isolating transformer

[matt-the-dj]

hi there .

I would like some input from you guys, a friend of mine is trying to get an input into his pc to be the signal for a sound to light progamme on his DMX lights, he is using an unbalanced (maybe balanced) signal from the signal output of his yamaha active speakers which then passes thru an isolating transformer which then plugs in via a jack plug into his laptop, he maybe uisng a mic input as im not sure it has a line in on its souncard. The problem is whenever he plugs the leads in, the pa system on that side shuts down and is muted, there are no fault light on the actives to suggest what is happening. I personally think the isolating transformer is showing a dead short to the amp and its shutting down. I have suggested he uses a di box to see if that helps, it maybe an impedance issue what do you think?

regards

matt.

[Peter Chivers]

This is a case where it's good to start with the usual suspects. Check the lead from the PA cab to the transformer (is that shorted); measure the (DC) resistance of the input to the transformer and see if there is a reasonable value showing, I would expect over 1k ohm as a starting point, but it could be considerably more. Check that the socket being used on the PA cab is actually a line level output, and that when a plug is inserted into it that is does not disconnect the internal system.

 

Hope this helps

Peter

[matt-the-dj]

This is a case where it's good to start with the usual suspects. Check the lead from the PA cab to the transformer (is that shorted); measure the (DC) resistance of the input to the transformer and see if there is a reasonable value showing, I would expect over 1k ohm as a starting point, but it could be considerably more. Check that the socket being used on the PA cab is actually a line level output, and that when a plug is inserted into it that is does not disconnect the internal system.

 

Hope this helps

Peter

 

 

thanks peter,

it gives me some where to start, ill look in the manuals as it may be the case that the pa shuts down for a good reason when they plug it in. It may well be a design characteristic.

regards

matt.

[KevinE]

Is the so-called isolating transformer the correct one? Ideally you need a 20K input otherwise the impedance may sink the signal. (Is it possible you have a 600ohm one meant for microphones?). The Line Out of the active speaker will almost certainly be in parallel (connected to) the signal input (from the mixer or signal source) so the wrong impedance transformer may kill the signal to both. What transformer is in use? It might be easier as suggested to splash out a few quid more on a proper singal isolating box (like maplin sell) or use a 1:1 DI box..which is perhaps an overkill but will be better and useful for other things.

[Charlie Jeal]

Surely it would be simpler to either take a direct output from whatever desk you're using to the laptop or maybe just Y-split the input to the cabs .

 

 

Charlie

[KevinE]

it would be simpler to either take a direct output from whatever desk you're using to the laptop

 

As has been discussed ad nauseam on BR before, many laptop users connect the audio I/O from a PA through some form of isolator to minimise either hum loops or power supply/processor/fan/disc noise.

[Charlie Jeal]

it would be simpler to either take a direct output from whatever desk you're using to the laptop

 

As has been discussed ad nauseam on BR before, many laptop users connect the audio I/O from a PA through some form of isolator to minimise either hum loops or power supply/processor/fan/disc noise.

 

 

I know that Kevin and in 12+ years of using laptops for work purposes I've never run into this being a problem.These problems are usually caused by laptops that don't have a decently grounded psu and can be simply fixed by using an external inteface or a decent quality DI or simply by running on batteries. In any case Matt isnt using the laptop as a sound source or for recording he's running some sound to light software.

 

Charlie

[bruce]

Surely it would be simpler to either take a direct output from whatever desk you're using to the laptop or maybe just Y-split the input to the cabs .

 

I suspect not, since the mixer output may well be balanced, or it may not be.... and it may be going into a mic input, or it may not be (there's more than a touch of vagueness in the original post...)

 

So it will quite probably be at the wrong level, and simply y-splitting and bunging it into a laptop would unbalance it, as well as overloading the laptop input.

 

 

Edited to add: To the OP: tell us more about the isolating transformer. Model number, details of EXACTLY how it's connected etc. When you say the "PA shuts down", what do you mean? No sound output? Actually powers off? Protection circuits kick in?

