# intercom power supply



## wyatt20019 (Aug 21, 2008)

we have some portacom units but no power supply, would a clear com power supply work? like this one 

NEW CLEAR COM PK-5 INTERCOM POWER SUPPLY - eBay (item 150285258622 end time Aug-26-08 15:07:52 PDT)


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## cutlunch (Aug 21, 2008)

They would probably work together. Someone will be a long soon who will know more.

The Clearcom puts out 24V as opposed to the Portacom units which use 22V. 

2 Volts shouldn't matter in this case. 

But you need to get hold of a manual for both the portacom and clearcom units to make sure they apply power on the same XLR pins. They should do but you need to check first.


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## waynehoskins (Aug 21, 2008)

I thought CC was 28 or 30 volts.

In any event, it's likely to work. Most all of the production intercom systems out there use a 28-32 volt dc power supply on pin 2 with respect to pin 1.

I also made my own from spare parts for free.


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## SHARYNF (Aug 21, 2008)

Clear com is 30 volts DC typically at 2 amps for the larger powersupplies (usually this spec if for 50 beltpacks or 15 stations)

Sharyn


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## wyatt20019 (Aug 21, 2008)

i dug through some old power adapters and found these.

adapter 1: input:100-120V, output: 24V----0.75

adapter 2: input: 120-127 69hz, 350mA, output: 30v---500mA (AC power adapter) http://cgi.ebay.com/AC-DC-POWER-SUP...1524815QQcmdZViewItem?_trksid=p3286.m20.l1116

would one of these work?


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## waynehoskins (Aug 21, 2008)

Old wall warts are pretty dirty .. but I have managed to power a CC-compatible pack directly from a 12-volt wall wart before (or even a nine-volt battery). Since CC PL has a dry audio bus, it's less critical how clean the supply is than on an RTS system.

Those power adapters, are those AC or DC outputs? It would be rather a simple matter to cobble together a filter capacitor bank and a regulator circuit to clean those up. The 317 takes about 3 volts off the top, so if that's DC I'd take the 30 volt supply because there's more to cut off. If AC, I'd take the 24 volt supply since that'll make about 32 volts DC, which will clean up nicely, and be able to supply more current.

If I had the choice, though, I'd get a good beefy iron transformer. The one I used came free with the chassis! (my schematic and photos at crossroadsofarlington.org/wayne/rts-psu )


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## wyatt20019 (Aug 21, 2008)

the 30V one is DC. not sure about the other one.


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## wyatt20019 (Aug 21, 2008)

so how do i set this up? can i just solder the hot wire to pin 2 of an XLR head? or do i have to have the capacitor thingy?


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## waynehoskins (Aug 21, 2008)

wyatt20019 said:


> so how do i set this up? can i just solder the hot wire to pin 2 of an XLR head? or do i have to have the capacitor thingy?



Hot, no. That goes to the transformer primary along with neutral.

No matter what kind of supply you use, you'll end up needing beefy filter capacitors: if a homebrew supply, to filter ripple; if a wall-wart or cheap supply, because they don't have much filtering if any inside there. I used a pair of 2000-mike electrolytics to provide 4000 mikes , and that should be pretty fair. That's just a plain linear supply: transformer, diode bridge, capacitor, and that's usually pretty good; but I threw the 317 on the end to clean it up even more.


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## wyatt20019 (Aug 21, 2008)

so what do i need on top of this? http://cgi.ebay.com/AC-DC-POWER-SUP...1524815QQcmdZViewItem?_trksid=p3286.m20.l1116 and how do i do it?


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## waynehoskins (Aug 21, 2008)

wyatt20019 said:


> so what do i need on top of this? AC/DC POWER SUPPLY ADAPTOR 30 VOLT 30V 30VDC 500MA - eBay (item 270261524815 end time Aug-31-08 18:24:05 PDT) and how do i do it?



That's not going to be regulated, but it at least may have some amount of filter capacitor in it. That might work directly as-is. Failing that, it's easy to build up a 317 and the other components of the regulator circuit onto a small board or some such.

Another thing you will want to do is terminate the intercom bus at that same connector with whatever resistor value they suggest (200 ohms or something like that).


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## wyatt20019 (Aug 21, 2008)

thanks, how should i wire it to an xlr head?


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## dramatech (Aug 22, 2008)

check mpja.com. They have lots of 24v dc supplies and their prices are the best I have found. I have purchased several from them to make scroller power supplies. They sell both switching regulated and linear regulated supplies. For scrollers, the switching units are best. For intercoms, the linear are best.


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## waynehoskins (Aug 22, 2008)

wyatt20019 said:


> thanks, how should i wire it to an xlr head?



pin 1 ground
pin 2 dc
pin 3 intercom bus (200-ish ohm terminating resistor)

And it's "XLR connector", by the way.


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## wyatt20019 (Aug 22, 2008)

what do i ground it too? and could i use a DMX terminator on the last beltpack?


