WWII Tank intercom question

Discussion in 'General Electronics Chat' started by spookydad, Nov 27, 2012.

  1. spookydad

    Thread Starter New Member

    Nov 2, 2012
    I am trying to set up an intercom in a WWII tank. It is an extremely high noise environment and the system employed throat microphones to help with this.

    There is no existing intercom but we do have some original headphones and microphones.

    We have been using a portable Nav-Data aircraft intercom and it functions fine using standard aircraft headsets. To me, there is nothing uglier than a wonderful looking WWII tank rumbling by with the crew wearing modern aircraft headsets.

    The problems we have been having are:

    1. The noise in the driver's position is so loud that the microphone is feeding the noise back into the system. The driver needs both hands to steer the tank at all times so it needs to be vox.
    2. The original headphones are 1000 Ohm per ear and I think the throat microphones are 300 Ohm. We can get either original equipment working or modern but not both at the same time. Obviously there is an impedance mismatch.
    3. The Nav-Data has a couple of external connections for an mp3 player and a recorder. I would like to use that to hook up to a GMRS/FRS/HAM walkie-talkie to communicate with other vehicles. This sort of works but it is erratic.

    Our goals are:

    1. Be able to use the original headphones for the two exposed crewmen.
    2. Be able to use original throat microphones or replace them with modern throat microphones designed for a GMRS radio.
    3. Allow the radio operator and/or the commander (co-pilot/pilot) to switch over to the GMRS radio to communicate with event staff.
    4. Allow music to be played over the intercom.

    As I see it, I will need to build some impedance matching circuit boxes to insert at the various change-over spots. Each crew position has a plug in box that can hide the circuits and multiple types of connection points.

    I have enough electronics knowledge to know that I am out of my depth. I have built some breadboard circuits but never made a proper circuit board.

    Does anyone have any suggestions?

  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
    Usually throat microphones have two pickups, one picks up the voice the other picks up background noise, then the background amp is inverted and added to the voice mike to get rid of the background noise.
  3. spookydad

    Thread Starter New Member

    Nov 2, 2012
    This is what we have:

    The Nav-Data (ND-74) on this page: http://nav-data.com/Products.html

    The FDC Fd-450 radio ( http://www.radioshop888.com/radioshop_product.php?id=102276 )

    and these throat microphones: http://www.radioshop888.com/radioshop_product.php?id=103687

    I just read that the radio is using 4 Ohms as a full volume load.

    Am I correct in thinking that I need to amplify the signal going in to the 2000 Ohm headsets?

    Should I be putting a resistor in line with the 4/8 Ohm modern throat mics?

    I think the standard resistance for an aviation mic is 150 Ohms and 300 Ohms for the headset.