Here's a Circuit for a Cheap Intercom
Ben Mehlman
Many times on this group I've seen people asking how to build a cheap
theatrical intercom. I have built several different systems but that was a
while ago (before I got my hands on a cheap used Clear Com system), but I
always used random parts that I had lying around, and not stuff I could
explain to a non-electronics-experienced person.
Well, this weekend I needed a system in a hurry, for a community theatre with
moderate means.. so I built them a system out of stuff entirely from Radio
Shack (I don't love Radio Shack but they're the only place you can get this
stuff on a moments notice, and they are all over the place). It works very
well, and I thought perhaps someone else can use this, because even if they
don't know a lot about electronics they can go into Radio Shack and get
absolutely everything they need. Here's the circuit.
-----------------
| | 50 ohm 10W 120V:12V 3A
| 12V DC 1.2A |+ -------/\/\/\/\----+-----------
| Power Supply | |+ ) # (--
| | 4700 uf ----- ) # (
| | 35V /---\ ) # (--
| | | ) # (
| |- -------------------+ _) # (__
|________________| | |
| |
| |
^ ^
Red Green
To Telephone Jacks
The power supply is the type sold to run your CB or car stereo inside the
house. It costs around $29. In the past I built the power supply portion
myself (usually 24V) but this is a neat solution, safe, saves time, and works
fine. The resistor and capacitor have a dual purpose: 1) Filter residual hum
from the (cheesy) power supply, and 2) protect the system from short circuits
(the resistor limits the current to around 250 MA). The transformer is
actually being used as a choke; Radios Shack doesn't sell a regular choke
that does the job. You use the (thick black) primary wires, and just tape
off the secondary wires. I suggest soldering all connections, for
reliability. If you want a portable system you can probably fit everything
into the case of the 12V supply. It's pretty empty in there. I used a
terminal strip and screwed the whole thing to the wall of their tech-booth
(in community theatre, that's the only way to keep stuff from walking).
The red and green outputs connect to ordinary modular telephone jacks. I
used shielded twisted-pair wire because I thought I might want to plug in my
clear-com system one day, but regular unshielded telephone wire will work
too. The jacks are all in parallel, daisy chaining is ok. You can use a
modular Y adapter to make one jack into two. Re telephones, I'm running the
system now with four headset telephones- ie a regular wired telephone except
that it has a headset instead of a handset and can clip to your belt etc.
Also purchased from Radio Shack. I also have a regular "500" style telephone
in the system (by the dimmers for patching), and a regular cordless phone.
Use any phones you like. The headset phones work well- It's not quite as
good as a professional system but it's darn close. They even have volume
controls- no mute button though.. You have to 'hang up' when you sneeze ;)
There is a definite limit to how many phones you can plug in- I have six now
and it's fine, but I'm guessing about 10 is where it'll start to be a
problem.
Anyway, the whole system as described, including wire, jacks, power supply
and other parts, and four headset phones, cost around $350, ie around the
cost of one clearcom station... Enjoy!
-Ben Mehlman
b...@staff.prodigy.com
PS: Any questions or success stories feel free to email me.
Jerry Musselman
In article <5681e9$m...@usenet1w.prodigy.net>, b...@staff.prodigy.com (Ben Mehlman) says:
>
>(great info on cheap intercom deleted)
This reminds me of the days when I was in High School. We did our shows at the junior high,
because the HS had no adequate performing space. Anyway, an old-timer from the
phone company donated a bunch of OLD carbon-mic headsets...the type operators
wore in the 40's!!! We used to string a bunch of cable to connect them, but on year
the electricians put in a bunch or 12/2 romex (in conduit) for us. For the station at the
lighting control desk, we had the old 'horn-mic-around-the-neck'....This stuff was old, but it was
all we had for years. I finally figured out how it all work (I was young), and made some improvements.
Bought a bunch of 'hands-free' speakerphone boxes on close-out at a place called 'Combs'
(remember them?), and a friend had bought a bunch of headset/mics for CB use.
I put the two together, and we had a much improved system. Only problem we had was the local
AM station was always on the air...on our system, at least!
