mrglass2626

Hello all:

Looking for help on the following issues.

Background: I have built my own "clarity aloft" style headset and added active noise reduction and MP3 input to the system. The unit I have created works flawlessly and I have received many positive comments from colleagues - a few of whom have expressed an interest in paying me to build them one too!

Herein lies my problem.

To build my original headset I used parts from a crappy old conventional headset I had lying around similar to this one:


I have since done a breakdown of original parts and believe I can source every component separately except for one pesky little thing:



This black chip runs in line from the electret microphone: red & white IN....red & white OUT....and then off to the PJ068 plug.

I believe it is a pre-amp and/or ceramic noise filter chip for the electrec condenser microphone and is used to match the level of the microphone to the avionics system of the aircraft radio.

For the life of me I cannot find out much information about it or where to buy / source one from.

The closest thing i've found is this:

But it has more pins and virtually no information on the application of the chip. Although the reference to "M87" is an aviation M87 microphone so it might be close but I am not sure.

Since it would be impractical to destroy a perfectly good headset each time I want to re-build one of these units I would like very much to be able to buy this little chip.

If anyone has any knowledge or further information on this item please let me know.

As a side note:

What is that resistor doing there and what purpose does it serve?

If I can get hold of that pre-amp and figure out the mystery of the resistor then I will be in a position to be able to replicate my entire headset design for anyone that would want one.

Not saying it's TSO'd or anything but it reduces noise in the cockpit VERY well.

Thanks for you help and knowledge. If you know a better forum that this could be posted on I welcome that information also!

FlightGear

Hi mate, I've taken the liberty of asking my HAM mates about your first question. I am 80% sure that black jobby [an IC not a chip] is the Auto Gain controller. I will update as soon as I get an answer.

Your resistor is simply protection from 'power' spikes caused by plugging / unplugging equipment that would otherwise dissipate through the microphone its self. You don't have to use a 472 Ohm 5% resistor but you will need to keep it well above 300 Ohms otherwise the resistor will act as a shunt.

Although this site is for home entertainment, you will probably glean enough info to answer simpler questions; DIY Audio & Video - FAQs, Tutorials, and Calculators for Speaker Boxes, Crossovers, Filters, Wiring and more

For more involved questions I would suggest a amateur radio forum. They contain a nice blend of electronics info and plain english.

mrglass2626

Further internet trawling has led me to the following descriptions of this IC:

"Ceramic hybrid active filtering chip"
"Pre amplifier with selective filter"

I found another person doing a similar project who also took a photo - his image clearly labels the IC as:

"M-65 V3.43"


Which is obviously a reference to the M65 Electret microphone which is common on many cheaper aviation headsets and was almost certainly the same microphone I pulled out of my headset.

I found the microphone replacement here: for $26. So i'm getting closer.....but $26 would be the most expensive component of the whole headset...so I would still prefer to buy just the chip and not a microphone boom that happens to come with the chip.

I see these cheap grey generic Avcom-200 looking headsets everywhere....there must be a factory with a bucket load of these chips, there must be a way to get hold of a supply of them.

Any info is appreciated.

mrglass2626

bump....
Anyone?

N9373M

Sorry I can't help you with your original question, but I have one for you that maybe you can help with. I have two Telex ANR headsets I use for general aviation aircraft. Depending on the headset and which jack (pilot/co-pilot) I get a pronounced squeal when transmitting. For example, the same headset squeals on the pilot side, but not the co-pilot. Is this an aircraft problem, or a headset problem? It's happened in two different a/c. Telex Stratus, and an older model.

Thanks!

mrglass2626

Sadly I do not know the answer to that question. My experimentation with preamps has shown me that if you connect the live voltage wire to the headphone output you get a very loud squeal so maybe the PTT switch is wired wrong and the Bias Voltage is going to the wrong place.

I am not sure though. Good luck finding the answer.

Cubdriver

Why don't you tear one apart and check it out with an oscilloscope and a low power microscope.

mrglass2626

Can't , the "chip" is covered in a ceramic material, impossible to scrape off. The only text on it is: "M65 V 3.4".

Do not own an oscilloscope.

I am almost certain this is a preamp with either a low or high pass noise filter, but would still like to know for sure and where to get them from or find a schematic.

CessnaVolador

I have the circuit you are looking for, just let me know or I can post it here if anyone need it.

Regards

mrglass2626

Yes, interested please post here or PM me. Thank you.