cedd Posted November 9, 2007 Share Posted November 9, 2007 More a stage management subject, but I'm particularly talking about audio here so thought sound would be a decent place to start. The theatre have hired in 3 beltpacks and a master station (Tecpro I think, though as far as I know they're all pretty much the same). but we're 2 packs short (mess up at our end not the hire company). I have my own built system at home, but I feel mine is slightly different to that at the theatre. Mine uses scrap desk telephone boards for the belt packs with a 2 wire connection on XLR's between units (No fancy call lights). All boxed up looks really quite well, and sounds crystal clear. Using astrolite headsets bodged on where the handset would go (earphones work fine, mics had to be swapped for the scrap telephone ones. The whole system is powered by the power supply at the bottom of the linked page here: http://www.epanorama.net/documents/telecom...e_intercom.html Mine uses a 24v PSU to get us nearer a domestic telephone voltage, though they'll work on most supplies. Now here's my issue; can I connect my system to the hired system? Thoughs so far are; Off the shelf system has an audio line, a ground and a supply line. Call light is DC sat on the audio line. My system is 2 wire - supply and ground with audio modulated on the supply line. So, my thoughts are, transformer with DC blocking capacitor between the theatre audio line (cap protecting the transformer against call light current) and ground. Seconday of transformer wired onto my 2-wire circuit, again using a cap (0.1uF I thought) to block the DC. My system then runs on its own PSU and the theatre system on its'. The big question is, will it work? Clearly I don't want to do anything nasty to the hired system (or my own particularly). A little light reading suggests line levels on a theatre system is around 0dB (1v). Anyone know what it's likely to be on my system? My scope is at work unfortunately! Am I talking nonsense? Will it never work? Have I done something really silly in the above text? If I can do this I'll be loved forever, if I screw up, I'll be shot. Instinct says stay away but I'd like to help if I can. Many thanks Chris Link to comment Share on other sites More sharing options...
mackerr Posted November 9, 2007 Share Posted November 9, 2007 Call the hire company and admit your faux pas in not ordering the correct number of stations, and ask if they'll comp you the 2 additional with your sincere apology, since your budget is all used up now. You are just asking for trouble trying to interface your carbon mic system. There are impedance and level differences as well to deal with. Mac Link to comment Share on other sites More sharing options...
boatman Posted November 9, 2007 Share Posted November 9, 2007 There's a lot of information about a Tecpro compatible system here. It is based on the original Technical Projects system which dates back to 1992. The original schematic used to be here http://www.beltpack.com/tec-bp1-schematic.pdf but that URL now returns a 404, so I don't know where it's gone. Techpro/Clearcom systems actually use a nominal 24V power supply, and the audio level is around -6dB. You may get away with just a capacitor between the signal lines but I would use something rather larger than 100nF. Closer to 100uf would probably be about right, but use a non-polarised cap such as VM76H from Maplin. If the audio levels are significantly different you will need a matching transformer. If you do a search on the BR for 'http://www.beltpack.com/tec-bp1-schematic.pdf' you'll find a thread dating back to 2003, which contains a lot of useful information. HTH. John Moderation: Topic moved to General Tech Chat, as it's a bit more general than just sound. Link to comment Share on other sites More sharing options...
boswell Posted November 9, 2007 Share Posted November 9, 2007 I've got a copy of tec-bp1-schematic.pdf if you want it.PM me and I'll send it to you Link to comment Share on other sites More sharing options...
cedd Posted November 10, 2007 Author Share Posted November 10, 2007 Great stuff! Thanks for all the replies folks! Impedence was an issue I'd neglected, rather short sightedly. I suppose I should really have a clearer idea of how my own set works, I just took the boards from the telephones, boxed them and it worked! Never really thought about interfacing with anything else! We have a work-around for the time being now in place (after a lot of head scratching done tonight after I posted. We're now having a secondcircuit of my cans for backstage, and the hired set for followspots and LD. The show is slightly abnormal in that it's called from FOH by the director, who's sat next to the LD. Lighting cans circuit is just for telling the followspots if anyone need filling in off their mark. Stage crew can manage with their own circuit just about. They don't take any queues from the director, it's all done by themselves, so best they'll need is vox from stage left to right. The director has my last belt pack at FOH if he really needs it but will have to change headsets. Bit of a rough workaround, but it should work! When I have a little more time (i.e. the show not starting on tuesday with dress tomorrow!) I'll take a scope (when I get it back from cal at work) to my system and a borrowed set and see what needs doing for a proper interface if the need arises again. Tecpro do a 2wire to 4 wire converter which seems quite interesting. May give that one a go. There is also an interface unit to connect a sony camera talkback (4 wire) system to a comclone system on the comclone website. Opamp driven so impedence is less of an issue, and means I get a line input and output for the future! Have to say, I'm very impressed with my system. I can say that cause all I did was box it up! Very clear audio! May need an additional output stageto drive the phones in a loud gig, but for theatre work they're fine! Driven them over 200m of cable with no problem. As I deal with dead telephones most weeks, boards are in ready supply (most have mechanical faults, i.e. have broken cases, not electronic) so I'll be making plenty more! Very very helpful posts, thanks all very much! Link to comment Share on other sites More sharing options...
pritch Posted November 10, 2007 Share Posted November 10, 2007 Have to say, I'm very impressed with my system. I can say that cause all I did was box it up! Very clear audio! May need an additional output stageto drive the phones in a loud gig, but for theatre work they're fine! Driven them over 200m of cable with no problem. As I deal with dead telephones most weeks, boards are in ready supply (most have mechanical faults, i.e. have broken cases, not electronic) so I'll be making plenty more! What sort of headsets are you using? Did you just connect them directly in place of the handset, or was there some impedance matching needed? Link to comment Share on other sites More sharing options...
Ben Langfeld Posted November 10, 2007 Share Posted November 10, 2007 Last time I hired comms, it cost me £40 for a rack-mount base station and 4 beltpacks (although I was sent 8, apparently it's a standard package). Surely hiring an extra two beltpacks would be so much simpler and wouldn't really put you out? Link to comment Share on other sites More sharing options...
cedd Posted November 11, 2007 Author Share Posted November 11, 2007 Headsets are ancient astrolite ones (ex air traffic control) with replacement mics (from the telephone handsets - carbon mics). Drivers remain as they were. No impedence matching used, works crystal clear without. Of course with it being the same mic capsule, there is no difference to it being a telephone handset. Too late to hire more now and we're pretty much sorted anyway. 2 seperate systems works quite well. Link to comment Share on other sites More sharing options...
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