Gary, you missed out on those days (email me sometime....jam@ameslab.gov)
Ben Mehlman
In article <56fa3o$c...@news.iastate.edu>, j...@ameslab.gov says...
>
>In article <5681e9$m...@usenet1w.prodigy.net>, b...@staff.prodigy.com (Ben
Mehlman) say
>s:
>>
>>(great info on cheap intercom deleted)
Hehe, similar story here. The first system I built was at summer camp
age 13 (Timber Lake, in Phoenicia NY, a few miles from Woodstock). I ran
telephones in series because although we built a 40V supply it didn't work.
We didn't know anything about output impedance then! I put RCA plugs on a
bunch of phones, and jacks at each location running to an old bank of those
great Switchcraft switches. When there was no phone plugged in you had to
flip the switch to put a resistor in it's place, or the system would go dead.
We had some lines running to the radio station a few hundred feet away and
sometimes we'd run sound effects from there... It worked very well for
several years I was there, and probably for years after (I'm sure sooner or
later either someone improved it or wrecked it).
In HS (Plainview LI) we had a total lack of support for the theatre.. It
was the kind of place where custodians would arrange the house instruments in
a wash (no gel) and tell you there was no need to focus for the show because
the stage was lit fine. Anyway, I was in the habit of 'dumpster diving' the
local factories and came up with lots of parts. Vicon was the best of these,
throwing out hundreds of perfectly good parts a week- lots of prototypes with
fun stuff in them. So a friend and I built some neat stuff, including a
couple of intercom power supplies with binding posts in the back and jacks in
the front for the type of carbon headset you are talking about. We didn't
have the 'horn' type ones- we had three or four of the slightly newer (1950's
style) carbon headsets that you can still buy from Markertek for the video
cameras that still use them. We only had four stations- two for the people
running the old Century/Ward-Leonard board backstage, one for the SM sitting 6
feet away from us outside our booth SL, and one for the spot op. We were
forbidden from running wires to the spot booth but we did it anyway ;)
We also focussed lights when they weren't looking...
I built another system that used parallel star-sets over mic cable. A central
amplifier provided phantom power for the mics, amplified the audio, and sent
it back. It was a good system because the volume was controllable and the
noise cancelling mics of the old star-sets.. good for loud shows.
That system was lent to a student production and was destroyed to the point
where I decided not to bother fixing it.
Then a couple of years ago I got a steal of a deal on an old but working
Clear-Com setup so that's what I use if I am stuck for an intercom.
This system with the headset phones I posted is for the theatre where my old
system got wrecked. Now they have their own and if they wreck it... I tried
to build it in a hard to wreck manner... BTW they have been using it for
almost a week now and it's been just fine- crystal clear and everyone is very
happy. A little bit more noise pickup on the mics during noisy scenes, no
sidetone adjust, no mute. The mute is the only thing I miss that I get with
the Clear-Coms. I have worked with Mini-Coms a few times and they are pretty
junky- very easily broken. And not all that cheap. Now that these headset
type phones are so cheap I think it's a much better option if you can't afford
the real thing.
(Now, the only question is, now that the SM and Electrician can't use "I
couln't hear him" as an excuse, will my cues be executed more promptly?!)
PS: Sorry for this long and winding tale- hope it was of at least minor
interest and/or entertainment value ;)
PPS: Could we please stop the flaming and get back to talking about
stagecraft?
dsou...@aol.com
I am the sound op at the Bardavon 1869 Opera House in Pokipsee NY. I had a
request from a stage hand to record to cassette the headset chatter of the
upcomming Nutcracker production. I know this can be done and have an idea
how to do it but I want to be sure I am thinking things thru properly. As
I under stand it, audio in a clear com system runs between pins 2 and 3.
PS voltage is also run on one pin and call light voltage on the other with
pin 1 as common for all. I need to check my old electronic books to figure
out values, but I would think a capcitor and resistor to ground from pins
one and two and another set of resistors on pins one and two before the
cassete deck inputs. Am I on track or totally off?
