[ARC5] Running the receiver rack with the sidetone/voice function

[WA5CAB at cs.com]

ARA and AN/ARC-5 receivers (and both modulators) audio impedance is what 
today is commonly called 600 ohms.  Early -A suffix SCR-274-N receivers and 
BC-456-A modulators have 4000 ohm audio output impedance.  The -B suffix 
receivers and the -B and -E suffix modulators have what was called early in the 
War a 250 ohm tap on the transformers.  Later manuals all seem to call it 600 
ohms.  Unless the units have a "connected for low impedance" tag or sticker 
on the front, you will just have to open them up and check.  The Lo-Z tap 
on the sidetone winding on the modulation transformer is #7.  I read 
somewhere that the choice of 250 ohms for the audio sources and (except for the 
HS-30's) 600 ohms for the audio loads was because you could put up to four loads 
on a source without exceeding a 2:1 mismatch.  Whether the actual 
transformer impedances were changed or they just changed what they called it, I don't 
know but suspect the latter..  

Robert Downs - Houston
wa5cab dot com (Web Store)
MVPA 9480

In a message dated 04/18/2017 18:56:32 PM Central Daylight Time, 
arc5 at mailman.qth.net writes: 
> Took the plunge and wired up the 6 pin plug cable that goes from the 
> BC-456 to the receiver rack.
>  It actaully functions pretty well , not objectionably loud on sidetone or 
> voice. However I'm using a speaker , not headphones and I'm going to 
> re-visit the impedance of same because I think the whole thing could be louder 
> than it is. Right now it's set for about 600 ohms for the ARC-5 receivers.
> 
> 
> 
> 
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