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[Tim Barber]

It looks like somebody tried to do their own isolation transformers on each line.  Radio Shack 273-1374 if I'm not mistaken:

http://www.amazon.com/NEW-RadioShack-Isolation-Transformer-273-1374/sim/B002M3KCGU/2

Good call.  That's it, almost certainly.  Radio Shack support lists the following specs:

1:1 Audio Transformer  (273-1374)    Specifications    Faxback Doc. # 9888
Coil Turns Ratio:......................................................1:1
DC Resistance:
   Secondary:..............................................60 Ohms +/- 10%
Impedance:....................................................600-900 Ohms
Frequency Response:........................................300 Hz to 5 kHz
Insulation Resistance:.....................More than 100 megohms at 250VDC

The frequency response raises my eyebrows a bit.  I wonder if these things are not helping my sound?

[Tim Barber]

We can only guess.  if you meter the wires and disassemble and provide a drawing of the unit than we could possibly find an answer. 

Issolation transformers or blocking of the phantom power.  Isolation to send to line input on second mixer for video production ?    where did the TRS 1/4 go to ?

many good reasons.  Any documents or history on the setup in question  two mixers ?

No documents, nobody left to ask.  TRS 1/4 goes to a line input on stage.  Clearly someone thought it was worth doing, although if RFI rejection was their goal it didn't work.

Here's the guts.  Red/black to transformer in; white/yellow transformer out to pins 2 and 3.

[Rob Spence]

No documents, nobody left to ask.  TRS 1/4 goes to a line input on stage.  Clearly someone thought it was worth doing, although if RFI rejection was their goal it didn't work.

Here's the guts.  Red/black to transformer in; white/yellow transformer out to pins 2 and 3.

Can you correlate modded cables with your problems?


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[Mac Kerr]

It looks like somebody tried to do their own isolation transformers on each line.  Radio Shack 273-1374 if I'm not mistaken:

They may have tried their best, but it wasn't very good. The transformers themselves are pretty "LoFi", and the installation is horrendous. Not only are the unshielded transformers not in a metal box, the leads are not twisted, and unless you removed it, the lack of shrink tubing over the ends of the loose wires leaves them open to all sort of issues from contact to the shell.

A good start would be cutting all on those ends off and wiring them properly.

Mac

[Scott Holtzman]

No documents, nobody left to ask.  TRS 1/4 goes to a line input on stage.  Clearly someone thought it was worth doing, although if RFI rejection was their goal it didn't work.

Here's the guts.  Red/black to transformer in; white/yellow transformer out to pins 2 and 3.

Your problems keep growing.  The shield is connected to pin 1  audio ground.  That means and induced currents are draining to the audio ground.

The solder job is so so.  As you mentioned the frequency response of the transformer may be rolling off the highs.

With the time you have spent on this you could have just cut the connectors off and redo it.

[Tim Barber]

Can you correlate modded cables with your problems?

Some but not all, sadly.  I'm still working my way through the stage pockets with my new Rat XLR Sniffer and finding other problems there, too.

[Tim Barber]

Your problems keep growing.  The shield is connected to pin 1  audio ground.  That means and induced currents are draining to the audio ground.

The solder job is so so.  As you mentioned the frequency response of the transformer may be rolling off the highs.

With the time you have spent on this you could have just cut the connectors off and redo it.

Which I have just about decided to do.  I just need to get through Easter week at church before I can tackle anything major.

So, are you saying that the drain wire/shield should float, and not be connected to Pin 1?  I did some searching on this question but did not really understand what I read.

[Rob Spence]

Some but not all, sadly.  I'm still working my way through the stage pockets with my new Rat XLR Sniffer and finding other problems there, too.

That's great. Finding problems if often the hard part. Once you know a connection is wrong then it is usually straightforward to fix... Rinse, repeat.


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[Scott Holtzman]

The shield gets connected to the connector body not Pin 1.  Pin 1 is for the ground.  If it's only a two conductor cable then you are low on options.

I am sure different opinions will be offered.  Not all XLR cables are shielded.  It's a balanced signal so a shield is not required, it gets complicated but look up the term common mode rejection ratio.

AES standards say pin 1 is chassis ground.  Since a ground is a single ended connection it can act as a big antenna. 

You have to ring out the connectors and see if you have an open.  That RF is getting in somewhere and a loose connection acts like a detector and boom you have a radio.  A poor solder joint can also have a diode effect.

Did you ever build a crystal radio as a kid?  It's the same theory.

When you started down this path did you think it would be such a deep dive?

[Jordan Wolf]

In accordance with Scott's "deep dive" statement, here is a great read on the subject of signal grounding, etc..




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