Headset connections (attn: Jim Weir)

I have a question I'm hoping you may be able to answer based on a
Kitplanes article (Dec 1997) I came across concerning the topic.

As part of an upgrade program in the Flight Simulation Lab here at the
University of Toronto, we purchased a new helmet mounted display system.
This helmet includes an integral mic and speakers. Oddly enough, even
though it is designed for aviation simulators, they used 32 ohm speakers
and a non-amplified microphone (and a DB9 connector). The problem is we
use an aviation (Telex, actually) intercom for communications between the
pilot(s) and instructor station. I have acquired a set of GA style plugs,
but have been unable to find a suitable circuit to make an adapter (i.e.
to amplify the microphone), and radios/headsets is not exactly our
area of expertise.

Based on the above article, in which you talk about radios that can
accept either microphone as input, I'm guessing you've done this exact
conversion. As the helmet manufacturer has not been able to suggest a
solution, any help you could provide would be greatly appreciated (a
circuit diagram? an adapter for sale? where to find the answer if there
is one?)

Thanks,

Bruce Haycock
University of Toronto
Institute for Aerospace Studies

I don't see where the 32 ohm speaker should be much of a problem. The 8 ohm
output should be able to drive it . If not get a stereo amp with selector
for mono output and combine the two outputs in series which would give you
16 ohm impedance and with the amp should not present a problem.
As far as the mic there are a lot of microphone amps that are comercially
avalialble that are designed to do that. I'm fairly sure Radioshack carrys
them.
John



Bruce wrote:

I have a question I'm hoping you may be able to answer based on a
Kitplanes article (Dec 1997) I came across concerning the topic.

As part of an upgrade program in the Flight Simulation Lab here at the
University of Toronto, we purchased a new helmet mounted display system.
This helmet includes an integral mic and speakers. Oddly enough, even
though it is designed for aviation simulators, they used 32 ohm speakers
and a non-amplified microphone (and a DB9 connector). The problem is we
use an aviation (Telex, actually) intercom for communications between the
pilot(s) and instructor station. I have acquired a set of GA style plugs,
but have been unable to find a suitable circuit to make an adapter (i.e.
to amplify the microphone), and radios/headsets is not exactly our
area of expertise.

Based on the above article, in which you talk about radios that can
accept either microphone as input, I'm guessing you've done this exact
conversion. As the helmet manufacturer has not been able to suggest a
solution, any help you could provide would be greatly appreciated (a
circuit diagram? an adapter for sale? where to find the answer if there
is one?)

Thanks,

Bruce Haycock
University of Toronto
Institute for Aerospace Studies

The easiest way is to replace the mic with a David Clark or Telex
aviation mic.
If you can not do this then you can strip the amp out of an old Telex
100TRA slim line mic if you have a dynamic mic element in your helmet.
Put this amp in a box and plug in the 9 pin D connector into it and
run the aviation style plugs from the box so you have a lumpy
interface cable. Nine times out of ten the old telex mic died due to a
bad switch which you don't need anyhow. Most intercoms will drive the
32 ohm speaker. If it will not drive it to the level you need get a
low cost speaker amp of a watt or two. See the National semiconductor
data book for circuits if you want to make your own.
John

On 15 Apr 2004 08:30:43 -0700, (Bruce) wrote:

I have a question I'm hoping you may be able to answer based on a
Kitplanes article (Dec 1997) I came across concerning the topic.

As part of an upgrade program in the Flight Simulation Lab here at the
University of Toronto, we purchased a new helmet mounted display system.
This helmet includes an integral mic and speakers. Oddly enough, even
though it is designed for aviation simulators, they used 32 ohm speakers
and a non-amplified microphone (and a DB9 connector). The problem is we
use an aviation (Telex, actually) intercom for communications between the
pilot(s) and instructor station. I have acquired a set of GA style plugs,
but have been unable to find a suitable circuit to make an adapter (i.e.
to amplify the microphone), and radios/headsets is not exactly our
area of expertise.

Based on the above article, in which you talk about radios that can
accept either microphone as input, I'm guessing you've done this exact
conversion. As the helmet manufacturer has not been able to suggest a
solution, any help you could provide would be greatly appreciated (a
circuit diagram? an adapter for sale? where to find the answer if there
is one?)

Thanks,

Bruce Haycock
University of Toronto
Institute for Aerospace Studies

Unfortunately due to how the helmet is assembled, replacing the mic
would require quite a hack job, so I'm trying to avoid that. I'm
pretty sure the mic element is electret rather than dynamic. Although
it will probably be more costly than building the amplifier circuit,
would the amp be contained in a david clark electret mic assembly
(i.e. part #22 in this:
http://www.davidclark.com/PDFfiles/19515p39.pdf)?
Bruce

(John) wrote in message ...
The easiest way is to replace the mic with a David Clark or Telex
aviation mic.
If you can not do this then you can strip the amp out of an old Telex
100TRA slim line mic if you have a dynamic mic element in your helmet.
Put this amp in a box and plug in the 9 pin D connector into it and
run the aviation style plugs from the box so you have a lumpy
interface cable. Nine times out of ten the old telex mic died due to a
bad switch which you don't need anyhow. Most intercoms will drive the
32 ohm speaker. If it will not drive it to the level you need get a
low cost speaker amp of a watt or two. See the National semiconductor
data book for circuits if you want to make your own.
John

Yes the amp in the david Clark mic "Should" work however it is most
likely that this is a hybrid circuit and is so small you would need
micro sized tools and a microscope to work with it.

