Adapting a Peltor Com. Headset for Other Applications

The company Peltor AB manufactures lots of safety gear. They are based in Sweden.

One of their products lines is head sets, which integrate hearing protection, and communications. These are typically used in the aircraft industry but also show up in many other applications. One place where Peltor's head set products are often used is in auto racing.

Communications gear is often used in auto racing. Speakers and microphones are integrated into helmets so that drivers can communicate with pit crews. In rally racing the car crew is often traveling between high speed stages via public roads. During these so called transits, at legal limit speeds, wearing a helmet can be hot, and tiring. However, rally cars, although required to pass local inspection laws, are quite noisy. Thus the use of a head set with communications gear would protect the user's hearing as well as provide need relief from constant use of a helmet.

Head sets suitable for such use can be found on the used aircraft equipment market. Aircraft often have the same noise and communications issues as rally racing does, the need for hearing protection but not necessarily head injury protection. Used head sets from the aircraft industry are similar in design, and much cheaper to purchase. They are often more plentiful as well. Unfortunately while the designs are quite similar, identical in many cases, the interconnects are not.

So... If one is going to use an inexpensive surplus aircraft heads set, how does one adapt it to work with a typical intercom system?

There is no simple answer, since everybody makes their stuff different from everyone else. It probably isn't so much a question of standards. The components used in most headsets are common in the electronic industry. It is more likely the manufacturer's trying to protect their market share.

Be that as it may, lets go through how to get a typical old style Peltor headset to work more or less in a standard manner.

Here's a picture of the headset we will be working with:


Overall view of headset

This headset is pretty simple. There is no noise canceling or volume control. However, it does have a preamplifier housed in the left earpiece.

It came from the surplus market quite cheap because the microphone had been broken off of the boom. In the above picture a typical computer headset microphone has been taped onto the boom in place of the missing original. The wires connecting the microphone are still hanging out in the breeze, since the modifications required to use this new microphone were still under test at the time the photo was taken.

The Peltor model number of this headset is unknown, since the ink had worn off the placard.

In the bottom right of the photo the NEXUS U-174/U (TP-101) four pole connector can be seen with its shroud pulled back. This connector mates with NEXUS series TJ-120, TJ-102, TJ-101 jacks. At least according to this Acousticom website. This is a mil spec. type MIL-C-9177 (M9177\2-1) connector. I'm told they are often used in helicopters... whatever... For us simple folk, they are just another PITA, since you can't bop down to Radio Shack for a mating connector.

This type of connector is also problematic in racing applications. Many competitors use the Peltor FMT200 intercom unit, which does not support this type of connector.

So how do we get this potentially useful headset working on a conventional computer or intercom system. Turns out it isn't all that difficult.

Working in our favor is the fact that Peltor hasn't gone out and built everything on its own, such that none of it can be expected to behave in a predictable manner.

Firstly, a look at the FMT200 manual, what little there is of it, indicates that both Dynamic, and Electret microphones can be connected to the headset input ports. Note that an Electret microphone is related to a Condenser microphone, and rightly or wrongly these days, the two terms are often used with no distinction. Fortunately, in many circuit implementations either microphone type (Electret/Condenser) will work fine. The only caveat is that Electret microphones, as manufactured today, do require proper polarity connections. Not for the mic itself but for the micro circuitry hidden in the microphone element body. What matters here is that the Peltor FMT200 does support an Electret type microphone, if it is wired to the correct pin on the input connector.

On the FMT200 the headset input connectors are five pole circular connectors. Unfortunately Peltor does not specify what type of connector this is. However, they will sell you something called an "FL2D Headset Cable" or an "FL2E", which is supposedly for headsets with an Electret microphone.

The headset connector pin out is as follows:

PIN

Function

1

Electret Microphone +

2

Dynamic Microphone +

3

Microphone - (earth)

4

Earphone -

5

Earphone +

It is nice of Peltor to have at least provided this information. I just wish they had also told us what type of connector mated with the ports on the back of the FMT200. Just to be clear, the two headset ports on the back of the FMT200 have pins, not sockets.

Headset Details

In order to get the headset to work on anything (Intercom, computer... what have you) it will need to be properly connected. So we need to map out how the poles on the connector get tied into the rest of the headset.

We know the microphone signal must end up on the connector. We know the earphone signal must end up on the connector.

