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

[J. Forster]

I think the headphone impedance change could well have resulted from an
earphone design change.

Prewar, earphones used a "U" metal structure wound with a coil on each leh
and a metal disk diaphragm. I suspect this design does not work well at
high noise levels as might be found in AFVs ot planes.

So, they went to the small loudspeaker design, and hence a much lower
impedance.

FWIW,

-John

==================





> 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)
> MVPA 9480
> ______________________________________________________________
> ARC5 mailing list
> Home: http://mailman.qth.net/mailman/listinfo/arc5
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:ARC5 at mailman.qth.net
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
>
>