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schematics offered

Posted: Fri Apr 06, 2007 8:37 pm
by rmrichesjr
I see the balanced vs. unbalanced issue got fixed after the instant email notification went out. It's cool how the forum site can fix typos like that. :-)

The newer installations have line-level inputs with a pair of RCA jacks and (IIRC) a gain control. They work great for line-level outputs from a piece of stereo equipment or the headphone jack from a CD player (through an adapter cable, of course).

It's very easy to build an adapter from line-level or headphone level to balanced mike level. I have always used the cheap 600-ohm audio isolation transformers from Radio Shack and have found them to suffice. I'm sure I have the schematics from a few of the boxes I have built over the years. If you'd like, you could PM me on this forum, and I'd be happy to email you a scanned image or a written description/netlist. Actually, if there's interest, I suppose I could attach a .png file. Anyone want the scanned image?

Posted: Fri Apr 06, 2007 9:23 pm
by garylm-p40
rmrichesjr wrote:It's cool how the forum site can fix typos like that.

It's called repentance, and I've been doing it a lot lately on this forum.

Like repentance, if you catch your typo quickly enough, you can fix it without too many people noticing. However, if you wait to edit your post, you get a nice little "Last edited by..." caption at the bottom of your errant post, and the whole world becomes witness to your folly.

Sure, I'd like a schematic. Thanks Riches Jr.

I don't know how many blown meetinghouse PA's we'll be responsible for with all the hardware hacking that will ensue, but we can always repent and nuke this thread.

Posted: Fri Apr 06, 2007 9:47 pm
by rmrichesjr
garylm wrote:Sure, I'd like a schematic. Thanks Riches Jr.

I don't know how many blown meetinghouse PA's we'll be responsible for with all the hardware hacking that will ensue, but we can always repent and nuke this thread.

It's hand-drawn, but hopefully it will be legible enough to communicate the idea. I had to scale it down and convert to .jpg to squeeze under the file size limits. You may need to adjust the resistor values to get the attenuation right for your building's system. Or, you could use a dual audio-taper potentiometer to make a volume control.

The two signal wires from the voltage divider resistors go to pins 2 and 3 of the XLR connector.

The switch is for hum elimination. Connect its wire to pin 1 and shield of the XLR connector. Use whichever switch position gives the least hum. If there's no detectable hum in either position, it doesn't matter.

If anything's not legible enough, I'd be glad to clarify. Of course, use at your own risk, your mileage may vary, etc.

Posted: Sat Apr 07, 2007 12:51 am
by russellhltn
Nice! All it needs is a volume control. (Remember to use a "audio taper" pot. Linear taper is too fussy.)

Microphone preamp schematic

Posted: Sat Apr 07, 2007 3:26 pm
by rmrichesjr
If you want to get fancy with a volume control, you might be like the schematic at of a one-channel mike preamp.

I have no connection with the company that makes it, other than a friend of mine recommended it in response to a question I had a while ago.

Not the Crab!!

Posted: Sat Apr 07, 2007 7:13 pm
by DARWINK-p40
The company you refer to is called Emtech Electronics. The device that you are refering to is known in church circles as a crab. There are a couple of different models that they have been used in lds installs. The EJ-10 comes in a gray zippered case with adapter cables wraped around the device. The EJ-8 comes in the same pouch but does not quite fill it. There is an adjusting pot for volume and a toggle that lifts the xlr ground pin. It is a handy little gizmo. I have purchased several replacement parts from Emtech over the years. The folks at Emtech are very easy to work with. If you watch ebay you can usually pick one up for $30 or so. In essence these little boxes are a transformer that drops the voltage from line level to mic level so that you can put any input into a standard xlr mic input. I use it like an insurance policy. I figure as long as that piece is that last in the signal chain before the amp in the locked cabinet, I am safe from scrutiny. I have used this device in several differnt buildings for numerous applications. I have never had any type of issue.

Re: The Crab

Posted: Sat Apr 07, 2007 8:02 pm
by garylm-p40
I got the EJ-2 Plus.

My mantra over the years has become: if the meetinghouse library has one, I'll be needing one.

Radio Shack

Posted: Tue Apr 10, 2007 8:59 am
by The_Earl
Radio Shack sells a canned balanced to unbalanced adapter for $15. I added my own 1/4" to dual RCA adapter, and now I can plug just about anything into anything. It is small enough that I can keep it in my bag for 'emergencies'.

Posted: Tue Apr 10, 2007 9:41 am
by rmrichesjr
There are two potential problems with that adapter setup:

First, you'll need some attenuation. The microphone level signal the PA system is expecting through the XLR connector is about 1-5mV. The line-level signal and headphone level signals you're likely to see on the RCA connectors will probably be about 300mV. If the 1/4" jack on that adapter is intended for microphone level signals, it's not likely to provide enough voltage attenuation, and you'll get distorted sound due to overloading of the microphone input circuitry.

Second, if the dual RCA connection is a hard (zero ohm) connection between the two RCA jacks, there's a chance the device feeding it won't like it, especially if using headphone outputs if there is no series resistance inside the device with the headphone output. Damage to the device isn't too very likely, but it is possible. It will probably work okay, but there's some likelihood of distortion if the driving device presents a very low impedance source.

Posted: Sat Apr 14, 2007 3:21 pm
by thedqs
garylm wrote:Do you bring gadgets to church, and where do you personally draw the line?

I have a Toshiba e740-NW (No wireless) on which I take notes (sacrament meeting talks) and read the manuals/scriptures (Priesthood and GD).

As for drawing the line, as long as it doesn't beep or and the keyboard is quite it is alright.

Oh I also have an old 8086 laptop that when I was younger I would take with me to take General Conference notes on, though it was loud, since my family had it's own private viewing room (the High Council Room) it didn't bother anyone and taking notes kept me on track.

garylm wrote:Do you WAN from church?


garylm wrote:What apps do you run at church and what materials do you access at church? Scriptures? Lessons? Conference talks? Church mags? Presentations? Attendance spreadsheet? Meeting minutes? Personal journal?

All except the attendance spreadsheet and personal journal. (I do make annotations in my digital scriptures that might be considered personal journal entries)

garylm wrote:What is the largest gadget you would feel comfortable powering up in a church meeting? Beeper? Cell phone? PDA? Tablet? Laptop?

Tablet in hand writing mode, unless the keyboard is quite too.

garylm wrote:What precautions do you take to keep from distracting yourself and others?

Since I am taking notes or reading along in the lesson, and I tried to read the lesson before going to church, I stay very focused. To avoid distracting others I make sure in sacrament meeting to be in handwriting mode only (people just assume you have a notebook or pad of paper if you have a pen in your hand) and in other meetings make sure that I look at the teacher to show that I am listening and can be called on at any time.

garylm wrote:Does the fit and finish of a gadget affect its church-ability?


Yes, small and un-obtrusive are great pluses, large and noisy are pitfalls.