Care and feeding of the EJ10 Crab

Conversations around originating a webcast for conference, including cameras and mixers.
quintonrhq
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Care and feeding of the EJ10 Crab

Postby quintonrhq » Tue Aug 31, 2010 5:13 pm

The EJ10 and EJ8 are affectionately known as the ‘Crab’ for all of the adaptor cables which are fastened to the case of the box. Over time the legs seem to disappear. The box is a passive device that allows audio devices such as CD players etc to be tied into the building sound system via a XLR connector.
The EJ10 has a telephone section which connects telephone signals into or out of the church sound system. The audio connection for the telephone connection is on the left side of the case, somewhat obscured by the zipper case.
The documentation shows a mini-stereo connector with two 45 ohm resistors. This is not the case. The connector is a mono connector going into a 100 ohm resistor. A sketch of the schematic is attached. If you are using a stereo connector there will be a short of the ring portion to ground. Use only the tip connection.
While the connection is more sensitive than the rest of the connectors, it suffers from a low input impedance that is below the typical audio output driver device. It really looks more like a speaker than a buffered input port.
As an audio telephone backup for a Webcast broadcast, an old PC desktop speaker amplifier was adapted with its low output impedance to drive the EJ10. As a side benefit, there is a volume control and a monitor speaker was gained.
Attachments
EJ10_Sketch.pdf
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russellhltn
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Postby russellhltn » Tue Aug 31, 2010 9:12 pm

starobelka wrote:Over time the legs seem to disappear.


The ones I've seen have the cables firmly fastened to the case at the midpoint. If someone removed that fastener so that all the cables were lose, then I could see them disappearing.
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danpass
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Postby danpass » Wed Sep 01, 2010 3:43 pm

starobelka wrote:Over time the legs seem to disappear.

RussellHltn wrote:The ones I've seen have the cables firmly fastened to the case at the midpoint. If someone removed that fastener so that all the cables were lose, then I could see them disappearing.



We have both an EJ8 and an EJ10 for our stake center. The EJ10 does have the cables secured as RussellHltn describes. Our EJ8, on the other hand, has the cables loose. On a few occasions, one of the cables (usually the RCA) has been stored with or left connected to another piece of equipment.

russellhltn
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Postby russellhltn » Wed Sep 01, 2010 4:14 pm

Odd, our EJ8 has secured cables as well.

I'll have to look the next time I'm there, but it may not take anything more then a phillips screwdriver to unsecure them. Someone might have done that to make the cables reach for a particular application.
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quintonrhq
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Postby quintonrhq » Wed Sep 01, 2010 10:36 pm

My goodness, the point of this post was not to discuss how cables are attached to the crab. It was a bit of humor. Over time someone with a screw driver sees a higher need to remove a cable and they slip away into forbidden paths and are lost.
There is a documentation error in the instructions. If you want to drive a telephone line, your average audio device won't give you enough signal.

michaelfish
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Help modifying attenuation of EJ-8 or EJ-10 (crab)

Postby michaelfish » Thu Sep 16, 2010 5:52 pm

starobelka wrote:If you want to drive a telephone line, your average audio device won't give you enough signal.


I've noticed this problem for many years - trying to get a line level feed plugged into a house system microphone jack. Unfortunately, unless you have access to increase the gain on the building's microphone's input you're out of luck; the crab attenuates so much that the signal can just BARELY be heard.

The crab was a great idea back in the 60's to connect a 16mm film projector or record player to the house system (connect speaker output to building sound). 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.

Can anyone make a suggestion of a good, INEXPENSIVE device similar to the crab the will attenuate properly (line level to mic level, unbalanced to balanced, with volume control and various jacks).

Or can someone tell us how to modify the crab (perhaps a different value resistor here or there...) to make it more effective?

russellhltn
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Postby russellhltn » Thu Sep 16, 2010 6:43 pm

michaelfish wrote: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.


It would be interesting to do a power-level comparison. I think you'll find that there isn't enough power in the typical line or headphone output to drive a telephone line. That immediately rules out a passive device. I suspect an amplifier is needed.

It doesn't have to be expensive or elaborate. I think most anything that will give you a small speaker output from the sound source will do the job. This product could probably do it, but may not be ideal in that it's not designed to take accept the typical line in, so you'd probably have to add some adapters and attenuators to the rig.

