That white thing is a deal gas discharge tube over-voltage protector that dissipates most of the over-voltage surge. It is followed with two other layers of protection (VDR and surgector)

Steve Smith wrote (put here with his permission):
“If you’re into making your own stuff, this will show up any fault with a mic cable when connected to phantom power. Three green LEDs shows a good cable, other combinations show faults.”

This looks like a quite nice and simple to build circuit if you have suitable dual color LEDs on your hands. The circuit is powered with 48V phantom power from audio mixer. The zener diode in this circuit is here to detect the situation that pins 2 and 3 have been shorted to each other. The resistor is here to limit the current through the LEDs.

This circuit looks to be in functions pretty similar to Sound Tools XLR Sniffer/Sender at https://www.ratsoundsales.com/mm5/merchant.mvc?Screen=PROD&Product_Code=soundtools-xlr-snifsend

I finally got all the components to test the circuit. This circuit design worked well on my initial tests with my circuit test prototype.

For comparison here is my circuit where I used three single color LEDs, three diodes and two resistors:

PA amplifier rack built in late 1990′s with practically very low budget: DIY amplifier (uses some Rowe jukebox parts with new transformer put yo rack case), repaired/modified Kenwood KA-7X amplifier for sub, car active crossover, wooden DIY rack, extension cord and ground fault interrupter. The main speaker are built to old home stereo speaker enclosures. The subwoofer was my own design based on Cerwin Vega subwoofer element.

Here i the front side of the amplifier rack.

Here is the backside of the amplifier rack with connector panels. The audio input is 6.3 mm TRS (Tip Ring Sleeve, balanced) jack on the active crossover (black box on the top). From there signals go to amplifiers (unbalanced 6.3 mm outputs that to 6.3 mm jacks or RCA on amplifiers). The speaker outputs are wired to Neutrik Speakpon connectors.
The power input is 230V AC 50 Hz using SCHUKO connector. The power distribution consists of extension cord, ground fault interrupter and power outlet splitter. Originally I had everything just plugged to this three output extension cord, but then I added later that ground fault interrupter for safety.

Here is the overview of the wiring inside the amplifier rack.

That Kenwood originally put out 110 watts rms output per channel at 8 Ohms and 560 watts Dynamic power output at 4 Ohms. With single channel used (other dead) I could get nearly 180W continuous and nearly 600-700W peak to sub.

The amplifier that drives the left and right speakers is based on old Rowe amplifier module form jukebox. This video shows similar looking Rowe amplifier I used in my DIY amp. I replaced original 110V transformer with 230V input transformer and threw out the broken preamp board. With all the tweaks I could get up to 2x120W power from it to four ohms speakers.

The left and right speakers were based on the cabinets from old home HIFI speakers (came with ASA StudioTrio system). The woofer elements had failed years ago, and I replaced them with new elements. First I used some cheap 8 ohms element from Bebek and later a 4 ohms car hifi element. The high frequency driver is original. I did measurements to the speaker performance and did slight modifications to crossover where needed.

The speaker accepts bananas, bare wires, 6.3 mm jack and wires with Speakon connectors.

Some old measurement results with old woofer element.

Subwoofer is my own design based on Cerwin Vega 12″ subwoofer element. The box was designed so that I could fit it to my car and built so that it can take some road use. I use around 65 liters inside volume with bass reflex tuning. I used the element manufacturer example recommendations as starting point and changed is to that I can play to somewhat lower frequencies (I used some speaker simulation software to simulate how it behaves). I use that subwoofer nowadays on my home theater system.

Here are some pictures of a prototype:

To get the idea of the signals what go on the VGA cable normally, take a look at this:

In good quality VGA cable at least RGB signals are small coaxial cables. The HSYNC andf VSYNC could be coax or just wires.
Other ID and control signals just go through as thin wires on the cable.

The signals were arranged to VGA cable so that the video signals were carried over R and G signal coaxial cables. The audio communications uses B coaxial cable for audio and one other “free” wire for it’s power.

All the remaining wires were reserved for camera power feeding so that several wires in parallel were used and + and -. A floating 24-30V DC (current limited to less than two amps max) was fed to the power feeding wires. On the camera end the power goes to a Mascot voltage 24V to 12V linear regulator module that converts to 12V DC that is right voltage to power many video production cameras. This arrangement allowed stable voltage to camera and the many volts of loss (load current dependent) in cable does not affect the output voltage to camera. Because the power – gets connected to video signal ground only at camera end (the power supply is floating on sending end), the noise caused by power feeding voltage drop on the cable does not affect the video signals.

A friend called from a big computer festival Assembly 1999 that was starting. It was one of the biggest this type of event in Europe with thousands of people coming with computers. Some of my friends was running big screen and video streaming for the event.

