[ARC5] Fw: Receiver current drain (AF output)

[WA5CAB at cs.com]

Les,

I'm not certain of your definition of "early" but in the context of Command 
Sets, what I'll define as "late pre-war" (roughly 1933-1938) most Signal 
Corps headsets were typically 20K - 24 K (for two receivers in series).  
Around 1939+/- the 4000 ohm R-14 receiver appeared and the standard Signal Corps 
headsets (except for two that only used one receiver) became 8000 ohms.  
Around the same time, the LS-3 loudspeaker appears, with an 8000 ohm to VC 
matching transformer.  Followed by the LS-7 with, for some unknown reason, a 
7000 ohm one (but the sets it was used with typically only had one or two audio 
loads).  So near the beginning of the War (regardless of how you define 
that), the new audio loads were 8000 ohms and the common Signal Corps receivers 
had 4000 ohm outputs.  The rational being, I think, that you could put one 
to four loads on them without exceeding a 2:1 mismatch.  And it does work 
pretty well, as long as you don't mix low-Z and hi-Z headsets!!!.  The US Navy 
and Naval Air switched to low-Z audio sometime earlier (I have no info on 
when).  The Army began switching in late 1942 but stuck with the 2:1 concept 
so the headset receivers (ANB-H1 and R-30) were around 300 ohms (times two 
in headsets) and loudspeakers (LS-9 and LS-11) were 500 ohm while the audio 
output transformers in the BC-312/342, BC-224/348 and B-model SCR-274-N got 
250 ohm taps.

In a message dated 1/6/2011 11:54:30 PM Central Standard Time, 
vk2bcu at operamail.com writes: 
> As I understand the original design, early sets were designed to drive 
> headphones with 2k ohms impedance.  Later this became 600 ohm headphones.  
> Since the output impedance of early transistor radios was about 500 ohms I 
> ran a raiding party on my local radio club store, and got a handful of output 
> transformers "rescued" from broken transistor radios.  I drive this 
> directly from the output transformer of the "command" set.  Works well with a 
> good 8 ohms speaker.
> 

Robert Downs - Houston
wa5cab dot com (Web Store)
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