SceneMaster Posted May 30, 2005 Share Posted May 30, 2005 Hi, Two problems. Recently there has been a new sound install at church all is working fine but when we go to plug a laptop into the mixer we get big problems. Firstly there is a massive screech when the jack plug is put into the laptop, current solution turn the level down on the unmanned mixer however this is inconvenient for the vicar on normal services, what would be a solution to this so that we could have the mixer level up without the massive screech when you plug in the laptop audio out cable? Obviously there will always be some clicking but this is fine but not a massive screech. DI box? What is the normal way of taking a laptop feed into a mixer? Both of our phone inputs are in use (rack mound 1u mixer) for a dvd player and CD/Tape Rack. Currently the laptop is feed by a very poor quality jack to XLR (just about to be replaced) cable into the stage box down the multi core then into one of the balanced mic inputs. Is this right or should I have a DI or should the laptop go into on of the phone line in, although the mic inputs are effetely line in. The sound produced from the laptop is fine but when there is no sound being output there is a background hiss. Currently solution turn the mixer level done, but this again is inconvenient. First thing which would improve it is to replace the poor unscreened laptop to stage box cable, but I don’t this will fix it. The hiss is significantly but not completely reduced when the loop amp is turned off. The loop amp cable runs with the new multi core (currently what the laptop is being fed down). Is this an issue? If so should I run a dedicated screen cable away from the loop wire for the laptop feed? Also is the loop wire and multi core run being the same going to effect normal mic use as well? If so serious words need to be had with the install company. The multi core is Van Damme and fully screened so hopefully I won’t be a problem for balanced mic loads. Any thoughts on the situation would be greatly appreciated… ;) Link to comment Share on other sites More sharing options...
mac.calder Posted May 30, 2005 Share Posted May 30, 2005 First things first - shove the laptop through a DI if it is not being plugged in directly to the mixer. Long runs of cable which are unbalanced (like that from a laptop) suffer from electro-magnetic inductance. The DI turns it into a balanced line, which cancels said inductance... Basically, Laptop -> DI -> Stage box -> Multicore cable -> Mixer 99.9% of the time there will be some form of noise when you plug the plug into the laptop - a sort of crunching sound. However a screech is something I have never come across. As an aside: If you are using stereo, you may wish to make a dongle to split the signal into individual channels - they are fairly simple, the stereo plug has a sleeve, ring and tip, the tip is the left channel, ring is right. The dongle should split the 3.5mm stereo headphone jack into two 5.25mm mono plugs. These are plugged into a DI for each channel... which makes things get expensive, but is something you may need to consider. Link to comment Share on other sites More sharing options...
SceneMaster Posted May 30, 2005 Author Share Posted May 30, 2005 First things first - shove the laptop through a DI if it is not being plugged in directly to the mixer. Long runs of cable which are unbalanced (like that from a laptop) suffer from electro-magnetic inductance. The DI turns it into a balanced line, which cancels said inductance... Basically, Laptop -> DI -> Stage box -> Multicore cable -> Mixer 99.9% of the time there will be some form of noise when you plug the plug into the laptop - a sort of crunching sound. However a screech is something I have never come across.<{POST_SNAPBACK}> Ok I will DI the laptop. I any suggestions on a decent fairly low-cost DI box for this situation for use with the laptop only. Will a passive DI be suitable as there is now power supply easily available for an active one. Will a passive one work with good quality or is active the only way to go. The multi core is 30m in length. The laptop lead needs to be about 3m and the multi-core goes straight into the mixer. Will the short unbalanced audio cable be ok from the laptop to the DI if it is well screened? As an aside: If you are using stereo, you may wish to make a dongle to split the signal into individual channels - they are fairly simple, the stereo plug has a sleeve, ring and tip, the tip is the left channel, ring is right. The dongle should split the 3.5mm stereo headphone jack into two 5.25mm mono plugs. These are plugged into a DI for each channel... which makes things get expensive, but is something you may need to consider.<{POST_SNAPBACK}> Why would we need to split the audio into two different channels? Unless we loose the stereo by DIing the laptop into one out put? If we do loose the stereo we will have to use two DI channels as we really need stereo output from the laptop. So are there any good two channel DI for this purpose with my above comments about passive/active in mind? Thanks and any comments on the loop system/multi-core setup would be greatly appreciated. Link to comment Share on other sites More sharing options...
