Comms PSU

[TomHoward]

Morning all

 

We have six ASL beltpacks & headsets which are powered from an ASL BS 181 power supply. We've got an upcoming need to use three packs at two different events - so we ideally need a second PSU. However, budgets being what they are, the ASL PSU is around £250, so I'm wondering if I could just wire an inline power supply up to an XLR or similar.

 

Would anyone know what kind of voltages I'd be looking at, and whether it's feasible / sensible?

[Brian]

Yes.

 

Just a 24V supply with a few extra bits.

[osal]

or hire

[boatman]

Yes.

 

Just a 24V supply with a few extra bits.

 

The audio termination on that power supply is very poor. In true Blue Peter style, here's one I made earlier:

 

Tecpro compatible power supply schematic

 

It just needs a 24V wall-wart.

[TomHoward]

Thanks for those. I had a look inside out existing psu and it's an off-the-shelf switching psu board, with a diode over the voltage out, an RC filter, and a switch that shorts the voltage to ground when off.

 

What power psu would you suggest? That comclone site suggests 100mA per pack, although that sounds a bit low to me I guess it doesn't need to be too beefy, 1-2A should cover it?

[boatman]

I ran six headsets with the above interface from a 24V/2A wall wart from CPC with no problems at all.

 

Edit to add:

 

I just checked inside the box and the pin numbers on the output connectors apply to the headers on the Veroboard not to the XLR connectors. The XLR pinout should be:

 

1 - Ground

2 - Power

3 - Audio

 

In order to avoid any confusion, a corrected schematic with XLR pin numbering can be found here.

[TomHoward]

Thinking about it, this is getting sent down mic cable, so it doesn't want to be too high.

 

A concurrent post has been automatically merged from this point on.

 

I ran six headsets with the above interface from a 24V/2A wall wart from CPC with no problems at all.

Excellent, thanks. Did you find the circuit was small enough to build into the back of an XLR plug, or did you use a project box?

[boatman]

It all fitted in a 75mm square project box with a DC input connector and two XLRs.

[Stan Hope-Streeter]

I had a look inside out existing psu and it's an off-the-shelf switching psu board, with a diode over the voltage out, an RC filter, and a switch that shorts the voltage to ground when off.

 

Two-wire comms systems work by common impedance coupling. The PSU needs to provide a low source impedance to audio signals, so any old DC PSU will not do the job. Maybe you need to look more closely at that "RC filter" to see what it's doing.

[dbuckley]

Any old 24V PSU with very low ripple and preferably regulated will do, but be aware that shorts are not unheard of with intercoms, so a PSU that can resist shorts and without delivering enough current to melt cables is recommended. Connect the +ve to pin 2, and the -ve to pin 1.

 

Pin 3 is the audio - build the line terminator shown here, the capacitor really is a tantalum, and make sure it is > 30V rated, and the resistors should be 1%.

 

And for giggles: heres the innards of a TechPro PSU I picked up on eBay, the tantalum caps are clearly visible, except for the melted cap and resistor. Looks like someone had an 15A to XLR adapter handy, and plugged it into the intercom ring...

 

http://davidbuckley.name/pix/fried_tp_psu.jpg

 

The PSU claims to be from the Hammersmith Odeon

[TomHoward]

I had wondered whether I would get away with just the audio termination if the PSU was good enough. I'm going to have a dig around and work and see what I can find.

[Stan Hope-Streeter]

I had wondered whether I would get away with just the audio termination if the PSU was good enough. I'm going to have a dig around and work and see what I can find.

Yes, the audio termination is what's needed to make it work properly.

 

<br /><br /><I>A concurrent post has been automatically merged from this point on.</I><br /><br />

 

And for giggles: heres the innards of a TechPro PSU I picked up on eBay, the tantalum caps are clearly visible, except for the melted cap and resistor. Looks like someone had an 15A to XLR adapter handy, and plugged it into the intercom ring...

Oh, what a nice picture.

Tantalum capacitors are renowned for going short circuit and burning up the circuit board and anything else nearby...

[TomHoward]

Thanks for all the input

 

I've put one together using an old laptop PSU, and using some of the Neutrik module bits that I had lying around (for making adaptors etc) I managed to fit it all inside an oversized XLR connector. The PSU outputs 20V instead of 24V but it seems to work, I'll test it down the full length of the muticore just to make sure though.

 

http://img832.imageshack.us/img832/1982/dscf8276y.jpg

[boatman]

Excellent job. Well done.

[dbuckley]

Of all the laptop power supplies, in all of the world, you have a Dell.

 

Dell PSU are horrible, as they have earth continuity between the mains earth and the output of the PSU.

 

If you plug this into a multicore it may work, or you may induce noise and buzz into the audio system.

 

The TecPro system couples the intercom ground (from power supplies from mains connections) to earth through a small capacitor 0.1uF if memory serves.

 

Other than that: I have to say that looks a lovely job, and 20V will do fine for the comms. However, that's 20V at several amps, so under the wrong conditions, that PSU could melt wire, as I noted above. Impressive way to trash a multicore...