Bridging a 70V line

In a few days, we're doing a single-camera remote from a large cathedral. The mixing position for the house PA system is far from where we'll be set up, and there's no practical way to get a proper feed to us. (Running a long cable back to the mixer is a no-no). We do have wireless equipment, but the path is not line-of-sight, other media will be there--possibly using wireless gear of their own--and I just don't trust it under those circumstances.

However, there is a speaker (70V line on the house PA) very near our position. Something that occurred to me is to bridge that line to derive our feed. (Yes, I know it's a kludge, but there aren't many other options). We have some old RCA bridging transformers, 20k:600/150. A 20K impedance bridged across the 70V line would only draw a quarter-watt, and the stepdown would provide something reasonably close to "line level" at the secondary.

I believe the voltage stepdown of these transformers is greater than the 15dB or so you'd expect from a nominal 20K:600 transformer, because the primary actually has buildout resistors in series; they're literally built into the transformer. IIRC, the actual loss is more on the order of 18-20dB but I don't have one of the transformers here at the moment to check. 70.7V is +39dBU, so with an (assumed) 20dB bridging xfmr stepdown, that would be +19dBU if the house system is running full-tilt, which it won't be. It's easy enough to pad it down if we need to.

I can't think of a reason why this wouldn't work... But I'm so strung out on muscle relaxants right now, I'm not sure how clearly I'm thinking. So I wanted to run this by you in case I missed something.

You'll get the EQ of the speaker affecting your line, but maybe that's okay. Any way to check its thumbprint ahead of time?

Wayne, I thought about using a proper 70V transformer (we have a few small ones laying around) but I wanted to draw as little power from the line as possible. I believe the lowest power tap usually found on those things is 0.5 watt. Your mention of autocouplers takes me back to my previous job in live news; we used a lot of them! In my current job, I don't have to deal with telco nearly as often.

Doug, you make a good point about the EQ. There really won't be any time to check it. Since it's just for voice, it should sound OK as long as the installer didn't do anything very radical. We're bringing along an FP33 and will probably put up an ambient mic to pick up the pipe organ when needed. (I'm sure we'll have to quick-fade between the two signals, rather than mix them, to avoid nasty comb-filtering effects).

We're practically sitting inside part of the organ (which totals 23,000 pipes, 874 stops!), so our pickup of the instrument is definitely not going to be very well balanced. Oh well.

This organ is nearly 100 years old and is quite well-known among people who are into that stuff. It's quite a sight to behold... If I weren't going to be so damned busy, I'd take some pictures. But there just won't be time for that.

As an adolescent, I was fascinated by the workings of pipe organs and I read a few books on the subject. During my site survey last week, I started a conversation with one of the organ tuners, only to realize (to my dismay) that I'd forgotten almost everything I learned all those years ago.

We had to run our fiber-optic and power through a passageway that included, among a lot of plumbing and some large trumpet-like pipes, a whole mess of antique electrical workings: wiring looms and terminal boards, relays, etc. But I noticed a new-ish looking solid-state circuit board mounted among all the ancient relics. I asked the organist about it, and he said that it's part of a recall system--controlled by a small box with a numerical display that sits atop the massive, four-manual console. Maybe it's an SSL product... I'll take a look next time I'm up there.