bassman Posted May 30, 2005 Share Posted May 30, 2005 Hi, With regard to the power supply issue for active DIs, if your mixer has phantom power on board, then this can be used to supply the necessary power to most DI boxes, providing all cables between the DI and the mixer are balanced correctly. If your mixer doesn't have phantom then 9V batteries are probably the solution. A short cable (or cables) between the laptop and DI(s) should be fine, go for a decent quality one though, not bell wire! If you need stereo from the laptop, you will need to proceed as mac suggests in the previous post as it is one channel to one DI box, i.e. one for left and another for right. Stereo signals are effectively 2 separate channels, hence the need for two DI boxes and for splitting the output of the laptop which I presume is on a single minijack connector? You can get 2 channel DI boxes, a certain German manufacturer makes quite cheap ones which seem OK. (Moderators - are we allowed to give makes/models?!) As long as you don't work for the manufacturer, of course. In Fact, even if you do, if it's justified in the context of the topic, go ahead - Bryson Hope this helps S Link to comment Share on other sites More sharing options...
SceneMaster Posted May 30, 2005 Author Share Posted May 30, 2005 Ok, this will reduce the hiss and maybe even reduce the issue with the jack making above normal screeching sound rather than the normal clicking sound when inserted into the laptop audio out. Would a DI20 from Behringer do the job well or not? It is small two channel and can be powered by phantom power which are mixer has (although it is only 15vDC on our mixer not the normal 48v will this make a difference. I would have though not as the unit can run on a 9v battery). I know Behringer aren’t the best make but for a laptop feed surely this can cut it? BTW our mixer is a TX8201 from Australian monitor. Website I have just thought the loop cable will not affect the balanced devices on the multi-core at all, so it doesn’t matter that it runs with it. I personally wouldn’t have run them together anyway but that is what the install company did. Thanks for you advice guys keep it coming, EDIT: Our mixer spec is at the bottom of the linked page. Link to comment Share on other sites More sharing options...
bassman Posted May 30, 2005 Share Posted May 30, 2005 Not too sure you will reduce the hiss by using a DI box, onboard soundcards often chuck out a certain amount of noise regardless of what you connect them to. Also, when you insert anything into a mixer where the faders/pots are turned up and the amplifiers are on, there will inevitably be some kind of pop, click or bang. If it is at all possible, the amps should be off until you have finished connecting source or at least have the levels down on the mixer. In the specs for the TD8201 it says something about remote level control in the VCA control bit of the page - might be worth investigating if you could implement this if your mixer is not easily accesible for normal use. Ask the install company... I see you guessed that I was referring to the DI20! The behringer spec for the unit states that it will run on phantom power of between 18-48V DC and your mixer only throws out 15V DC according to the specs. I guess you would have to check with Behringer tech support on that one to confirm whether it would still work at the slightly lower voltage. Or use 9V batteries and remember to turn it off when not in use! S PS - still curious about the screech, never come across that before in this situation! Link to comment Share on other sites More sharing options...
SceneMaster Posted May 30, 2005 Author Share Posted May 30, 2005 Ok, well a DI box should reduce the hiss as it is mainly from the use of an unbalanced line as when the loop amp is off the hiss is dramatically reduced. Although PC card always produce a little bit of hiss it is nothing like this hiss. Unless you think otherwise I think it would be wise to make the line balanced regardless. As to the noise I agree with you the best thing to do it to have the amp muted and the remote main master control may be an option but radio mics all need to be used before and after the laptop’s use in the service off the same mixer. Adding an external main control would just add another possible thing to go wrong. The system is mainly being used by people who don’t know about sound. This may be a stupid question but surely 15v DC from the phantom on the mixer is a higher voltage than the 9v DC from battery (quite obviously yes). So surely this would work, unless the battery is pushing far more current round than the equivalent phantom voltage? Thanks, Link to comment Share on other sites More sharing options...
bassman Posted May 30, 2005 Share Posted May 30, 2005 I am just going off the specifications in the Behringer manual for the DI20! Incidentally, when you don't have the laptop connected, is the hiss still present through the system and does it change when the loop amp is switched off? Link to comment Share on other sites More sharing options...
