I just noticed this thread, and thought I'd jump in since I have comments and questions on several of the posts. First of all, thanks Starobelka for the schematic. That is very helpful, since I've always been curious exactly what was in the box, but never have gotten around to actually doing any measurements or opening one up.
The documentation shows a mini-stereo connector with two 45 ohm resistors.
I'm curious, I've never been able to find the official documentation, so your sketch is the first schematic I've seen. Anyone know where I can get a copy of the "official" documentation?
It's sad that the church doesn't have a better solution for adapting the typical headphone or line level jacks (laptop, ipod, wireless microphone, inexpensive audio mixers, etc.) to the building's sound system.
This seems very surprising to me. I've never noticed any problems with using the crab box to inject line level signals into the building systems of many different buildings. I've always found this device to be incredibly useful with no commercially available equal. The only alternative, as has already been mentioned, is to buy a device that will specifically do the job you need.
As Russell mentioned, the gain structures of different church buildings vary quite a bit- sometimes by design, sometimes because of problems with the system or setup. Anyway, I suspect your problems might be more than the design of the crab box.
Regarding telephone line interfaces:
We've tried several different ways to use telephone line interfaces to connect 5 or 6 buildings in our broadcasts with backup audio, so here's my experience:
We have found that the EJ-10 is marginally useful for use as a backup audio device. When receiving audio, the gain is weak, but we have been able to use it in a pinch.
We also have several of the "TAI" (telephone audio interface) boxes that you can get from the church. These are those small black boxes that can send or receive audio from the phone line. If you connect this device directly from its XLR output to the building system, they also have very weak gain. BUT
we have had excellent luck taking the RCA line out from the TAI into a crab box, and then into the building system. While kind of crude, this method seems to give us tons of gain to work with.
To confirm previous posts- it has definitely been our experience that the key to getting good backup audio is to drive the front end hard. Over the years, we have tried using the EJ10, our TAI boxes, a homemade device that I built (similar to rmrichesjr's post) and now we are using another device (JK Audio "THAT-2"). None of these provide a lot of gain when injecting audio onto the phone line. We have always found it necessary to drive the audio input with a much higher level than your typical line level audio. In the past we have used a small audio mixer to boost the gain, but in our current setup we have an RDL audio DA that allows us to crank the gain of the backup audio output, so that seems to work well. In any case, we have been able to get good results by injecting a hot signal from our source location.