Film-Tech Forum ARCHIVE: Radio-Cine LPB Model T-7B Output Impedance

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Posted by Daryl C. W. O'Shea (Member # 1303) on 09-26-2004, 03:54 AM:

Would anyone happen to know the line level audio output impedance of this prehistoric (well, likely pre-me anyway) artifact?

It's a Model T-7B Theatre Transmitter Control Center manufactured by "LPB".

Come to think of it I don't actually know if it's even intended to have an output (it currently does, it's only being used as a mixer) since it really only has outputs to feed the AM transmitter which is happily sitting disconnected in the bottom of the rack. I didn't want to take it apart during the show to look myself.

So, if anyone's got schematics for the thing or a (an educated - Rick? Gord?) guess at what the output impedance may be, from wherever the output was sourced from, that'd be great, and I'd owe you folks yet another round.

Posted by David Buckley (Member # 2600) on 09-26-2004, 08:30 AM:

This sounds a bit like a silly suggestion, as I'm sure you would have thought of it, but you could give the makers a call, the contact details are on their website, http://www.lpbinc.com. They seem proud of their history, so may be very helpful.

The Radio-Cine trademarked logo was used between October 1981 and cancelled in July 1989, which dates your bit of kit.

Posted by Tim Reed (Member # 137) on 09-26-2004, 11:52 AM:

Although I think it's just to feed the xmtr(s), I'm almost certain I fed field speaker preamps with that unit, once upon a time. It's probably on the order of 50K to 100K.

Posted by Stan Gunn (Member # 554) on 09-26-2004, 06:32 PM:

Hi Daryl

There were a few drive ins that had them here in OZ.
I have seen many set up to drive FM mono transmiters, even seen two hooked up as stereo {one for each channel} to a home brew fm transmitter, in my dealings with them they seem happy to drive almost any thing hung off them, I would have said the output would be 600 ohms.

Watch out for the transitter tho its out put can be connected to the mains supply. [Eek!]

Stan.

Posted by Jack Ondracek (Member # 1466) on 09-26-2004, 10:15 PM:

Wow... what an antique! I've never seen one of those, and LPB had a lot of their stuff around S. Oregon, back when I was down there. Bet it's got all the right decoding stuff in it, yah??

If it's 10k and you hook it up to a 600 ohm input, it won't drive it.

If it's 600 ohm and you hook it up to a 600 ohm whatever, great! You're a true old time broadcaster!
If it's 600 ohm and you hook it up to a 10k whatever, it's probably balanced and won't know the difference... at worst, you'd have to load it with a resistor.

If it's unbalanced, you can ring that out with a vom. It's probably too old to have an active output.

... but you probably already knew all that, eh????? [Big Grin]

quote: David Buckley
The Radio-Cine trademarked logo was used between October 1981 and cancelled in July 1989, which dates your bit of kit.

I wonder if that was some kind of marketing competition toward the " Cine-Fi " AM system? There's still one of those installations in our area... 6 screens worth of them, actually... all tuned to the same frequency, with those antenna clip wires hanging off of all the speaker poles. I think the old window speakers sounded better.

Posted by Daryl C. W. O'Shea (Member # 1303) on 09-26-2004, 11:37 PM:

David, thanks for the website link (I was to tired to even think of 'lpbinc' last night, and I wasn't even sure they were still around). I checked out their manual and schematics links to their "tech site", but the script that processes the logins for that part of their site is broken. There's also no mention of the product on the site. I'd rather not call them since the unit in question clearly wasn't designed to do what it is doing and I don't really like to bother people asking them how to do something with something the stopped selling years ago, when it's not even intended for you to do so with it. As for the "Radio-Cine" trademark dates, I guess the thing doesn't really pre-date me, oh well, I tried.

Tim, they used to feed the field speaker transmitters, until they were disconnected (hoo-ray!). I couldn't open one (triple bills were in progress) to see if there were any labeled outputs inside. There very well may be outputs labeled, possibly noting their impedance too. I thought'd be more fun to play trivia though. [Wink]

Stan, if they are actually intended to provide an audio out, I would also expect that they would be 600 ohms. The problem is it is currently connected to a 600 ohm balanced input and it doesn't sound right at all. I'm pretty sure it's not a gain issue on this units output or the transmitter's input (both are pretty much cranked).

Jack, I thought you might like it. Maybe I'll send you one in the spring when I rip them out, as a gift or maybe a curse. It's feeding what's supposed to be a 600 ohm balanced input of a Decade TRC-800 transmitter (which I'm looking for a manual for [Wink]

, I have one for a similar digital model but not that exact one. I don't really need it anyway, unless I have to change frequency's, but even then I could probably figure it out). I'm thinking that the output impedance is higher, by quite a bit, but is still driving the transmitter input. The FM radio audio levels suck though, and I have no reason to doubt that the transmitter isn't setup to achieve full modulation, save for the audio signal source problem.

I have no idea if it's a balanced or unbalanced output coming out of the LPB unit. I couldn't take anything apart to see how it was wired. In the event I can get out there again before they close for the season, while they're not running film, I'll be able to check to see what's really going on, and measure the output impedance if it is in fact unbalanced.

I've seen a Cine-Fi system somewhere. I can't remember where though.

Thanks for all the input folks!

Posted by Stan Gunn (Member # 554) on 09-27-2004, 05:16 AM:

Hi Daryl

I have just dug out my T-7A and its AM-30P transmitter.
The output of the system is 600 ohm unbalanced your preamp switcher is much nicer looking than mine, hope this info is of help.

Stan. [beer]

Posted by Daryl C. W. O'Shea (Member # 1303) on 09-27-2004, 04:56 PM:

Hmm, I guess it's not an impedance mismatch then. Must be a -10dBm output going into the -4dBm XLR balanced in. Easy fix then, I believe there's a -10dBm TS phono unbalaned input on their transmitters.

Does the bottom of your unit look similar? The audio output is coming from within, in that bundle of wire.

Posted by Rick Long Jr (Member # 541) on 09-27-2004, 11:19 PM:

Daryl,
I have the TRC-800 manual. I'll try to e-mail it to you tomorrow as I don't have a scanner here. Definitely try the 1/4" 10K input though. As well, just to confirm what Stan said, the manual for the T-7A says 600 ohms unbalanced.

Posted by Daryl C. W. O'Shea (Member # 1303) on 09-27-2004, 11:52 PM:

Thanks Rick! Whenever you've got some time... I've got to go north for this weekend for a wedding, so it's no rush.

Somehow, maybe it was your name written on half the stuff there, I knew you'd have the manuals for them.

I'm definitely going to try the unbalanced input, I should get at least a 6 dBm boost doing it. I'll especially do it on the one rack that's using the RCA outputs of a Shure mixer.

I would have tried them when I was there, but I only stopped for some food on my way back from Detroit, and didn't have anything with me since I don't like having a vehicle full of junk when going across the border, unless I specifically need the stuff.

Out of curiosity, what's the difference between the T-7A and the T-7B?

Posted by Tim Reed (Member # 137) on 10-03-2004, 12:49 AM:

quote: Stan Gunn
The output of the system is 600 ohm

Oh, of course! Duh! What the heck was I thinking?! Hi-Z inputs, I guess... [Roll Eyes]