Odd amp to restore

[Quayhog]

I acquired an ARINC AFM-101 monitor amp.  It has some curious features and some impressive parts.  As near as I can tell it’s a monitor amp used by FAA facilities.  I think it’s designed to take BALANCED HiZ mic signal and route the signal through either or both 600 Ω or 8 Ω outputs.

The mic jack shares the same type jack as Electro Voice 666 mics.  The signal gets routed through a UTC H20 15k:80k interstage transformer, Then to a 6AU6 input tube.  The rest of the tubes are 12AU7 (PI) and push/pull 6AQ5.  Tube rectifier and 6E5 eye tube complete the tube count.

The output transformer is a UTC LS52 and has dual outputs.  The freq response is 7hz-50khz.  Way overkill for its intended purpose. L-Pad attenuators on the outputs.  The dataplate has a march 1960 date on it.

Now the question,  why would HiZ signal be transformer balanced and why did ARINC use such expensive parts?  After I get it working what would you modify it to do?

 

[mjrippe]

Can't think of a balanced HiZ microphone off the top of my head.  Perhaps it is a bridging input for line level signals?  Odd that they used the cannon xl connector though.

 

[abbey road d enfer]

I acquired an ARINC AFM-101 monitor amp.  It has some curious features and some impressive parts.  As near as I can tell it’s a monitor amp used by FAA facilities.  I think it’s designed to take BALANCED HiZ mic signal and route the signal through either or both 600 Ω or 8 Ω outputs.

The name suggests it was designed to receive line-level signals and drive either a loudspeaker or a tie-line.

Now the question,  why would HiZ signal be transformer balanced

The high input impedance means it could be used for eavesdropping on a 600-ohm line that's already loaded. At the time, broadcast systems were 600-ohm matching; you cannot put two 600-ohm loads on a 600-ohm output.

and why did ARINC use such expensive parts?

At the time, broadcasters were very rich and typically used what was best. 

After I get it working what would you modify it to do?

I wouldn't mod it, and use it as it was designed, anywhere a medium power amp is needed.