Buzzing throught cans system

[Alec97]

Hi all Basically when we use the cans system we always get a really low frequency buzz through every headset, it's a tecpro system and all the cabling is run internally in the building. I've tried changing cables etc but can't seem to get rid of the buzz. This buzz is not just a little hum it's really loud and makes it hard to communicate with one another.

The power supply is a tecpro PB75. Any ideas what it could be. There is a transformer right beneath it?

[Brian]

Your wiring MUST NOT connect to earth ANYWHERE in the system. That includes the screen. All three conductors must float everywhere. Your PSU will take care of any earthing required. The usual gotcha is having a connector body connected to pin 1.

[pjrails]

Hi

I have the Tecpro master station and had this problem and found that an earth was connected in the mic snake box that we were running through to the stage.

I found the only way was to run a dedicated cable just for the headsets and it has now been running fine for the past 15/20 years with no hum.

 

 

HTH

PJ

[Alec97]

Right Sussed it now I have un plugged channel A send and return and running on channel B and the buzz has gone maybe the wires crossed somewhere :))

[Bazz339]

Right Sussed it now I have un plugged channel A send and return and running on channel B and the buzz has gone maybe the wires crossed somewhere :))

 

 

The usual cause is earth loop on the line input program feed if it has one. Signal 0v is connected to mains earth and case inside.

[Wingwalker]

My old school is located on the south coast right by the sea. We had a comms feed from the PSU backstage for the SM and crew which went up and through the roof to the back of the theatre linking Lighting, Sound and Followspots. Every once in a while particularly during the summer months we would receive the dulcett tones of Radio Luxembourg playing through the headsets as the cable acted as one giant aerial which gave us hours of endless fun.

 

Happy days... :)

[Richard P-W]

Right Sussed it now I have un plugged channel A send and return and running on channel B and the buzz has gone maybe the wires crossed somewhere :))

 

Based on that statement, have you got the belt packs connected in a ring, with a cable returning from the last pack to the master station? If so - that's your earth loop!

Although it's called a ring, you actually daisy chain the belt packs - the last pack in the line only needs one cable connected.

[indyld]

Bit of thread necromancy, couldn't really work out which was the optimal thread on buzzing comms systems to add this to but this seems to be Google's favourite.

 

One probably for the electronics gurus on here.

 

Today, I had a brief look at a comms base station which wasn't Tecpro but similar and seemed compatible with the standard Metro type beltpacks and Tecpro headsets. I wanna say Eurolight(?) but forgot to take a pic while at the bench.

 

The base station has a mains hum even with a single beltpack/headset connected and 20M cable all locally. This isn't an installed system and isn't being interfered with by anything else, the base definitely has a buzz all of it's own.

 

Significant things I've found out so far:

 

There isn't a really obvious physical fault like dry caps or blown up stuff etc. but I did think that a some of the solder joints looked pretty dry in places.

 

The system XLR3s on the station Pin 1 definitely have full continuity to the chassis/earth lug but this isn't obviously connected at the XLR chassis or bridged in the cable. It's somewhere 'invisible'.

 

The station makes a deafening capacitor discharge sound down the headset when powered down.

 

(Less significant: The local headset connection doesn't work at all, but I saw a lot of dry looking joints on that part anyway).

 

Anyone have thoughts on further investigations for tomorrow?

 

Cheers

 

R

[timsabre]

Possibly bad ground connection between beltpack (pin 1) and 0v of the power supply (this will not necessarily be chassis, in fact should not be connected to chassis)

 

What is the power source? If a transformer, the smoothing capacitors could need replacing.

 

With the power off try feeding a 24v dc power supply into pins 1+2. 12v would probably work too. If buzz goes then it's a psu fault. If not then a wiring fault.

[indyld]

Possibly bad ground connection between beltpack (pin 1) and 0v of the power supply (this will not necessarily be chassis, in fact should not be connected to chassis)

 

What is the power source? If a transformer, the smoothing capacitors could need replacing.

 

With the power off try feeding a 24v dc power supply into pins 1+2. 12v would probably work too. If buzz goes then it's a psu fault. If not then a wiring fault.

 

Power is via an internal toroidal with a board including rectifier, regulator and a few caps. Decided that this was a key area for investigation.

 

Like the idea of skipping the internal PSU, had been thinking about how to isolate the different bits of the system.

 

Will also check the 0v / GND. Kinda had started some of that by trying to work out which bits were connected to the chassis.

 

Thanks.

[timsabre]

You can feed 24V DC to the +/- terminals of the rectifier without disconnecting anything

[pmiller056]

I had similar problems when building a 'blueclone' system. Initially I used a double insulated class 2 plugtop-type 24V DC power supply with no earth connection anywhere - lots of hum and buzz on the headphone system. Connecting pin 1 (common and supply negative) to mains earth produced a practically silent system when idle. In your case, check that there is a connection to mains earth in the power supply. Bear in mind that metal cased, but double insulated unearthed equipment is now quite common.

 

Peter

 

[Brian]

Comms systems are an unblanced audio system so there must be one earth, and one earth only, in the system. This is usually done at the power supply. By the same token, all cable screens and connector shells should never be tied to earth anywhere else, even if run through patch panels.

[indyld]

In the end, I had higher priority fixes to spend time on so the base station didn't get a ton of attention. I think it was a fairly old built-to-order that appeared to be made from stock RS parts rather than a production unit.

 

What I did discover was how Pin 1 was connected to the chassis: the 0v Com of the PSU was tied back the earth lug. Oddly enough, temporarily disconnecting this tie made small improvements to the hum level in the main system (I.e the result of removing the continuity between Pin 1 and main Earth - I guess that it just created a sort of ground lift.)

 

Along with the 50Hz hum, the 'local' headset system (a board that appeared to be basically the guts of a beltpack in the base station) induced a lot of higher frequency buzz particularly with the local mic on. What I'd describe as guitar pickup buzz of the kind that varies in loudness when you wiggle stuff. This whole 'local' board had connection issues to the main system which were resolved and that eliminated the capacitor discharge noise on power down.

 

The unit has been shelved as 'functional for use with beltpacks and low expectations'. Lampies cans systems are never known for their quality - haha. Although I'd love to chase all the issues down and turn it into a silent, 100% lovely thing, there are just too many R2 Beams with lamp connection issues and 1T Lodestars that need reviving - all the stuff where the money comes from during a busy summer.

 

I'd still be interested to hear any expert thoughts for my own interest and in case I find myself with more time to look at it again. I assume that due to the unit's age, some capacitors have gone senile.

 

Cheers.

[Brian]

IIRC the Eurolight(Eurolite?) beltpacks had their metal cases floating but connected to 0v via a small value capacitor to bleed any RF away.

 

I do have a big pile of spares/broken packs down in the store and functional units on the shelf. I think I also have schematics somewhere.