Malcolm Gordon Posted May 30, 2005 Share Posted May 30, 2005 Re: ScreechDoes your laptop have an an integral mic ? If it does, and it's open, there's a chance you'll get instant feedback as soon as you plug in your speakers. Try muting the mic input in the Windows volume control mixer (you may also have a volume control mixer associated with your sound chip). As a final resort try inserting an unconnected 3.5mm plug into the mic input to switch out the integral mic. Another thought: Unless I am mistaken, the laptop chassis is floating with reference to earth, possibly even when the psu is connected (although you could try reversing the figure-of-eight lead to the psu). As the TRS plug is pushed in the tip and ring will make with the sleeve and ring contacts, with unpredictable results. Link to comment Share on other sites More sharing options...
Andrew C Posted May 30, 2005 Share Posted May 30, 2005 Lots of people have had lots of problems with laptop sound. Have a trawl through previous posts... Link to comment Share on other sites More sharing options...
Solstace Posted May 30, 2005 Share Posted May 30, 2005 Hi... As previously mentioned, the squeals you're hearing could be feedback from the laptop's mic - check the settings in the mixer applet, make sure the mic input is turned down or muted. They could also be coming in the form of feedback from the induction loop. I'm betting here that a long unbalanced connection is allowing the cable to become a nice long antenna for the induction loop - and that the laptop output is mixed into the induction loop along with everything else. The result? Horrible, horrible noises. Solution? Use a DI (preferrably active) on the laptop output. If memory serves correctly, I think Behringer do a cheap active 2-channel DI that is useful for this kind of thing - somewhere around £30 last I saw. Again, it will draw phantom power from the mixing desk, but I think it can also run from a 9V PP3 cell at a push. There are others but that's the one I've seen most often. DI'ing in this case probably won't solve the hiss, as most laptop sound devices are built for convenience rather than pure audio quality. In the long run, I'd be pushing for the use of a dedicated rack-mounted PC for church presentations and the like - at least you'd then get more choice in the kind of sound-card used, without spending over the odds for a USB or Cardbus audio device. Hope this helps... C. Link to comment Share on other sites More sharing options...
J Pearce Posted May 30, 2005 Share Posted May 30, 2005 I have previously experienced a half plugged in jack plug demodulating the loop signal at church, I guess the theory is as above, when one end is shorted it becomes an open loop aerial, and components in the preamp demodulate it. Link to comment Share on other sites More sharing options...
SceneMaster Posted May 30, 2005 Author Share Posted May 30, 2005 Ok, I will see if I can get a DI20. This should do the job nicely, will it run of the 15vDC our mixer gives out even thought the spec calls for 18vDC minimum for the DI I presume it would be fine as it run of a 9v battery ok? (I posted this above but I really need to know before I order). If not I will have to by a external phantom power supply I seem to remember you can get them but preferably I would rather just use the 15vDC already available. Anyone know about the DI20? I still don't get how it will work ok at 9v but not at under 15v on a phantom power unless the current push is different, could anyone answer this for me? Link to comment Share on other sites More sharing options...
SceneMaster Posted May 30, 2005 Author Share Posted May 30, 2005 Will this do for the feed from the laptop to the DI? It is two core so will carry stereo audio fine along with the common/ground given out by the laptop. It is screened as well and is only needed to be about 3m if that long. On end will be adapted into two mono jacks for the DI the other will go into our pc point box which has a audio input just with no cable going to it. EDIT: Formatting corrected. Link to comment Share on other sites More sharing options...
J Pearce Posted May 30, 2005 Share Posted May 30, 2005 To make it neater when you split it to 2 jacks four core + screen would be a better cable. Or buy one, stereo minijack to double mono jack cables are easily found on the pages of maplins, adam hall etc Link to comment Share on other sites More sharing options...
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