I also suspect that when the crab boxes were originally selected, they only expected the telephone function to be used for receiving Priesthood Broadcasts in case the satellite system failed. I don't think they really expecting anyone to be originating a feed.
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michaelfish
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Postby michaelfish » Thu Sep 16, 2010 7:56 pm

RussellHltn wrote:I also suspect that when the crab boxes were originally selected, they only expected the telephone function to be used for receiving Priesthood Broadcasts in case the satellite system failed. I don't think they really expecting anyone to be originating a feed.


I may have mislead you...I was discussing using the crab at the receiving end.

For example, I have never been sucessful converting any line-level audio source (+4 dBm or less) such as the output of a telephone conversation, laptop, DVD, boombox, cassette, iPod, etc. and by using a crab, connnect that source to the building's sound system without having to increase the gain of the building's microphone input.

Typically, in order to be sucessful using a crab requires:
  1. Connect the equipment
  2. Locate the building's audio mixer controls
  3. Gain access to the controls (which typically is UNAUTHORIZED)
  4. Increase the gain of the mic input to MAX
  5. When done, remember to reset the microphone's level to what is was originally
The amplifier you suggested would work but I was hoping for a solution that requires less equipment. I've found alternatives to the crab with inexpensive passive direct boxes (example here) that do a great job but lack the telephone input and variable output option.

I was hoping to find a way to modify the crab to have less attenuation between the source (unbalanced line level) and output (balanced mic level), resulting in an increase of signal on the balanced microphone output.

rmrichesjr
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Postby rmrichesjr » Thu Sep 16, 2010 8:12 pm

michaelfish wrote:...

Can anyone make a suggestion of a good, INEXPENSIVE device similar to the crab the will attenuate properly (line level to mic level, unbalanced to balanced, with volume control and various jacks).

Or can someone tell us how to modify the crab (perhaps a different value resistor here or there...) to make it more effective?


Rather than modify the crab box, I would recommend you build a device to exactly meet your specific needs. For line level to mike level, this hand-drawn schematic shows how I have built one that has worked very well for feeding the PA system from a CD player:

http://tech.lds.org/forum/attachment.php?attachmentid=7&d=1175921212

I would recommend against reducing the resistance on the input side by much lest it cause distortion. Feel free to change the resistor values on the output side to suit your needs. If you would like variable attenuation, substitute a stereo potentiometer instead of the fixed resistors on the output side. I suggest a stereo potentiometer to maintain balance to reduce capacitive or inductive noise pickup. The transformer is an inexpensive 600 ohm 1:1 audio isolation transformer available at most neighborhood electronics stores. A metal box would be ideal, but I have used a plastic box without any noise problems.

For driving an outbound phone line, I concur you will likely need amplification. I would recommend an ordinary amplifier designed to drive a speaker. Put a non-inductive (non-wire-wound) 8-10 ohm resistor across the speaker terminals to properly load the amplifier. Then, connect a 600 ohm audio isolation transformer with the primary across the amplifier output and the secondary connected directly to the phone line. If you need attenuation, use a resistive voltage divider between the amplifier and the transformer primary. You'll need to disconnect the transformer from the phone line for the phone line to go on-hook. You'll get loud pops/clicks if you connect or disconnect phones to the line while the receiving site is live.

rmrichesjr
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Postby rmrichesjr » Thu Sep 16, 2010 8:33 pm

michaelfish wrote:I may have mislead you...I was discussing using the crab at the receiving end.

For example, I have never been sucessful converting any line-level audio source (+4 dBm or less) such as the output of a telephone conversation, laptop, DVD, boombox, cassette, iPod, etc. and by using a crab, connnect that source to the building's sound system without having to increase the gain of the building's microphone input.


Typically, in order to be sucessful using a crab requires:
  1. Connect the equipment
  2. Locate the building's audio mixer controls
  3. Gain access to the controls (which typically is UNAUTHORIZED)
  4. Increase the gain of the mic input to MAX
  5. When done, remember to reset the microphone's level to what is was originally
...


Well, in the case of going from a phone line to the PA system, just connect the primary of the transformer in my earlier schematic directly to the phone line. (That's basically what the transformer was designed for.) With a phone line as the source, a stereo potentiometer on the output side would definitely be a good idea to allow the attenuation to adjusted in real time.

The newer meetinghouse PA systems don't have controls. All gain settings are done inside a DSP, and control of the DSP is password protected. I cannot recommend trying to do anything unauthorized with that type of equipment.


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