Sound Juice Posted February 16, 2009 Share Posted February 16, 2009 Hi BR, Might be a silly question, but having never constructed a 3phase amp rack or had enough amps to overfill a 63A single before...How do you configure amp racks to run across phases?Do you need an active DI or a ground lift cable?Or do you not suffer as badly from ground loops in this part of the signal path? Cheers inadvance guys (and gals) Link to comment Share on other sites More sharing options...
Rob_Beech Posted February 16, 2009 Share Posted February 16, 2009 Just plug them in and go. If you get any problems then you need to PREVENT and CURE the problem, not MASK and COVER UP the problem with a Cable and / or GND lift on a DI. Under normal circumstances there should be no problems. Link to comment Share on other sites More sharing options...
J Pearce Posted February 16, 2009 Share Posted February 16, 2009 Why would a 3 phase rack suffer from ground loop more than a single phase rack? Ground is still ground, and the amps are all dc inside and won't care or know what phase they're on. Link to comment Share on other sites More sharing options...
Sound Juice Posted February 16, 2009 Author Share Posted February 16, 2009 Cheers RobGig isnt till tuesday, but thought it good to check well inadvance after my power maths Link to comment Share on other sites More sharing options...
Rob_Beech Posted February 16, 2009 Share Posted February 16, 2009 It still makes my brain hurt trying to work out amps being DC inside Link to comment Share on other sites More sharing options...
Sound Juice Posted February 16, 2009 Author Share Posted February 16, 2009 It still makes my brain hurt trying to work out amps being DC inside I clearly didnt put 2 and 2 together in my head... Link to comment Share on other sites More sharing options...
J Pearce Posted February 16, 2009 Share Posted February 16, 2009 Well yes they are ac, but derived from a dc rail. So their ac output has no direct link to the mains ac, hence the supply phase is largely irrelavent. Link to comment Share on other sites More sharing options...
Dmills Posted February 16, 2009 Share Posted February 16, 2009 I never understood where the whole "Audio must be on the same phase to prevent ground loops" thing came from, it is blatantly complete nonsense. Mind you, I am fairly firmly of the opinion that most of the claims by audio "Engineers" that hums and buzzes are due to bad venue power are dubious at best, and are probably far more indicative of someone who does not understand correct system design (or has not practised it) on the audio side then of a problem with the venue power. Three phase amp racks work fine, just be careful about the neutral connections. Now, what can be a problem is gear with a pin 1 problem, and that can be made worse by large earth loops as might be found when plugging front of house into a socket far removed from the one the racks are plugged into, but that is not a three phase problem, that is a numpty designed the gear badly problem. The solution is to add a fairly heavy supplemental bond running alongside the snake between front of house and the on stage racks (And then fix the pin one pain back at base). Regards, Dan. Link to comment Share on other sites More sharing options...
Sound Juice Posted February 16, 2009 Author Share Posted February 16, 2009 I never understood where the whole "Audio must be on the same phase to prevent ground loops" thing came from, it is blatantly complete nonsense. So the ground lift function on a DI box is pointless? And backline power should be a different phase to FOH? Or am I totally loosing the plot today? Link to comment Share on other sites More sharing options...
Rob_Beech Posted February 16, 2009 Share Posted February 16, 2009 Not totally. The ground lift function is to mask problems encountered on gigs. When all of the equipment if functioning correctly of course, there will be no need for a ground lift switch. They are a there to mask a problem in a hurry so to speak. Backline power phase shouldn't matter. When your FOH position is a long way from the stage (often) then sometimes plugging into a local mains socket at FOH can cause problems. This is regardless of phase, this is an earth loop problem which can be found on the same or neighbouring phases. An earth bond should fix this but all but 1 of my multicores has 16A mains (on 4mm cable) attached to save having to plug in locally. Link to comment Share on other sites More sharing options...
Dmills Posted February 16, 2009 Share Posted February 16, 2009 So the ground lift function on a DI box is pointless?Sort of, on one side of a DI you have an unbalanced line, on the other a balanced one, you could almost certainly build a DI that always isolated the unbal side from everything and it would work fine. However there is more then enough half arsed gear out there that having that switch is a useful get out of trouble feature. And backline power should be a different phase to FOH?Who gives a ###### what phase the backline is on? It makes no difference to anything[1]. Now I would probably keep all the backline on the same phase, just because I am paranoid about creative 'modifications' that may have been made (usually to guitar amps), which you don't always catch in the heat of the action, but given my amp room is sprawled across all three phases, my FOH gets plugged into whatever hard power socket is nearest, and the assisted listening system is powered from fused spurs off whatever ring main is closest to the radiators I see little reason to be overly precious about phasing, and you know what? The system is completely silent! Three phases, one neutral and ONE earth! Earth loops can exist irrespective of which of those three phases a particular bit of kit happens to have tied to its input, they all share the same earth wire! Of course, if you do it right you can have dozens of earth loops, and it has absolutely no real effect at all on the system noise floor. Regards, Dan. [1] Yes, I know about that guidance note, which seems to me to be squarely aimed at pub bands who in general don't use (or need) three phase amp racks! Link to comment Share on other sites More sharing options...
Rob_Beech Posted February 16, 2009 Share Posted February 16, 2009 The system is completely silent! You should probably get that looked at...... sorry.... it's getting late and I have to put up with dozens of people telling me they can hear the music in their motorhome for the next 5 days at the NEC. Link to comment Share on other sites More sharing options...
Dmills Posted February 16, 2009 Share Posted February 16, 2009 sorry.... it's getting late and I have to put up with dozens of people telling me they can hear the music in their motorhome for the next 5 days at the NEC. My sympathies. I despise that place, but I have to ask, motorhomes? Regards, Dan. Link to comment Share on other sites More sharing options...
Rob_Beech Posted February 16, 2009 Share Posted February 16, 2009 National motorhome show. People travel in theirs, and stay over night on a car park. Entertainment is put on in the evenings, with a PA provided by yours truly. Staying in a motorhome on a car park. In February. Needless to say, with my 3 beds, aux heating tv and dvd player in my van you can guess where I'm staying for the week. In a bloody travelodge 5 minutes up the road that's where. Link to comment Share on other sites More sharing options...
dbuckley Posted February 16, 2009 Share Posted February 16, 2009 Now, what can be a problem is gear with a pin 1 problem, and that can be made worse by large earth loops as might be found when plugging front of house into a socket far removed from the one the racks are plugged into, but that is not a three phase problem, that is a numpty designed the gear badly problem. The solution is to add a fairly heavy supplemental bond running alongside the snake between front of house and the on stage racks (And then fix the pin one pain back at base). The solution is to run the mains for FOH from the stage and then there is just the single earth point on stage. Can't do that? Got to run FOH from local power? Use an mains isolating transformer at FOH to break the ground reference. Go to whatever lengths are necessary to keep and maintain a single earth point, it saves so much time in the long run :blink: Link to comment Share on other sites More sharing options...
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