Care and feeding of the EJ10 Crab

Using the Church Webcasting System, YouTube, etc. Including cameras and mixers.
russellhltn
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#11

Post by russellhltn »

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

For example, I have never been successful 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.
Ah, that does change the picture.

The old EJ-10 wasn't very loud. I find I like the EJ-8 better. Somewhere I think I've heard the newer EJ-10 is a bit louder. One day when I get good and board, I may take a few apart and see what the differences are.

It sticks in my mind there's a resistor in series with the pot that effectively limits max volume. I think changing that would solve the problem, but would want to look at the whole picture to make sure it's not going to affect something else.

Typically, the building's sound system is set for the maximum volume you can get before feedback for the model of microphones issued during the last sound system renovation. If that model happened to be "hot" it might cause the sound system to be set low.
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shanebankhead
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#12

Post by shanebankhead »

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.
quintonrhq
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Link to manufacturer

#13

Post by quintonrhq »

Emtech makes a number of familiar products to us. This is a link to the EJ-10.
http://www.emtechelectronics.com/docume ... Manual.pdf
russellhltn
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#14

Post by russellhltn »

RussellHltn wrote:One day when I get good and board, I may take a few apart and see what the differences are.
I checked two different ones. I didn't see any obvious differences that would account for difference in sound level (unless it was the different transformers used). But I did find something significant about the telephone audio input (where you attempt to feed an audio signal into the phone line.)

The newer "Rev B" models appears to have a input impedance of a little more than 400 ohms. Way too low for consumer gear.

But what was more interesting was the "Rev A". It uses a stereo mini jack with the two channels connected to the "bus" by 100 ohm resistors. That means, if you use a mono plug, one of the "stereo channels" will be grounded. And the source will see an impedance of less than 200 ohms and you'll have created at least a 3dB attenuation of your signal.

If you're using your crab to feed a phone line, you might want to keep that in mind.
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shanebankhead
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#15

Post by shanebankhead »

I’ve always wondered about what was under the hood of these things too and have always been curious to open one of these up.

I didn't notice that Russell had also gone and made some measurements, but last week this thread got me thinking about it. So I took some measurements at work the other day that you guys might find interesting. It turned into kind of a long post, so I started a new thread here before I noticed that this thread was continuing:

http://tech.lds.org/forum/showthread.ph ... #post49884

As suspected, there is a big output difference between the two crab boxes. The level difference is due to the much heavier loading of the volume pot on the EJ10. Also, the same transformers are used, but wired differently for more attenuation on the EJ10.

I'll also add that on my EJ10, the telephone input was a mono 1/8" jack with 100ohms to the bus. But a "stereo" version with a mono plug would definitely make a bad situation worse.

Check out the other post & let me know if you have any questions.

-Shane
michaelfish
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Solution - Perfect audio control for the EJ-8/10

#16

Post by michaelfish »

After experimentation, I found that about 30 db of amplification is needed to match the volume of the crab's telephone feed to our church mic jacks (our microphone gain controls are not accessable). Since the crab is a passive device, it cannot increase the output more than the input. An amplifer was necessary.

The solution was found by using a microphone preamplifier (thank you RussellHltn for the amplifier suggestion). The model I used was the Art Tube MP Studio microphone preamp and was able to purchase a brand new unit off of ebay for $30.

I connected the telephone line and telephone to the crab and the mic cord of the crab to the preamp. The preamp was connected with a microphone cord to the mic jack on the wall.

The A/C powered unit has both a balanced 3-prong XLR microphone input (switchable low/high impedance) and an unbalanced 1/4" line level input. Two separate knobs adjust input sensitivity and output gain.

The result is the ability to increase gain as much as 70 db. The sound was clean and undistrorted at almost any level (loud!).

Thanks for all the suggestions!
russellhltn
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#17

Post by russellhltn »

Silly me, the spec sheet on the EJ-8 has this to say:
The EJ-8 is similar to the EJ-10 except it does not have
telephone line capability. Without the telephone line
capability, the output signal from the EJ-8 is 22dB higher
than the EJ-10. It also has a wider frequency range (20-
20K Hz),
For those who are not familiar with dB, the human ear perceives 10 dB as "twice as loud" (although it takes 10 times the power to do it). So 20 dB would be perceived as four times as loud. Quite a difference.
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Aczlan
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#18

Post by Aczlan »

michaelfish wrote:After experimentation, I found that about 30 db of amplification is needed to match the volume of the crab's telephone feed to our church mic jacks (our microphone gain controls are not accessable). Since the crab is a passive device, it cannot increase the output more than the input. An amplifer was necessary.
We have a stake conference coming up soon and I will have to take a look at our "broken" EJ-10 and see if that is the issue. Using that would be more elegant that my current method.
My current method of providing backup audio is an RJ45 jack, a telephone handset cable and a pair of female RCA jacks which are connected together and plugged in between the handset and the phone. It was made by yours truly at 11:30PM on a Sat night after the priesthood leadership where I decided that the other system (a pair of cheap computer speakers sitting in front of a Polycom) didn't sound good enough for me to be comfortable using it.
To use it, I feed all of the mics on the stand through my mixer (a Mackie VLZ1042PRO that my wife got me for Christmas) and send the Left channel to a mixer in the room where the phone is. I send the Right channel into an XLR connector under the sacrament table. Then I can add mics (such as choir mics pointing at the crowd during a Q&A session for the priesthood leadership session to catch the responses of those who don't use the wireless microphone) that are ONLY sent offsite and thus provide better sound while preventing feedback

Aaron Z
rmrichesjr
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#19

Post by rmrichesjr »

aczlan wrote:...
My current method of providing backup audio is an RJ45 jack, a telephone handset cable and a pair of female RCA jacks which are connected together and plugged in between the handset and the phone.
...
Your description didn't mention any kind of isolation transformer in the above-described setup. I have always been advised to use an isolation transformer between a phone line and anything that is standalone and itself isolated from anything else. I have used an inexpensive 600-ohm 1:1 transformer from Radio Shack when doing similar things, and it has worked well.

The mixer setup you described with the two channels to avoid feedback sounds like a very clever idea.
Aczlan
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#20

Post by Aczlan »

rmrichesjr wrote:Your description didn't mention any kind of isolation transformer in the above-described setup. I have always been advised to use an isolation transformer between a phone line and anything that is standalone and itself isolated from anything else. I have used an inexpensive 600-ohm 1:1 transformer from Radio Shack when doing similar things, and it has worked well.
This goes in parallel with the microphone and speaker in the handset of a standard issue church phone. An isolation transformer would probably be a good idea, but I didn't have one handy that night.
The mixer setup you described with the two channels to avoid feedback sounds like a very clever idea.
Thanks. That setup should work with most any decent stereo mixer. It also works with the Peavey that is in the building, but I prefer my Mackie to the Peavey (more inputs that I can use and more options to play with the signal).


Aaron Z
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