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Posted: Fri Mar 20, 2009 11:18 am
by lajackson
RussellHltn wrote:Is that just a common 1:1 transformer like this one?

No. It includes one, but the levels needed for line and microphone are different. So if you try to put the line into the microphone, the signal is too hot. The box steps it down.

I don't know the technical terms. Level, padding, impedance. It all has to do with that stuff in addition to needing the isolation.

Posted: Fri Mar 20, 2009 11:32 am
by russellhltn
lajackson wrote:No. It includes one, but the levels needed for line and microphone are different.
Hmmm, I guess I should have been clearer - what are the specs for the transformer pictured in the schematic? Kinda important to know if you want to build one.

Posted: Fri Mar 20, 2009 3:43 pm
by lajackson
RussellHltn wrote:Hmmm, I guess I should have been clearer -
My fault. I took a more expansive view of "that" than you meant. I don't know the answer, but you are right, it would be important to know the specs of the internal transformer in the schematic. [grin]

Posted: Fri Mar 20, 2009 8:25 pm
by rmrichesjr
RussellHltn wrote:Is that just a common 1:1 transformer like this one?
Yes, that's exactly the transformer I used. Sorry I forgot to mention that. That transformer does a pretty good job.

If you have access to the official crab box, then I would advise using it. However, if a crab box is not available, the schematic I posted works quite well.

The first two resistors in the schematic serve to mix the L and R channels if using a stereo signal. The other resistors attenuate the signal levels from line level or headphone level down to mike level. To go from higher-power speaker level, you'd want to use more attenuation before the transformer. How much attenuation would depend on the voltage/power involved.

I built my unit in a common plastic project box. A friend of mine expressed concern about noise pickup in an unshielded box, but I have never heard any noise.

Posted: Sat Mar 21, 2009 8:57 pm
by techgy
lajackson wrote:This is a case where that "crab" box (which I have never seen) would come in handy. You would connect the sound from the source into the box, and the box to a nearby microphone input. Our chapel has two in the front on each side of the pulpit that are sometimes used for testimony meeting.
Here's a couple of photos that I took with my cell phone today when I was at the stake center. They're not the greatest but they do give you some idea as to what the EJ10 (crab) looks like. Size is approximately 6x8 inches.