They had all the stuff rented and set up for the event by the AV contractor. Everything worked flawlessly on the previous day. But on the first day the audio and video system started picking up lots of noise. The audio signal had mains humming in it, and video signals had “humming bars” on them. As more computers get powered up on the hall, the worse the noise becomes. We are not talking about few computers, there was over two thousand computers on the hall.

Same event 2010

I walked in to the even with my tool-set. Solving the audio problems involved sorting out the lines that picked most noise. Most of the audio connections were analogue (both balanced and unbalanced) and most video connections used composite video interface. I walked in to the event with necessary tools. A combination of audio signal isolation transformers and “ground lift” on some balanced audio lines made situation OK.

Video signals were a tougher problem. There were some very long video line runs from the main video mixer to different locations (projectors for big screen, smaller screen etc..). Those long video lines seemed to have lots of noise in them. I measured form 100 mA to 1 ampere mains AC current on coaxial cable shields on the cables with most humming bars! Disconnecting some of those longest lines made the video system to work noise free, but what to do because we can’t live without connections to video projectors.

The main program on the event was starting on the next day, so this problem needs to be solved so that everything works flawlessly or at least “well enough” by tomorrow. I know I would needed some type of video signal isolation devices for those problematic video lines, but our equipment supplier did not have them. It was already quite late night, so it was impossible to try other sources.

Fortunately I had one prototype of video “humbugging” transformer with me. It was a common mode choke built by winding thin 75 ohm coaxial cable over a suitable toroid transformer core. This device reduces the amount of current flowing on coaxial cable shield and reduces the noise pickup. I built the device based on some old documentation (someone sent me old document from BBC on hum bugging transformers). I tested this DIY “humbugging” transformer on one of the video lines, and it worked very well. The amount of noise on that video line was reduced to point that it could no longer be detected on the big screen.

The problem is that there were also other lines that seemed to need the same kind of treatment. I went to home lab, trying to gather whatever parts I could find to build more such devices. I ended up building three more on the night from the part I could find – I even managed to find nice plastic cases for them. Next morning I came with my brand new “humbugging transformers” to the event. They worked and saved the show next day. What I made looked pretty much like this inside (this is a later made version that uses the same box).

In the end the video equipment provider that rented most of the video gear for the event end up buying my boxes after the event at decent price. I ended later making some more for this company. I ended up making more on then when needed for some video companies. Quite often when I first rented few of them to solve an emergency, they wanted to buy them for themselves.

They have performed very well on many video systems to solve ground loop problems. There are passive hum suppressor transformers that will very effectively remove the hum from the video signal, but do not affect the video signal otherwise. Those special transformers act like a common mode coils, which stop the annoying ground loop currents on the shield of the coaxial cable, but provide a straight path for the signal inside the cable. This type of device is capable of passing the signals from DC to tens of MHz without problems. The hum suppressor transformer both reduces the current flowing on the cable shield and compensated the voltage differences that would otherwise be between cable ends and eventually get to the signal.

You can find more information on my ground loop noise reducing boxes at
https://www.epanorama.net/blog/2009/09/06/build-humbugging-transformer/ and https://www.epanorama.net/blog/2009/09/15/build-video-isolator/ and https://www.epanorama.net/blog/2010/08/12/audio-isolation-transformers/.

I ended doing more audio and video tricks to keep this event running,

“Quickly troubleshoot faulty mic lines and cables with this set of remote end cable testers. The XLR Sniffers can also utilize phantom power to test snake lines and cables in live environments, making it simple to diagnose issues and get your show up and running.

When you need to know quickly whether it’s the mic or the line, the XLR Sniffer/Sender duo will give you an instant result. 

The Sniffer unit has 3 LED lights. When attached to an XLR cable that is powered by a phantom power source (either a console or the Sender unit), the Sniffer will light up in either green or red LEDs to show whether the line is good (all green) or has errors (all red or a combination of red and green). The XLR Sniffer/Sender can even test for a pin 2-3 short. 

Sniffer and Sender units are not intended to replace a multi-meter during your repair phase. These are designed as a fast way to test for faulty XLR cables and phantom power in the field.”

I saw some videos of this product also:

I started wondering how it works. I did not figure out all the details based on just web pages and some idea. Then got idea to design my own version of idea.
Here it it:

Results:

All LEDs output light = Everything OK

Left and right on, center off = Short between pins 2 and 3

Left LED off = pin 2 not connected or shorted to ground

Right LED off = pin 3 not connected or shorted to ground

All LEDs off = not plugged in or phantom power missing or ground connection missing

Here are the circuit details of my idea that is designed to work with standard 48V phantom power supplied by an audio mixer:

The circuit is based on the idea that it takes phantom power from the mic line to light two LEDs. There are somewhat different load from pins 2 and 3 to ground. This causes some voltage difference between those pins, which is enough to light up the third LED if everything is correct. That’s the basic idea. In addition to LEDs and current limiting resistors, there are three diodes (1N4148) that protect the LEDs agains too much reverse voltage that cold occur on some wiring errors cases.