But what goes where?

With no documentation on the headset, and no info on how a typical NEXUS U-174/U is wired for this type of service, we will need to open up the headset and have a look see. Fortunately this is easy to do.

Opening the ear piece

This is pretty straight forward. Gently work your fingers around the back side of the ear cushion, then gently pull upward or away from the ear piece housing.

Removing the earpiece

The ear cushion should pop off, it's just a snap fit. You can now pull off the gray foam.

Remove Foam

At this point the left side ear piece should look something like this:

Inside the left earpiece

Next the two Phillips head screws next to the walls of the ear piece should be removed.

Removing the mounting plate

The screws which hold the white speaker element alone. All we are trying to do here is get to the back side of the black mounting plate.

Back side of mounting plate

The photo shows the business end of the electronics in the left hand earpiece. The wires from the connector enter at the bottom of the earpiece, which can be seen in the photo on the edge of the housing to the left of the white speaker element. The wires run up to a circuit board, which Peltor decided to paint black so that we could not read the component types used. Fortunately the paint can be scraped off the components without removing too much of the manufacturer's original markings.

If we think about what needs to happen in a headset we don't necessarily need to identify each and every component in detail. 

First of all Peltor, at the time this headset was manufactured, seemed to like to use Dynamic microphones. A look at their latest product lines suggests that they still do.  There are some good reasons for this. One is that Dynamic microphones are more resistant to electronic noise. Dynamic microphones are also pretty rugged, and they can handle getting a bit damp. So it is likely that whatever this circuit does, it will be working with a Dynamic microphone. The clincher comes from how the missing microphone was connected to the circuit.

1) One pole went to circuit ground (the YLW wire).

2) One pole went to a capacitor (3.6 uF, Tantalum, polarized).

3) There is no power going to the microphone.

The third point is crucial. Electret microphones, by and large as commonly available, must have power. Dynamic microphones do not need power. It's pretty clear the missing microphone was a Dynamic type.

Armed with that fact, and the overall structure of the circuit, despite the paint, we just need one more clue to confirm what the circuit is probably there to do. That clue comes from identifying the one IC (Intgrated Circuit) on the circuit board. It is a TL071 JFET op-amp.

IC op-amp detail

This particular IC was manufactured by Motorola (the circle with a hint of an "M" in it), although the TL071 was originally a Texas Instruments design, I believe.  Motorola no longer makes these devices.  The majority of their useful product lines got sold off.  Many of their IC products are now manufactured by ON Semiconductor, and another company called Freescale.  Motorola has now become a cell phone producer... as sad a corporate story as the demise of  Hewlett-Packard.

This particular IC op-amp looks to be configured as an amplifier. This is consistent with the use of a Dynamic microphone. Dynamic microphones don't generate a whole lot of signal, so a preamplifier is used to boost the signal before it gets sent anywhere else. Preamplification helps reduce noise in the overall system.

By closer inspection of the board we can see that the circuit doesn't do anything to the speaker elements it just passes the signals through.

Main bus connections

At the top of the photo the red wire on the far right of the upper edge of the circuit board, and the white wire next to it, carry the speaker drive signal into the headset. At the bottom the signal is distributed to both speaker elements, although the wiring is cropped in the photo.

This speaker signal does not share the same ground as the microphone, in this wiring harness. The microphone ground is the yellow wire, in the bundle of six, which comes from the NEXUS U-174/U connector. Two of the wires in the bundle (BRN and GRN) are unused.

From the tip of the NEXUS U-174/U connector to the sleeve, this is how the signals are patched:

Location

Color

Function

Tip

Yellow

Ground (Microphone & Amplifier)

Tip Ring

White

Earphone +

Sleeve Ring

Black

Amplifier Power & Amplifier Output

Sleeve

Red

Earphone -

Notice that with the existing wiring, it is not possible to connect the earphones to support a stereo signal. Rewiring the headset, retaining the NEXUS connector, will support separate earphone channels but connector compatibility with devices which use the NEXUS connectors will be lost. If the headset were wired to support a stereo signal, the NEXUS connector should be removed, and separate mini or sub-mini, three pole phone plugs should be substituted.  To retain backward compatibility with the NEXUS connectors, a patch box with appropriate circuitry and connector should be built.

If the wiring on the NEXUS connector is not altered, it may be possible to use a standard converter plug to connect the headset to the FMT200.