Stuart Wheaton
And you say Soundgod as a name???
Its cute, but altar_boy_of_sound might be more accurate.
Try looking at the back of the base station, you should be able to lift
it there with a 1/4" plug. Or if you have a spare box, lift the signal
at the 4 pin jack for the headset. two pins are mic, two pins are
earphone. I soppose you could even tee off a headset in use, then just
match speaker level to tape input and you are in.
P.S. This tape could prove to be a bad idea for some people... and
don't forget that recording someone without their knowledge and consent
is a violation of the law.
dsou...@aol.com
In article <19961209182...@ladder01.news.aol.com>,
dsou...@aol.com writes:
>SNIP!
> I need to check my old electronic books to figure
>out values, but I would think a capcitor and resistor to ground from pins
>one and two and another set of resistors on pins one and two before the
>cassete deck inputs. Am I on track or totally off?
WHOA!!! Ok so I'm totally off. I must have been on some pretty good drugs
when I wrote that one. :)
Any way, what I meant to say was a resistor to ground with a capacitor
before the cassete deck input on pins 2 and 3.
----------II---{cassette}
I
/
\
/
__I__
___
_
Louis Shapiro
In article <19961209182...@ladder01.news.aol.com>,
dsou...@aol.com wrote:
>I am the sound op at the Bardavon 1869 Opera House in Pokipsee NY. I had a
>request from a stage hand to record to cassette the headset chatter of the
>upcomming Nutcracker production. I know this can be done and have an idea
>how to do it but I want to be sure I am thinking things thru properly. As
>I under stand it, audio in a clear com system runs between pins 2 and 3.
>PS voltage is also run on one pin and call light voltage on the other with
>pin 1 as common for all.
On ClearCom, pin 1 is ground for DC and audio, pin 2 is 30 vdc, and pin 3
is audio. A shielded cable from pins 1 and 3 right to your tape input
should work just fine.
Lou
E. Riley Casey
Pin three also carries the DC signal for the call light. The cassette
deck is NOT interested in seeing that signal and could get very unhappy
about it. Yow may also create a ground loop or other loading problems on
thecomm circuit.
If you don't have an audio output on the base unit I'd make an adaptor
to take audio from the headphone output of a beltpak and test this out
very thoroughly before the show.
*************************************************************************
In article <lshapiro-ya023480...@news.interport.net>,
lsha...@interport.net (Louis Shapiro) wrote:
> In article <19961209182...@ladder01.news.aol.com>,
> dsou...@aol.com wrote:
>
> >>>>>>snip<<<<<<
Carl Zwanzig
Louis Shapiro <lsha...@interport.net> wrote:
>dsou...@aol.com wrote:
>
>>I am the sound op at the Bardavon 1869 Opera House in Pokipsee NY. I had a
>>request from a stage hand to record to cassette the headset chatter of the
>>upcomming Nutcracker production. I know this can be done and have an idea
This would make me VERY nervous. Most people assume that the headset
chatter is no going any farther than the crew. Things are said, cues
discussed, etc. Some of this stuff should never be said, let alone
be preserved on tape.
>On ClearCom, pin 1 is ground for DC and audio, pin 2 is 30 vdc, and pin 3
>is audio. A shielded cable from pins 1 and 3 right to your tape input
>should work just fine.
The call lamp is activated by a 30kHz signal superimposed on the audio lead.
z!
Carl Zwanzig
Douglas Boyd
e
>Pin three also carries the DC signal for the call light. The cassette
>deck is NOT interested in seeing that signal and could get very unhappy
>about it. Yow may also create a ground loop or other loading problems on
>thecomm circuit.
I thought Clearcoms used a 20khz tone for the call light switching not
DC - on DC operated call light switching you lose the coms audio
signal momentarily while its pushed, then cap & transistor circuit
conyinues flashing.
---------------------------------------------------
'Give us the toys, and we will play with them.
We will take the manual and throw it in the garbage
and turn the knobs until it blows up.'
Doug Boyd
Stageworks Theatrical Equipment
do...@nznet.gen.nz