[MRCA] Audio Impedance Matching on USAAF aircraft

[Mark K3MSB]

Hi Everyone

As some of you know, I'm using my BC-453B LF/MF receiver to see how many
NDBs I can log this season – I'm up to 123.

I don't want to use modern post audio processing (my own self imposed
limitation) and I remembered I have an FL-8-A filter on the shelf.    Since
that was used doing WW II I decided it was fair game for
audio post processing

So, I hooked it up and gave it a whirl.   “Both” worked fine,   “Voice” was
OK, but I needed to increase the audio,  and “Range” seemed dead.... but I
found if I cranked up the audio to max I could hear strong signals.

Hmmm.   After thinking about this a few minutes, I wondered if I had not
taken impedance into consideration.   So, I went to K4CHE's web site where
he has some excellent info on the FL-8 posted.... and yep,  I had some
nasty impedance mismatches and I could see why the “Range” position acted
like it did.   The FL-8 needs 8K on input and output.

All my SCR-274N radios are configured for “low” impedance – 600 ohms.   I
send the audio output to a 600 to 8 ohm transformer then to my 8 ohm
headphones.   Well,  I plugged the FL-8 into the 8 ohm output.... and
thence was the problem.

Handling the impedance issue is not a problem, but I have a question of
they did it “back then”.

All of the SCR-274N receivers I have are all configured for low impedance
(600 ohms).   The receiver output on the planes had to eventually make it
headphones, and I assume the headphones were all high impedance (8K or
more) which matches the FL-8 output.

But the FL-8 input was also 8K.   That means all the receivers needed to be
configured for high impedance output (8K),  or if configured for low
impedance (600 ohms) there needed to be another box or boxes between the
receiver output and the FL-8 input.  What where those boxes?

Or... were most receivers configured for Hi-Z and all of mine just happened
to be configured for Lo-Z (perhaps by a secondary non military user) ?

73 Mark K3MSB
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