Multiple PSUs on comms

[samchurchill]

Happy new year!

Is anyone able to tell me whether there’s a problem with there sometimes being two PSUs on a standard TecPro-compatible party line comms system?

I’m putting together a couple of racks which will often be used independently, but occasionally be within the same production, and need to know whether them being connected together would be a problem (and, if so, how big a problem it’d be!).

Thanks

Edited January 1, 2023 by samchurchill

[sandall]

Provided each PSU has a diode on its +volts output I don't see a problem (I may of course be missing something🤔). Tecpro do (or did) make a combiner, but not cheap.

[Brian]

The only danger is creating a hum loop. The comms audio line is unbalanced and must only be ever grounded at one point, usually the PSU. Some PSUs/Master Stations connect the 0V line directly to ground, some via resistors/capacitors.

 

Let me think on it.

[J Pearce]

Normally the PSU provides line termination. You may need to disable line termination on all but one PSU or audio levels may be off and call buttons may behave erratically.

Or use a 1:1 transformer to link only the partyline audio between the multiple racks.

[timsabre]

This is correct, the double termination would cause problems. The termination has an ac component for the audio and a dc component for the call lights - using a transformer would fix the dc problem but the double audio termination would still be there. 

[sunray]

3 hours ago, timsabre said:

This is correct, the double termination would cause problems. The termination has an ac component for the audio and a dc component for the call lights - using a transformer would fix the dc problem but the double audio termination would still be there. 

Hi Tim

That has taken me a bit by surprise. I set up a second bluecom system in a rush for cue lights into into dressing room without a termination, I can't say we noticed any difference.

The system was piggy backed on the intercom but didn't include pin 3

[boatman]

2 hours ago, sunray said:

Hi Tim

That has taken me a bit by surprise. I set up a second bluecom system in a rush for cue lights into into dressing room without a termination, I can't say we noticed any difference.

The system was piggy backed on the intercom but didn't include pin 3

There are two circuits in the Bluecom power pack. Each circuit has an audio termination in the power pack but, if they are joined together using the "Common" button, one of the audio terminations is removed so there is still only one termination.

[samchurchill]

Thanks everyone, sounds like although nothing will be blown up, it could cause some odd effects. I think I’ll just make it easy to unpatch the PSU on one of the racks.

[timsabre]

It doesn't die or anything with 2 terminations. The effect is that the call light doesn't work or only flashes if you hold the button down (normally it would continue for a time after you release the button). And the sidetone settings (how much you hear yourself in your own headset) all become different to what they normally are.

With no termination the call light sometimes flashes all the time once the button has been pressed, or flashes by itself, or doesn't work at all. And the sidetone becomes totally unpredictable, lots of headset feedback etc.

[sunray]

23 hours ago, boatman said:

There are two circuits in the Bluecom power pack. Each circuit has an audio termination in the power pack but, if they are joined together using the "Common" button, one of the audio terminations is removed so there is still only one termination.

Not sure what's happened, I thought I'd posted a reply to this already.

 

The very quick additional service was added in a rush, it was to add a 'call light' to the dressing room with a 'special' pack with strobe and buzzer facilities, where the intercom was not being monitored.

The extra pair of packs were piggy backed on the existing system with a Y split and a cut pin3.

We weren't aware of any misoperations other than a minor click on the other 4 stations

[sunray]

On 1/3/2023 at 10:49 AM, timsabre said:

And the sidetone becomes totally unpredictable, lots of headset feedback etc.

As it happens I have encountered a faulty connector on a PSU. The bad audio performance and sidetone problemis across the whole system was a fairly obvious symptom.

On 1/3/2023 at 10:49 AM, timsabre said:

With no termination the call light sometimes flashes all the time once the button has been pressed, or flashes by itself, or doesn't work at all.

My one attempt without it and without using any audio, the call light appeared to be fully functional. Looks like I was lucky, Thanks

[timsabre]

On 1/5/2023 at 1:50 AM, sunray said:

My one attempt without it and without using any audio, the call light appeared to be fully functional. Looks like I was lucky, Thanks

Might depend on the manufacturer maybe? Normally the call light button puts a DC offset on the audio line. This is ramped up and down fairly slowly to avoid a click on the audio. The DC termination is involved in the time constant of this ramp, without the termination it goes up very quickly (hence the click you heard) and down very slowly, so the call light might stay on for ages.

[sunray]

20 hours ago, timsabre said:

Might depend on the manufacturer maybe? Normally the call light button puts a DC offset on the audio line. This is ramped up and down fairly slowly to avoid a click on the audio. The DC termination is involved in the time constant of this ramp, without the termination it goes up very quickly (hence the click you heard) and down very slowly, so the call light might stay on for ages.

Thanks Tim

For what it's worth in my original sketch the pack on right is a Techpro panel mount version all others are bluecom (with 10 to 22uF cap for call) apart from the strobe unit which is based on bluecom board:

I also have an isolating transformer and cap  in there for picking off audio to an amplifier typically added to show relay in dressing rooms but not used on that show.

Bearing in mind the bus lead for the intercom was not extended to the additional pair of units I assumed it was passing back through the DC part, however the click was, I think,  more prominant than when the unit is used in conventional manner. I'll be getting some kit out soon for panto season so will have a little play if I get chance.

The unit does in fact create a bigger click on the system than a normal BP and is dependant where it appears in the system (although not as much as this time) which I assume is due to the relay and I've been considering altering it to an optoisolator.

[sunray]

On 1/6/2023 at 5:20 PM, timsabre said:

Might depend on the manufacturer maybe? Normally the call light button puts a DC offset on the audio line. This is ramped up and down fairly slowly to avoid a click on the audio. The DC termination is involved in the time constant of this ramp, without the termination it goes up very quickly (hence the click you heard) and down very slowly, so the call light might stay on for ages.

Welllll.......  Thank you very much for the heads up.

 

I've started prepping for a couple of events and pulling intercom out started having a play.

The short xlr lead I cut the wire in was labled and stored with packs and headsets, very easy to link from PSU to BP and a cable to second (special strobe) BP, totally and utterly unuseable, initial call worked as expected but then random flashing and sounder.

Adding 4 bluecom BPs as in initial sketch

altered the flashing but didn't stop it,

changing the remote BP to the techpro unit

nearly tamed it but it would continue for maybe 5 seconds after calling, that didn't make any sense to me as no pin3 connexion between the 2 services.

Up to that point the leads were all up to 5m or so, making the call system 50metres so now longerthan the installed system

Seemed to be fine except the the call continued for something like a second

Adding a resistor to the end

Made the delay virtually disappear and the clicking in the headsets is also minimised.

So yes I was lucky at first use and thanks again for the help.

Up until this thread it had not crossed my mind in any way that the termination had any relevance to the call lamp, if I'd tried to use a similar system again it may have taken ages to sort it.

[timsabre]

Correct termination for audio as well has a 200R resistor + 10uF capacitor in parallel with the 4K7 resistor. You should probably include that to prevent any audio weirdness on your branch line

http://www.dmx512.com/web/comms/tecpro/tptech.htm