More fun with crab boxes

Discussions around meetinghouse sound systems, microphones, assisted listening devices, and translation equipment
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More fun with crab boxes

Postby shanebankhead » Mon Oct 18, 2010 10:33 am

The crab box thread over in the building communications forum got me curious to take a look under the hood of both crab boxes- something I’ve always wanted to do, but never got around to it. So, the other day I grabbed both an EJ10 and an EJ8 on my way into work, took them apart on my bench, and made a few measurements. Since Starobelka already posted the EJ-10 schematic in the other thread, I traced out a schematic for the EJ-8, which I’ve attached. As you would expect, there’s not much to the EJ-8, but it is interesting to note the design differences, which explain some of the issues we have noted with these boxes.

Here’s what I measured:
(on an AP System 2, all measurements with volume knob at max, XLR output driving a 600ohm load)

XLR OUTPUT LEVEL (+4dBu Line Input)
EJ-10: -44.1dBu
EJ-8: -14.6dBu

Line Inputs
EJ-10: 4.9k
EJ-8: 4.9k
Telephone Audio Input
EJ-10: 250 ohms

FREQUENCY RESPONSE (+4dBu Line Input, response is +/-3dB from 1kHz level)
EJ-10: 12Hz- 4kHz (The response is not very flat, so don’t expect hi-fi performance)
EJ-8: 23Hz-57kHz (With much flatter response than EJ10

There are several interesting observations about how these two units are designed, but I think going into a technical analysis would probably be overkill here (I think I’ve already passed the point of “overkill”). Anyway, here are a couple of the main points that I learned from this exercise:

The EJ-8 does have significantly greater output than the EJ-10, so if you’re having trouble getting enough volume from the EJ10, you’ll definitely get better results with the EJ8. The amount of attenuation in the EJ10 is actually pretty typical for a line-mic conversion, and the EJ8 would typically be considered “hot”. But this gives the EJ8 plenty of range on the volume control to adapt to different situations.

As was noted in the other thread, the telephone audio input impedance is very low. It is apparent that this was a design compromise to avoid additional attenuation at the input. This is probably a big part of our frustrations trying to get a good volume level for backup audio. As mentioned before, it is still usable if you drive it with a low impedance and/or larger input signal. It may also need some additional gain when using it as a receiver. We have used the EJ10 successfully many times for backup audio- it's just not an ideal option.

Anyway, that’s way more detail than anyone ever needed, but it satisfied my curiosity, and I hope maybe the info is useful to someone else. Please let me know if you have any questions.

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Re: More fun with crab boxes

Postby elijahdell » Sat Dec 13, 2014 12:36 pm

Wonder if you might comment on the EM-j2 for connecting an iPad audio out to a Church sound system? Could you provide a schematic of that as well?

Many thanks for your enlightenment

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