Ground Loop Isolators

[sleah]

I have a need for a couple of GLI's. One is for a PC output to mixer. I already have some of those common stereo types which works a treat.

The other is to go between a mixer output and a Symetrix processor. I've used one of said stereo types to find out if that was the issue and it has worked. However it is unbalancing the (I would assume) balanced XLR out from the mixer then feeding in to the (again I assume) balanced line input on the Symetrix as an unbalanced signal.

I suppose if it works....then stick with it, but part of me says I should endeavor to keep the signal path balanced.

 

Cutting to the chase... short of going with pro-kit for proper money, is it possible to be cheapskate and use a pair of the aforementioned stereo GLI's but instead use 1 per channel. So instead of Left & Right I connect them to an XLR or TRS as Line and Return, with the two screens connected together on pin 1?

I don't see why it shouldn't work. Maybe I should just try it and see! :)

 

Hope that makes sense. I think it does to me :blink:

[bruce]

If source and destination are balanced:

 

Take one of the cheap transformer based isolators.

Chop off the phono plugs

Solder on some XLRs, connecting them to pin 2 and 3. Leave the ground unconnected.

 

In fact, if source and destination are balanced, you could try just losing the screen on the lead, without a transformer - just connect pins 2 and 3.

[Jivemaster]

I might play and hope but I'd be looking hard at RS for an OEP transformer intended for the job.

[richardash1981]

The other is to go between a mixer output and a Symetrix processor. I've used one of said stereo types to find out if that was the issue and it has worked. However it is unbalancing the (I would assume) balanced XLR out from the mixer then feeding in to the (again I assume) balanced line input on the Symetrix as an unbalanced signal.

I suppose if it works....then stick with it, but part of me says I should endeavor to keep the signal path balanced.

 

Cutting to the chase... short of going with pro-kit for proper money, is it possible to be cheapskate and use a pair of the aforementioned stereo GLI's but instead use 1 per channel. So instead of Left & Right I connect them to an XLR or TRS as Line and Return, with the two screens connected together on pin 1?

Even a balanced Ground Loop Isolator only has one transformer per channel. It's just wired slightly differently, without connecting the winding to screen / pin 1. So provided you can get into the box and separate all the connections, you only need one isolator to do stereo.

• The primary (input) of the transformer goes between Line and Return on the source signal.
• The secondary (output) of the transformer goes between Line and Return on the destination.
• You connect the screen round the transformers (if present) to pin 1 one side (either will do), and nothing at all to Pin 1 on the other!

You now have balanced out and balanced in. Note that you have doubled the signal level through the transformer compared to using it unbalanced. Not generally a problem, but using little transformers on high level signals you might find that large low-frequency signals give you distortion due to the transformer core saturating.

Or get an Orchid transformer isolator: http://orchid-electr...al_isolator.htm (I've got one, it just works)

Maybe a pair of these: https://www.studiospares.com/studiospares-red505-line-splitter-458240.htm (no idea if they are any good).

If the inputs and outputs are both balanced, have you checked whether inserting a 100R resistor in series with the pin 1 connection works to cure the original problem? The idea is that the resistance stops the current circulating (without being completely open circuit), and the balanced signal should cope with the small AC voltage between grounds OK.

 

[Sound Man]

If source and destination are balanced, you could try just losing the screen on the lead, without a transformer - just connect pins 2 and 3.

 

I can saw that I have tried this method and it did work.

 

David

[sandall]

The other is to go between a mixer output and a Symetrix processor. I've used one of said stereo types to find out if that was the issue and it has worked. However it is unbalancing the (I would assume) balanced XLR out from the mixer then feeding in to the (again I assume) balanced line input on the Symetrix as an unbalanced signal.

I suppose if it works....then stick with it, but part of me says I should endeavor to keep the signal path balanced.

The old-BBC purist in me says "keep it balanced", but unless your mixer & processor are a (very) long way apart in a really "noisy" environment I wouldn't worry about an unbalanced link for anything "Line" level (unless it's an old-school broadcast mixer your output is probably "quasi-balanced, i.e. floating, rather than balanced anyway). You can even run unbalanced mic cables a surprising distance without trouble.

[sunray]

I have a need for a couple of GLI's. One is for a PC output to mixer. I already have some of those common stereo types which works a treat.

The other is to go between a mixer output and a Symetrix processor. I've used one of said stereo types to find out if that was the issue and it has worked. However it is unbalancing the (I would assume) balanced XLR out from the mixer then feeding in to the (again I assume) balanced line input on the Symetrix as an unbalanced signal.

I suppose if it works....then stick with it, but part of me says I should endeavor to keep the signal path balanced.