There are some problems though:

This headset no longer has a microphone.

The microphone it did have was a Dynamic mic.

The new mic is an Electret type.

Converting to an Electret microphone

The circuit in the headset cannot be used with an Electret microphone. The reasons for this revolve around the need for DC power at the microphone, and the conflict between both the amplifier using the positive power line to send the microphone signal back to the intercom, and the Electret microphone trying to do the same.

Electret microphones send their signal back out the same line they receive DC power from. But the amplifier in this headset is doing the same thing with the same power line. Both devices can't use the microphone power signal in the same way. Among other things, this causes feed back into the amplifier circuit. The amplifier's own output signal will find its way back into its input. This is what causes the screeching and howling noise sometimes heard at concerts, and over other public address systems.  Although the route the feedback takes is different, the effect is the same.

So the amplifier must be dropped out of the circuit.  We have to keep the microphone in the circuit or else there won't be able to support two way communications.

This is easy enough to do. Just unsolder the black wires from the circuit board. There is one in the bundle from the NEXUS connector, and one coming from the microphone boom. This black wire is on its own on the opposite edge of the board. Once free, solder these two wires together, and tape or shrink wrap them so they won't short out on anything. 

Connecting Electret Mic

Now you can put the left hand ear piece back together, there's nothing more to do in there.

The next thing to do is mount the new Electret microphone to the boom, and properly solder the wires together. Unlike a Dynamic microphone, which doesn't care which wire goes where, the Electret mic must be connected with the proper polarity preserved. On the microphone used in this case the hot wire was colored red and the ground side was natural copper. This means:

Headset YLW to microphone natural.

Headset BLK to microphone red.

For the time being just tape the new mic to the old boom with a generous amount of electrical tape.  Since the end of the mic boom was flared, I wound some tape around the end first, so that the body of the boom was about the same diameter as the flare.  With that in place a portion of the new mic's flex shaft was firmly taped down parallel to the boom.

New Mic Mounting

The new mic and old headset can now be tested on an intercom system if the correct adapters are used, and the proper signal patching is used to bring the Electret microphones signal to the correct pin on the intercom unit.  In the case of the FMT200 the mic positive signal (BLK wire, sleeve ring on NEXUS plug) needs to get patched to pin 1 on the appropriate headset port on the back of the unit.  Of coarse the proper connector will be needed to do so.  

It is hard to say whether the FL2E cable will be compatible with this particular headset wiring.  There appear to be at least two different wirings that Peltor has used, and whether this wiring is compatible with the FL2E cable is unknown.

The known alternative wiring of the NEXUS plug is as follows:

Location Function
Tip Mic -
Tip Ring Earphone +
Sleeve Ring Mic +
Sleeve Earphone -

At the time of this writing the above table shows the other known wiring of the typical NEXUS U-174/U used by Peltor.  Since the whole mess is based on military requirements, there is likely some mil spec. (MIL-C-9177 (M9177\2-1) does not appear to dictate wiring of the connector) which covers how such things must be wired... whatever.

So you want Stereo... so do I...

As mentioned before wiring the headset for stereo is a possibility.

What is not known is how well the driver elements will work.  In Hi-Fi applications one would strive to use devices which have a flat response over the audio band.  The audio band, by convention, is 20 Hz to 20 kHz.  What's a Hz?  What's a kHz?  It's a unit of frequency or cycles per second, for more info look here.  The term "flat" response means we don't want our audio reproduction exaggerating or attenuating any particular frequency or set of frequency.  We want to hear things as they are.  Now flat response is not something our ears have by nature but their is no use adding to the complexity of system if it is not necessary.  Suffice to say it would be nice if the drivers in the headset worked in a useful manner over the audio band.

In any case Stereo wiring can be successfully accomplished no matter how limited the audio response.   Thus the distinction between the terms Stereo and Hi-Fi.  The term Hi-Fi is a contraction of high fidelity, which implies full use of the audio band without adversely effecting the original signal.  The term Stereo simply means that there are two channels which are capable of acting together or independently.

Incompatibility with existing wiring

As mentioned before the existing NEXUS connector and wiring does not lend itself to independent channels in both ears.  As currently wired the both earpieces are driven from the same two wires.  