I am still waiting to build final version inside an XLR male plug.

Passive summing works generally ok, both with unbalanced and balanced circuits if all connected circuits are same type (mixed balaced and unbalanced is mixed business if it works as wanted or not). When the devices connected to all inputs are powered on, things work typically very well. In some cases if you turn off some devices connected to one input, it can start to introduce distortion to sound.
I have used my DIY passive mixer circuits several times with event PA system and TV broadcasting setup.

For my passive summing boxes I have used resistance values from few hundred ohms (professional equipment balanced/unbalanced interfaces, potentially long lines and audio transformers on system) to 10 kohm (consumer equipment short distance to amplifier) depending on case.

Typical audio outputs have output impedance in around 20 ohms to 2 kilo-ohms range.

Balanced passive thing would be used to combine a couple of identical sources. Microphone or mixer.

Here is one of my DIY passive mixer circuit I have used for balanced signals from professional equipment (600 ohm or lower impedance).

Triax cables are used in TV broadcast industry for TV camera interconnections (connecting camera to CCU and supplying power to camera). Triaxial cables are constructed with a solid or stranded center conductor and two isolated shields. I was told (by a guy that was wondering what the heck I was doing) that national TV broadcasting company YLE typically sent their triaxial cables abroad to special company for repair.

I have correctly re-terminating triaxial cable to it’s connector on the field with leatherman multitool, small soldering iron and some basic tools. The cable end was damaged on use because the connector was originally installed incorrectly (one piece was put in wrong way). This camera cable in use in on location TV broadcasting setup and was was used in 15 minutes after repair in live TV broadcast.

I also made some makeshift adapter connectors from old antenna coax connectors that allowed me to make connection to triaxial connectors. This later allowed to use coax cable testing tools with them and run coax video signal over long fixed triax cable infrastructure wiring at location.

Saving one arena event video system by building several DIY get rid of video ground loop noise boxes from parts I had in hand one night and selling them to audio/video rental company after the event. I have written about those ground loop noise reducing boxes at
https://www.epanorama.net/blog/2009/09/06/build-humbugging-transformer/ and https://www.epanorama.net/blog/2009/09/15/build-video-isolator/ and https://www.epanorama.net/blog/2010/08/12/audio-isolation-transformers/.

I have also built some adapters to transport video signals over twisted pair wiring as described at https://www.epanorama.net/blog/2013/09/29/video-over-utp/. I also used clamp meter to troubleshoot ground loop problems.

One other McGyver trick was tracing where some unmarked connectors on the broadcasting connection point go and mark them correctly. I used DIY signal sender and DIY inductive amplifier to trace down the other end on the BIG media cross-connection central panel (few rack cabinets pretty much full of XLR connectors). I used the circuits I have described at https://www.epanorama.net/blog/2013/03/05/cable-tracing-inductive-amplifier/ in the process.

Closer look

Circuit diagram

This is very simple design. It just needs two resistors (I used three because I did not had exactly 24.9 kohms resistor in hand so I built it from two resistors in series). This is a way simplified version of my earlier PoE Powered Christmas Lights circuit.

The 24.9 kohms resistor connected between power input wires is the IEEE 802.3af PoE identification resistor that makes the PoE power supply to know that there is class 0 PoE capable device (can take up to around 15W power and does not specify exact power it needs). The IEEE 802.3af power supply probes the resistor at lower than 20V voltage low current pulses, so at those voltage levels the LED string does not pass any current (it has many LEDs in series and is designed to work at 31V DC). When power supply detects proper PoE device, is starts to feed 48V DC to the line. That 48V is feed through 390 ohm resistor to the LEDs, this resistor will limit the current an “burn” the voltage 48-31V=17V.

This circuit worked well with two different PoE power supplies with the 31V LED string. I tested that the circuit worked also with LED string that was designed to work at 24-26V DC voltage. Do not try lower voltage LEDs in the output, because trying that risk of overheating the resistor and messing the IEEE 802.3af device detection.

PS. In full standard compliant design there should also be a bridge rectifier between RJ-45 connector and rest of the circuitry to handle situation that the wiring can sometimes change the order of power carrying wires.

Circuit diagram:

The circuit operation:

Initially circuit presents around 23 kohms impedance when low voltage is on line (PoE PD is expected to be 19-27 kohm at 2.8-10v). Power supplying circuit can detect my load (PD). The 22k and 1k voltage divider keeps transistor off when voltage is 10V or lower.

When PeE power supplying circuit detects PD, it turn on 48V DC to line (+ to 7,8 pins and – to 5,6 pins of RJ-45). When voltage is 48V, the transistor can turn on passing power to the DC-DC converter. My 12V indicator LED will turn on getting it’s power through VDR that that takes around 35V voltage drop.

This was a “quick hack” that worked this time. It is not a reliable circuit recommended for any real application.