 output is wired so that when you are in stereo / dual mono mode, the output of channel 1 is on the 1+/1- connectors and the output of channel 2 is on the 2+/2- connectors. So, if you connected a bridge cable to it, you would have gotten the + side of channel 1 and the + side of channel 2 running into your monitors. There are 2 ways I can think of that this could have caused your problem. 1: By tying the 2 + sides together, you were causing the amp to go into a protection mode. 2: The content in both monitor mixes was very similar which would result in both outputs moving in the same directions at the same time. This would result in a low voltage difference between the two which means essentially no real amplitude in your signal. Link to comment Share on other sites More sharing options...
Members ashivraj Posted September 30, 2004 Members Share Posted September 30, 2004 Originally posted by Angry Jonny I think I may have an explanation for this. On the RMX amplifiers, the channel 1 output is wired so that when you are in stereo / dual mono mode, the output of channel 1 is on the 1+/1- connectors and the output of channel 2 is on the 2+/2- connectors. So, if you connected a bridge cable to it, you would have gotten the + side of channel 1 and the + side of channel 2 running into your monitors. There are 2 ways I can think of that this could have caused your problem.1: By tying the 2 + sides together, you were causing the amp to go into a protection mode.2: The content in both monitor mixes was very similar which would result in both outputs moving in the same directions at the same time. This would result in a low voltage difference between the two which means essentially no real amplitude in your signal. Good thinking! But - seeing as he didn't swap any cables during the break (at the gig) - or during the test in the garage - it wouldn't explain why the amp worked fine for a while and then not. FWIW, I'm leaning towards the 'some kind of protection' option. AS Link to comment Share on other sites More sharing options...
Members Angry Jonny Posted September 30, 2004 Members Share Posted September 30, 2004 I agree, given the fact that the problem takes time to show up, it almost has to be a protection thing. Maybe the miswiring causes the amp to dissipate too much power which results in thermal protection being activated? The more I think about it, the more I think that's what it probably is. This is really taxing my transistor memory but... This wiring will get you in big trouble if / when the outputs start going in opposite directions. The basic picture this creates is: channel 1 o-----------+ | 4ohms |channel 2 o-----------+ | 4 ohms |ground o-----------+ Kirchof's current law tells us that the sum of the currents at any point must be 0, so when channel 1 heads towards the positive voltage rail (+Vmax) and channel 2 heads towards the negative voltage rail (-Vmax), you have (Vmax-(-Vmax))/4 (2Vmax/4)going from channel 1 to channel 2, and -Vmax/4 going from channel 2 to ground which results in a 3Vmax / 4 going towards channel 2. This means that in this situation we've essentially created a load on channel 2 of 4/3 or 1.33ohms instead of the 2 ohm load it is designed for, sending the amp into overcurrent protection mode. I think this is right and if anyone wants, I'll try to explain further but I'm guessing this is what happened. Link to comment Share on other sites More sharing options...
Members tlbonehead Posted September 30, 2004 Members Share Posted September 30, 2004 Originally posted by Angry Jonny I agree, given the fact that the problem takes time to show up, it almost has to be a protection thing. Maybe the miswiring causes the amp to dissipate too much power which results in thermal protection being activated? The more I think about it, the more I think that's what it probably is.This is really taxing my transistor memory but... This wiring will get you in big trouble if / when the outputs start going in opposite directions. The basic picture this creates is:channel 1 o-----------+ | 4ohms |channel 2 o-----------+ | 4 ohms |ground o-----------+Kirchof's current law tells us that the sum of the currents at any point must be 0, so when channel 1 heads towards the positive voltage rail (+Vmax) and channel 2 heads towards the negative voltage rail (-Vmax), you have (Vmax-(-Vmax))/4 (2Vmax/4)going from channel 1 to channel 2, and -Vmax/4 going from channel 2 to ground which results in a 3Vmax / 4 going towards channel 2. This means that in this situation we've essentially created a load on channel 2 of 4/3 or 1.33ohms instead of the 2 ohm load it is designed for, sending the amp into overcurrent protection mode.I think this is right and if anyone wants, I'll try to explain further but I'm guessing this is what happened. But doesn't that particular amp have a protect light that would come on? Link to comment Share on other sites More sharing options...
Members dboomer Posted September 30, 2004 Members Share Posted September 30, 2004 Have you tried another send from the mixer? It could be a problem there just as easy. Maybe you could drive the 850 and monitors directly with a CD player for a while just to rule out anything upstream in the chain. Link to comment Share on other sites More sharing options...
Members Angry Jonny Posted October 1, 2004 Members Share Posted October 1, 2004 Originally posted by tlbonehead But doesn't that particular amp have a protect light that would come on? Actually no, it only has signal and clip LEDs. I'm pretty sure there's a combination of these that would indicate protect mode though. Link to comment Share on other sites More sharing options...
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