First you need to determine what kind of Mic it is. It should say in
the documentation for the helmet. If not you need to use an ohmmeter
to determine what it is. Use a DVM and measure the mic resistance in
both directions. If it is the same when you swap the probe polarity
and is less than 1000 ohms then you have a bare dynamic element. If
it changes with the polarity of the meter leads then you have an
eletrofet or a amplified electrofet or an amplified dynamic mic.
I assume that you have already tried to hook the mic to the intercom
and it does not work so that leaves the bare electrofet element as the
logical choice for the element. ( A bare electrofet element puts out
only a few milivolts into about a 1k bias resistor.) If so you can
go the National semiconductor web site
http://www.nationalsemiconductor.com/ and pick a audio amp in the one
to two watt range to power the relative low impedance headphones if
your intercom will not drive the 32 ohm headset to the level desired
and a pre amp that is available from parts suppliers like
www.digikey.com. If you power these from an separate 12 volt
regulated wall wart power supply then you do not have to worry about
many of the power and stability issues where the amp is powered by the
same line that puts out the audio. You MUST use a regulated power
supply or include a three terminal regulator as part of your interface
or the 60 hz hum will be very bad. National has application notes
that give typical designs. Most bare electrofet elements need a 1K
to 4.7K pull up to 6 to 10 volts as the bias power supply to make them
work. AC couple the electrofet mic audio to the preamp input then use
the preamp IC to get the gain necessary to get about 1/4 to one volt
of audio into the intercom. AC couple the preamp output into the
intercom mic audio input through at least a 1 UF cap and it will work
fine.
I cannot tell you exactly what you need since you do not have any
specifications and you must measure what you have as a starting point.

On 19 Apr 2004 07:13:03 -0700, (Bruce) wrote:

Unfortunately due to how the helmet is assembled, replacing the mic
would require quite a hack job, so I'm trying to avoid that. I'm
pretty sure the mic element is electret rather than dynamic. Although
it will probably be more costly than building the amplifier circuit,
would the amp be contained in a david clark electret mic assembly
(i.e. part #22 in this:
http://www.davidclark.com/PDFfiles/19515p39.pdf)?
Bruce

(John) wrote in message ...
The easiest way is to replace the mic with a David Clark or Telex
aviation mic.
If you can not do this then you can strip the amp out of an old Telex
100TRA slim line mic if you have a dynamic mic element in your helmet.
Put this amp in a box and plug in the 9 pin D connector into it and
run the aviation style plugs from the box so you have a lumpy
interface cable. Nine times out of ten the old telex mic died due to a
bad switch which you don't need anyhow. Most intercoms will drive the
32 ohm speaker. If it will not drive it to the level you need get a
low cost speaker amp of a watt or two. See the National semiconductor
data book for circuits if you want to make your own.
John

The only description of the mic in the documentation is the frequency
badwidth (100Hz to 3kHz). Mic resistance is 5.82kOhms one way,
5.56kOhms with reversed polarity, and yes I already tired to hook up
the mic to the intercom, so its definitely a bare electret mic.

(John) wrote in message ...
Yes the amp in the david Clark mic "Should" work however it is most
likely that this is a hybrid circuit and is so small you would need
micro sized tools and a microscope to work with it.

First you need to determine what kind of Mic it is. It should say in
the documentation for the helmet. If not you need to use an ohmmeter
to determine what it is. Use a DVM and measure the mic resistance in
both directions. If it is the same when you swap the probe polarity
and is less than 1000 ohms then you have a bare dynamic element. If
it changes with the polarity of the meter leads then you have an
eletrofet or a amplified electrofet or an amplified dynamic mic.
I assume that you have already tried to hook the mic to the intercom
and it does not work so that leaves the bare electrofet element as the
logical choice for the element. ( A bare electrofet element puts out
only a few milivolts into about a 1k bias resistor.) If so you can
go the National semiconductor web site
http://www.nationalsemiconductor.com/ and pick a audio amp in the one
to two watt range to power the relative low impedance headphones if
your intercom will not drive the 32 ohm headset to the level desired
and a pre amp that is available from parts suppliers like
www.digikey.com. If you power these from an separate 12 volt
regulated wall wart power supply then you do not have to worry about
many of the power and stability issues where the amp is powered by the
same line that puts out the audio. You MUST use a regulated power
supply or include a three terminal regulator as part of your interface
or the 60 hz hum will be very bad. National has application notes
that give typical designs. Most bare electrofet elements need a 1K
to 4.7K pull up to 6 to 10 volts as the bias power supply to make them
work. AC couple the electrofet mic audio to the preamp input then use
the preamp IC to get the gain necessary to get about 1/4 to one volt
of audio into the intercom. AC couple the preamp output into the
intercom mic audio input through at least a 1 UF cap and it will work
fine.
I cannot tell you exactly what you need since you do not have any
specifications and you must measure what you have as a starting point.