 

It could be something as simple as XLR pins 2&3 being shorted when plugged in to the transformer, effectively muting the input...

 

More info needed - without that anything else is just speculation.

[Charlie Jeal]

If that's the case then the only way this is likely to work is with an external interface or a DI either way I suspect it would be better not to derive the necessary signal from the speaker itself. Plus whether the signal is balanced or unbalanced shouldnt make any difference to the input used provided the cable is wired correctly ( any output link from the speaker is also highly likely to be balanced anyway).

 

Charlie

[matt-the-dj]

thanks for all your replys

I cannot use an output direct from the desk as all available outputs are used, it has no rec out that I saw and all 3 aux outs are used, I have suggested a 'y' lead to split a signal and see if picking an input to the actives instead of an output from them helps. I could suggest they change the mixer but they like it there way as its been like that 4 ever....these band people dont like change. I will endeavour to get all the relevant info on all the kit, I think the transformer they are using is not fit for purpose, I have suggested a di box and maybe they will listen to me or just stick a mic near the speaker as usual.Its a difficult problem for me as I do not work reguarly with them so I am trying to help them from a distance.

regards

matt.

[sleah]

it would be simpler to either take a direct output from whatever desk you're using to the laptop

 

As has been discussed ad nauseam on BR before, many laptop users connect the audio I/O from a PA through some form of isolator to minimise either hum loops or power supply/processor/fan/disc noise.

 

 

I know that Kevin and in 12+ years of using laptops for work purposes I've never run into this being a problem.These problems are usually caused by laptops that don't have a decently grounded psu and can be simply fixed by using an external inteface or a decent quality DI or simply by running on batteries. In any case Matt isnt using the laptop as a sound source or for recording he's running some sound to light software.

 

Charlie

 

Strangely, I have found the hum problem is mostly from laptops that DO have a grounded PSU! Particularly one's from the last 3 to 4 years or so.

 

I first discovered it when I changed my DJing laptop to one with a 3 pin clover leaf plug. I only discovered it at the start of the night (in a club). The cure was a 'bodge-it-and-scarper' affair - I won't go into details here :)

I had no other choice at the time and it was ONLY me near the kit.

The 'proper' fix was swapping the PSU for one that just had the 2 pin connector, and/or using a GLI on each channel (2 x stereo) :D

 

There is no way of saying wether or not a laptop will cause hum, earthed or not, it's just a case of try it and see and go equiped with GLI's :)

 

 

* just in case..... GLI = Ground Loop Isolator

[Sound Man]

I would say that the best thing to do would be to have something like a Behringer MX882 splitter connected to the mixer outputs. The active speakers are connected to the splitter's main outputs and the transformer/laptop is connected to one of the other outputs. Other outputs are available which could be used for other things.

 

David

[matt-the-dj]

I would say that the best thing to do would be to have something like a Behringer MX882 splitter connected to the mixer outputs. The active speakers are connected to the splitter's main outputs and the transformer/laptop is connected to one of the other outputs. Other outputs are available which could be used for other things.

 

David

 

 

I have one on my rig, it allows me to run multiple amps without the need for splitting signals but as this is not my rig and there rack is full they are not inclined to change they dont want to spend on something to them that is not a problem, as long as the lights work the band dont care how, so my mate has to set all his sequences up using tap sync,ill suggest the use of a splitter but I guess £60 ish quid will be frowned upon.

[KevinE]

I know that Kevin and in 12+ years of using laptops for work purposes I've never run into this being a problem

 

Then you've been lucky. I've supplied many ground loop isolators (some with custom leads) to bands and discos where it very much IS a problem. The noise manifests in different ways and is almost always cured with a signal isolator. Earthed PSU designs can of course inject hum loops (by definition) and floating PSU's tend to radiate disc drive or processor noise/harmonics onto their chassis.

 

If a hum loop necessitates a signal isolator (eg audio transformer) then it doesnt matter if it's an input or an output if it's in the main signal path..the results will be the same.