Earphone +, the white wire (tip ring)

Earphone -, the RED wire (sleeve)

Not only is the nexus connector not wired for Stereo, it physically will not mate with common Phone plugs (6.35 mm, 1/4") or Mini Phone plugs (3.5 mm, 1/8").  So the wiring must be changed along with the connector.

Common Mini Phone wiring

Typical Mini Phone 3.5 mm pulgs, also called TRS connectors, have a ground connection on the sleeve with the right channel on the ring, and the left channel on the tip.

In the headset the black wire of each speaker element is soldered to the ground terminal, with the two yellow wires of the speaker elements soldered to the node driven by the WHT drive signal.

Board speaker interconnect detail

What we need to do is keep the black common wires together, separate the two yellows, and patch them to the correct location on a Mini Phone plug.  With that done we can test the operation of the headset using a computer audio power output (not a line output) or an iPod, Stereo amplifier, MP3 player or whatever.

There is now a choice to make.  Use the existing flex cord or not?  I have chosen not to use it.  It's big and heavy not really necessary.  I have never been a big fan of flex cords anyway.  Furthermore I prefer to keep the NEXUS connector intact on the flex cord so that it could be used again if need be, so removal of the flex cord is called for.  This could be done by simply cutting the cable but I prefer to clip off the strain relief, and slip the now unused cable out of the left side housing.

In place of the flex cable I used a headphone cable with a mini phone jack, cut from a pair of old rotted out KOSS headphones.  This cable has a couple problems.  The first problem with this cable is that it is not shielded, the second is that nylon was wound with the conductors, which makes soldering a bit messy.  On the other hand it is fairly lengthy being about 2 m in length.  I'll live with its short comings for the time being.

Also added was a piece of "24 AWG Audio Cable" from Radio Shack.  It's a bit stiff so I used only a short piece (about 10 cm) long enough to work with outside the housing.  I can always use something better from there.

Mono Mic wiring

The black 3 conductor cable is the new Stereo mini phone cable.  The white wire in this cable is the right channel (plug tip), the red wire is the left channel (plug ring), the black wire is the ground (plug sleeve).  

The gray two conductor with braided and foil shield is a 10 cm or so length of 24 AWG "Audio Cable" from Radio Shack.  This will be used to connect to an Electret compatible microphone port on a computer or other device.  The braided shield and the foil sheild will be used as the "Mic -" signal.  This isn't the best way to do things but this way the white wire and the red wire of this cable can be used as the left and right microphone channels.  There is only one mic in this application, so both lines will be soldered to the same node.  This may not be the best thing to do, since each channel has its own "phantom power" but we'll give it a try and see what happens.

Good grounding practice dictates that the shield should be connected only at the originating device (a star topology).  In this case the originating device will likely be a computer or perhaps a radio.  Since the shield is also being used as the mic signal reference it isn't acting solely as a shield, although arguably it is being used as a typical shield is used in RF work.  It is also sharing the same connection, at the originating device, as the ear piece drive common.

To use the braided shield the YLW microphone wire was soldered to it where a small portion of it protruded from the gray jacket.  It's a bit fuzzy in the picture but it can be seen in the crook of the black microphone cable on top of all the other cables in the photo.

Since the original grommet rotted out.  I used some hot glue to provide strain relief on the two cables.  Be careful when hot gluing.  I was not the first time.  As a result the was not enough length to position the speaker element bracket properly.  I had to gouge out the glue and try again.  Second time's the charm.  Try not to get hot glue in the grove where the ear cushion will sit.  If you do, it should be fairly straight forward to work it out of the groove.  The cushion won't snap in, and seat properly if you leave glue in the groove.  Hot glue on the foam will be more or less permanent.  The only way you will get it off will be to tear the foam.

Results

As it turns out the drivers don't do so well on the low end.  This is not unexpected since they are, after all, primarily designed for voice communication, and hearing protection.  It's not surprising that they don't do too well with respect to Hi-Fi.  It is possible that using better drivers may help... whatever.

I still need to test the microphone.

Hope this helps with your head set project....

73  DE  AA1ZB

Resources

Peltor parts:  enviro safety products

My local supplier is N.H. Bragg.  They don't have the "Hygiene Kit" and what-not in stock but some of the hearing protectors don't cost much more than a "Hygiene Kit".  It may be possible to buy an inexpensive headset and steal the parts from it to upgrade one of the older microphoned aviation sets.