 

Cutting to the chase... short of going with pro-kit for proper money, is it possible to be cheapskate and use a pair of the aforementioned stereo GLI's but instead use 1 per channel. So instead of Left & Right I connect them to an XLR or TRS as Line and Return, with the two screens connected together on pin 1?

I don't see why it shouldn't work. Maybe I should just try it and see! :)

 

Hope that makes sense. I think it does to me :blink:

It makes a lot of sense. Assuming you have something like this: https://cpc.farnell....-rca/dp/AV26879 and a multimeter check for isolation between ALL screen wires, especifically check between both plugs screens and then between both socket screens, does that make sense?.

Assuming they are all isolated [like all of the units I've used] then chop the plugs and connect the centre core to pin 2 and the screen to pin 3 of your XLRs. The short length of 'coax' cable won't be a problem [as it seems you may have already found]. This will give you a stereo balanced isolator.If you particularly wish to alter the cable, ie to extend them:

The units I've linked to consist of 2 metal case halves which are an interference fit and covered with heatshrink, If you look carefully at the picture you can just make out a bump half way along the body. Removing the heatshrink and carefully working the 2 halves they can be separated.

Internally there is a single PCB, basically wrapped in insulation and wedged in place, containing the 2 transformers. the cables are easily replaceable. I have modified several for very different applications.

One of the hospital radios I used to help had a balanced o/p running down standard cat2 cable, each ward/section used a TOA 500 or 1200 series style of amplifier with balanced I/p. At each location one of the above transformers was modified by replacing the lead and terminating the cat2 directly onto the pcb, all wired as stereo but only one channel used, there were something like a dozen of those transformers hanging directly on the single balanced feed.

 

 

Then the unexpected use of the second channel... they used to do little 'outside broadcasts' around the wards using a cassette recorder and playing it later... until Sunray was on the scene and using a radio mic RX and buffer amp on the spare channel they went live.

 

 

I've always stuck with them because I lnow them, a 'competitor' has the same opinion about these: https://cpc.farnell....ator/dp/AV12774I have not tried these https://cpc.farnell....ator/dp/AV19954 but another has said to avoid them, no idea why.

 

Sorry gone OT but it may be useful somewhere.

 

 

For the PC have you seen these: https://cpc.farnell....m-to/dp/AV25544 I havn't tried thiem but they seem to be made for the job.

 

The old-BBC purist in me says "keep it balanced", but unless your mixer & processor are a (very) long way apart in a really "noisy" environment I wouldn't worry about an unbalanced link for anything "Line" level (unless it's an old-school broadcast mixer your output is probably "quasi-balanced, i.e. floating, rather than balanced anyway). You can even run unbalanced mic cables a surprising distance without trouble.

Balanced is always the better option where possible/sensible.

Funny thing is balanced systems are like Marmite, some people seem to hate transformer I/p & o/p and others rue the day electronic versions became popular.One guy I know insists he has never had a single issue connecting balance ports together but then he didn't know his soundcraft mixers o/p's were unbalanced, but called; ground compensated [I just treat them as unbalanced]. I habitually insert GLI transformers for the sake of it it if the o/p's are not transformers.[I managed to fit 6 o/p transformer in my first Spirit F1 and made up a 1U box of 12 transformers and XLR & jack sockets and a multicore to go with the Soundcraft SX o/p's & returns]

In the very early days our mics were all very low impedance, think ribbon mics/big yellow cast iron Trix etc and our first cables were mostly TRS twin flex.

 

Then mics like the Foster range became popular which were mostly 50KΩ and suddenly we didn't need the add in transformers and then the Jap mics with a reversable 4P connecter for 600Ω/50kΩ [We used Yu-Brothers electret mics with 1.5V battery @ £8 from Norman Rose in mid 70's]. We knew the maximum spec'd cable length was 20ft or so but we regularly ran 50m of cheap single spiral screened into the arena without too much trouble. Yes there was handling noise etc but generally the MC was trainable.

 

The bigger problem was the silly idea of using DIN plugs which a lot of our kit seemed to use and then the even sillier idea of changing over to jack [which a lot of Jap mics came with as standard]. It wasn't until mid/late 80's that I started using XLR and mid 90's when getting more modern kit that I went hell for leather changing over to XLR.

 

An amazing journey from holding a DIN plug insert as little more than a toddler while Dad soldered it with an iron heated in the gas cooker flame and buying screened cable from Woolworths.

 

EDIT:Reading the thread again I realise I misunderstood OP and some replies. I'll make no apologies for repeating some of the advice.

Edited October 28, 2020 by sunray