[ARC5] Audio Impedance Matching on USAAF aircraft

[Craig Cantwell]

 Mark: I just pulled the wiring diagrams for my Fairchild and looked. All the receiver audio goes to the control box, and then to the RC-36 Interphone amp. Form there, it gets fed out to various locations, including the FL-8 filter. In the diagrams, the FL-8 has a single headset output jack for the pilot to unplug from the interphone and plug into the filter directly and run in an isolated manner. If you want, I'll see about copying that section the page and posting.
CraigKF5JOT
    On Tuesday, December 30, 2025 at 02:42:16 PM CST, Mark K3MSB <mark.k3msb at gmail.com> wrote:   

 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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