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## waynehoskins (Aug 22, 2008)

wyatt20019 said:


> what do i ground it too?


You're right, ground isn't quite the right word. That's actually DC common and signal common.


wyatt20019 said:


> and could i use a DMX terminator on the last beltpack?


While that would work, you really want the terminating resistor to be at the power supply. Its function is different to that of a DMX terminator. While a DMX terminator serves to suppress any reflections from the end of the transmission line, the purpose of the intercom-bus terminating resistor is to load the bus at a static level. Without it, the bus level will be very very high and will be very sensitive to the number of devices on the bus: when you add another pack, the bus level will drop noticeably. This resistor, which could be more properly called a loading resistor, is of significantly lower value than the impedance of a pack, and thus sets the operating level of the bus at something equivalent to 30 or 100 devices. You don't want this loading resistor to be a removable device like a DMX terminator.


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## wyatt20019 (Aug 22, 2008)

so the by the "ground" you mean the wire that isnt hot? and where can i find the resistor/terminator.


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## waynehoskins (Aug 22, 2008)

wyatt20019 said:


> so the by the "ground" you mean the wire that isnt hot?


Well, we don't really use the term "hot" to describe DC. "Hot" is used to describe a particular conductor of an AC circuit or the "true" of a balanced signal. Your cable coming out of the brick there has two wires. One corresponds to the big "plus" symbol on the brick, the other to the big "minus" symbol. The "plus" wire is positive with respect to the "minus" wire; the "minus" wire is negative with respect to the "plus" wire.

For a positive power supply circuit, we take the "minus" wire as common (this is also a "negative ground" arrangement like your car), "common" being the signal that measurements are referenced against. We also call this "ground", though "ground" can mean a bazillion different things.

The wire that's positive with respect to ground is usually given a label like "+30" or, in the case of a single-voltage power supply, simply "power" or "dc".

To find which wire is which, first make sure you have a DC voltmeter, then chop off the end of the DC cable. Separate the conductors of the cable a ways and strip them. Plug in the power supply. Put the red probe on one wire and the black probe on the other. Look at the meter. If the reading has a leading minus sign, then the wire your red probe is touching should be used as ground. If the reading does not have a leading minus sign (that is, no sign or a plus sign), then the wire your black probe is touching should be used as ground.


wyatt20019 said:


> and where can i find the resistor/terminator.


Radio Shack will have something close enough, though they'll overcharge you.

But based on your electronics knowledge, I'd really caution you to simply buy a proper CC power supply. Power supply circuits are simple, but it takes more knowledge and experience than you've got, no offense. "Which wire is which?" and "What's ground?" are huge red flags that you shouldn't be poking around inside PS circuits.


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## icewolf08 (Aug 25, 2008)

wyatt20019 said:


> we have some portacom units but no power supply, would a clear com power supply work? like this one
> 
> NEW CLEAR COM PK-5 INTERCOM POWER SUPPLY - eBay (item 150285258622 end time Aug-26-08 15:07:52 PDT)



For all the suggestions here, why not just get the Porta-Com Base station. The PC-100 base station supports 2 channels of communications, and for about $350 it is worth getting the real thing rather than kloodging something together. I am sure that you can get it at other places, but I found it for sale from B&H Photo & Video, who are very reliable. Link


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## icewolf08 (Aug 25, 2008)

More info for you. If you are insistent on building your own power supply, it does sound like Porta-Com can interface with Clearcom, so things like termination would be the same. As people have mentioned, the pinout for the system is:
1- common
2- +30VDC
3- audio

Termination for this system requires one 10K Ohm resistor across pins 1 and 3, AND in parallel with that: a 200 Ohm resistor in series with a 1uF capacitor.


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## waynehoskins (Aug 25, 2008)

icewolf08 said:


> Termination for this system requires one 10K Ohm resistor across pins 1 and 3, AND in parallel with that: a 200 Ohm resistor in series with a 1uF capacitor.



Interesting. I'd always read or heard about just a single low-value resistor. But I can see the advantage to capacitor-coupling the thing. The high-impedance resistor provides constant dc-through-daylight loading of a rather modest value on the line, and the the capacitively-coupled low-value resistor loads at audio frequencies but not at DC, which I imagine improves the call circuit by not loading that nearly as heavily. Cool stuff.


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## icewolf08 (Aug 26, 2008)

waynehoskins said:


> Interesting. I'd always read or heard about just a single low-value resistor. But I can see the advantage to capacitor-coupling the thing. The high-impedance resistor provides constant dc-through-daylight loading of a rather modest value on the line, and the the capacitively-coupled low-value resistor loads at audio frequencies but not at DC, which I imagine improves the call circuit by not loading that nearly as heavily. Cool stuff.



You got it! I didn't spend a semester of school studying com systems with the man who wrote the book on "Sound Design for Theatre" for nothing! The big thing to remember is that you should only have one terminator in the line, so if your PSU has termination, you shouldn't put your own terminator on the line.


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