Tech Forum

Rather than tack onto an already lengthy thread, I thought I'd start a new one. This past week I built my own cable and was able to successfully use it this past Sunday for a webcast using a cell phone. There might be a pre-made cable out there that can do that, but it's not easy to find and I'd have to order it.

I apologize in advance for no pictures, but it would have delayed this post.

A number of design criteria needed to be met:
1. It has to connect to the sound system which uses a 2-conductor 3.5mm jack (tip and sleeve).
2. In my case, it has to connect to a cell phone which uses a 4-conductor TRRS 3.5mm connector.
3. It has to attenuate the line-out of the sound system down to mic-in level.
4. It has to present a DC resistance to the cell phone of 1K "or higher", otherwise the phone will not recognize the microphone as "present".

Item #1 is easy enough.

Item #3 and #4 was done by building an L-pad, using the values of R1(series resistance)=120K, R2(shunt)=1.2K. That gives an attenuation of 40dB, which experimentally proved to be the minimum attenuation that didn't cause problems. Why 1.2K instead of 1K? The sound system output is transformer-coupled (yea, no ground loops!) As a result, the DC resistance seen by the phone will be R1 in parallel with R2. Plus, I didn't want to get too close to the 1K threshold specified in the Android specs. (From what I can find, iOS seems to be closer to 800 ohm.)

Item #2 is a bit tricky. I wasn't able to find a TRRS connector in my area. I ended up using a pre-made TRRS to three RCA adapter cable used to connect video and stereo audio to a camcorder. (White, Red, Yellow). They can be found in stores and even thrift stores. Connecting the output of the attenuatior to the Red lead accomplishes that.

That fixed my situation, but what about yours? There's no less than 3 different pin-outs for 3.5mm that can be called "microphone":
1. Cell phone/computer single-jack "headset" input using a TRRS connector. (2ed ring = ground, sleeve=mic-in)
2. A computer input that has a "mic in" jack using a TRS connector. (sleeve=ground, tip=mic-in)
3. A camera stereo mic input using a TRS connector. (sleeve=ground, tip=left mic, ring=right mic)

I've already outlined what I did for #1. The other two can probably be found at a local electronics store, but you can also used pre-made cables. So, if you build an attenuating 3.5mm to female RCA, you can use pre-made cables to adapt to the device in question making the attenuator "universal".

I'll give more details on that later, but that should be enough to get you started acquiring the necessary materials. Just remember that 25ft cable that gets you from the sound system out to the location of your device should be shielded cable.

I would point out the one piece of direction that we have received from the First Presidency regarding the broadcasted meetings: https://newsroom.churchofjesuschrist.or ... 091020_CTA

"Since a sacrament broadcast does not include the administration of the sacrament, consider initiating the broadcast following the sacrament,or alternatively, holding the sacrament portion of the meeting last,so those viewing the sacrament meeting virtually see the program without interruption."

Not a requirement, but a suggestion, however that doesn't leave people at home wondering what's going on for 12 minutes. (although that time would provide them the opportunity to do the sacrament at home)

-- and now I realize that I posted this on the wrong thread... --

Yes, I think the leaders learned their lesson on that one.

What I'm interested in is how many "makers" do we have that can make this cable? I wish I could point people to something commercial off-the-shelf, but I'm not sure where to find them. I might try dropping into a local supply store for professional installers to see what they have.

russellhltn wrote:What I'm interested in is how many "makers" do we have that can make this cable? I wish I could point people to something commercial off-the-shelf, but I'm not sure where to find them. I might try dropping into a local supply store for professional installers to see what they have.

Good question. I've got a couple here, but they're all hand-made, I haven't seen anything off the shelf.

mrbitsch wrote:Good question. I've got a couple here, but they're all hand-made, I haven't seen anything off the shelf.

Well scratch that, this MIGHT work https://www.amazon.com/Alilong-Micropho ... 088LNFXDN/

mrbitsch wrote:Well scratch that, this MIGHT work https://www.amazon.com/Alilong-Micropho ... 088LNFXDN/

It might be a "step", but it appears to adapt the old "computer mic" to the TRRS. No attenuator.

But this looks promising. Just need to add a 3.5mm to 3.5mm 25' cable and you should be good to go.

russellhltn wrote:

mrbitsch wrote:It might be a "step", but it appears to adapt the old "computer mic" to the TRRS. No attenuator.

Yes, I've been looking through Amazon at attenuators, but I your find looks like a perfect fit.

Gentlemen,

Have you considered using an ALS (Assisted Listening System) receiver to get sound? The receiver has a volume control so you can turn it down. You may be able to do that and get by without an attenuator.

I did find a commercially built cable that does exactly what you are trying to do. I believe I picked it up from Performance Audio (https://www.performanceaudio.com) in Salt Lake City, but I'm not certain of that. I've had it for quite some time. I used it to feed an "Audio Only" backup for stake conference. I input sound directly from the chapel sound system into a cell phone. If you haven't found a solution, I can try to track down what the cable is called and where I acquired it.

davidanderspatten wrote:Have you considered using an ALS (Assisted Listening System) receiver to get sound? The receiver has a volume control so you can turn it down. You may be able to do that and get by without an attenuator.

It's possible, but it also raises technical issues. I know of at least 3 different models of receivers made by two different companies.

As I recall, past efforts showed that it takes some effort to get it right.
- The device may use the electrical properties rather then the physical presence to switch on/stay on.
- The device was designed to drive a earphone at high volume, so I think an attenuator is still advisable.
- I've noticed a fair amount of hiss in the receiver, especially the older ones.
- Transmitters sometimes fail in a way that makes them intermittent. Unless you have active users of the system, it may be failing and you don't know it.

By all means, try, but just don't count on it until you've tested it and confirmed it works. I've learned the hard way what happens when you make assumptions about how easy an approach will be.

russellhltn wrote:

mrbitsch wrote:Well scratch that, this MIGHT work https://www.amazon.com/Alilong-Micropho ... 088LNFXDN/

It might be a "step", but it appears to adapt the old "computer mic" to the TRRS. No attenuator.

But this looks promising. Just need to add a 3.5mm to 3.5mm 25' cable and you should be good to go.

Just bought 4 of these-- should be here early next week. Will let you know if they work.

Great find!