Surface Supplied Diving Comms Box to 3.5mm Jack Adapter

[RMunoz]

I would like to connect a standard 4 wire (full duplex) communications box, used in surface supplied diving operations, to:

1) A standard TRS-Type 3.5mm (1/8") jack on a mobile phone
2) The two (mic and speaker) standard TRS-Type 3.5mm (1/8") jacks on a computer

The binding post on the "comms box" I want to connect to is standard for most surface supplied diving operations (Photo 1). Two black binding posts (0.1V) are used for connecting the two wires used for the divers speakers. Two red binding posts (12V) are used for connecting the two wires used for the divers microphone (Photo 2 for basic wire diagram).

I already know I can connect it to a telephone. I cut the wire between the telephone handset and base. When I hooked the telephone handset to the "comms box" using the binding post I was able to carry on a conversation. The volume from the microphone was very low and easily drowned out by anyone else patched into the "comms box." I also attached the binding post to the telephone base. I was able to make a phone call but there was an overpowering high pitch background noise that made communication unpleasant.

I have very little background with electrical circuits and was hoping to purchase something off the shelf or some easily assembled solution. I have soldered once or twice and have access to a digital multimeter.

Any other sites where I might post this are also welcomed. Some additional specifications from the product's manual are:

Operating Voltage: 12 V DC recommended (Between 7 and 16 V DC allowed)
Idle Current: 0.29 Amps
Max Overload Current: 3.0 Amps
Audio Output Power: 8 watts max
Audio Output Protection: Over load and short circuit
Maximum Output Voltage: 7 VRMS @ 12 V supply
Microphone impedance: 100 ohm
Microphone: Dynamic microphone or preamp microphone

Thanks for the help. If this works, any Ipod recommendations for diving music would be welcomed.

 

[BradtheRad]

Microphone: Dynamic microphone or preamp microphone

Can you find out for sure: Whether your mic already goes through a preamp? Or is it by itself?

Typically a dynamic mic puts out less than 0.1 V amplitude. This is barely strong enough to be heard.

A single op amp is sufficient to boost it to a volt or two. That would be sufficient volume for more than one listening device.

You could power the circuit from your 12V supply. You would run the mic cable into it before it goes to any other equipment.

Additionally you should use shielded mic cable, to prevent noise from corrupting the weak signal. This type of cable is a bit more expensive than telephone line.

 

[RMunoz]

BradtheRad, the "comms box" is able to "detect the type of diver microphone, and automatically adjust the audio level." That is what the manual says. I assume that is why it list both dynamic and preamp microphones as compatible? I did a quick internet search for other "comms boxes" and the two I found have a preamp or amplifier card built in (Aquacom MK2-DCI and Amcom 2810A).

Good idea regarding the External Supply. I thought it was for connecting a 12 V battery, more of an input for an external supply. I just tested it with a DM, while plugged into a AC outlet, and sure enough I got 12 V.

Does anyone market an off the shelf product similar to the solution you described? Also, I have some "comms boxes" without and external supply. I could probably connect it to an external 12 V battery. I would prefer something that did not need an external power supply. Is this even possible?

 

[BradtheRad]

A 9V battery could serve as a power supply for short periods, if there is no equipment drawing heavy current.

Try further tests with the microphone. You need to find out where the noise is coming from. It's possible your comms box has automatic gain control, and is amplifying noise as well as voices.

Try a short cable instead of the long telephone line. Even though you're away from 60-cycle hum, there's still a chance electrical noise is present in the water. Or noise might be broadcast from an engine, generator, alternator, etc. A shielded cable is essential on land or sea.

Alternate ideas:

To amplify a mic signal I have used a cassette recorder, taking the signal from the earphone jack.

Or the same might be done with a laptop computer. There should be a way to set it to accept incoming audio (or maybe it has a built-in mic), amplify it, and output to the earphone jack. Either from the Sound control panel, or from a program.

 

[RMunoz]

BradtheRad, was hoping more for an off the shelf solution at first. I brought the "comms box" with me to Radio Shack last night and talked with one of the employees about what I wanted to do. No luck. I also posted this question on another forum:

**broken link removed**

Two of the most promising solutions have been a Direct Box or Sound-Card/Radio-Interface. I spent some time on the internet looking for a DI Box that would do the trick. All this audio/communications tech is kind of new to me, but I did not find anything that looks like it would be directly applicable.

As for Sound-Card/Radio-Interface, I found a Tigertronics product via a recommendation:

**broken link removed**

The $45 cost is a plus. However, the RJ-45 connection has 8 wires. When I did some internet searching to see if I could wire this up I noticed the word PTT (Push to Talk). Since the device is for radio (half duplex - one person/device talks/transmits at a time), will it integrate with the full duplex functionality of my device? The divers breathing is always audible while operations are in progress. Auto-PTT may never give the device an opportunity to kick in?

I want to try and exhaust all possible off the shelf solutions before I look at more complex solutions that involve soldering and circuit design. Thanks for the help.

 

[BradtheRad]

Whatever amplifier you use, success begins with getting a clean signal from the microphone to the surface.

Looking at your pictures...

The mic post has '12V' written next to it. This would not apply to a dynamic mic, but it would be used by an electret. An electret can put out an adequate signal, and it would not be overpowered by noise, yet your initial post speaks of an 'overpowering high pitch background noise'.

If you hear this then you are getting amplification somewhere. And the first thing an audio technician would say is 'Can't hear speech but you hear high-pitched noise? The amp must be okay, so the likely culprit is a bad ground connection in the mic feed.'

Can you send in a clean signal and verify whether it gets through your system the way it should? It can come from a radio, Ipod, auxiliary mic, etc. To do the test right you should reduce it to mic level.

Likewise, can you plug your mic into a different amplification system, and see if clear speech emerges?