Re: [Phys-l] Why no hum?

[Brian Whatcott <betwys1@sbcglobal.net>]

Bernard Cleyet wrote:
> Back in the middle ages I repaired radios for pin money.  The "big"  
> ones often used a field coil instead of a PM in the loud speaker for  
> the voice coil "to work against"  [correct physics expression].  The  
> coil did double duty as part of the filter for the HV supply.  I  
> don't remember if they were L or C input filters, but either way I  
> was surprised -- no audible background hum; still am.
>
> Anyone out there have an idea why.
>
> bc
> __
Carr's Old Time Radios! ...

<http://books.google.com/books?id=KkvHhhJcGIwC&pg=PA101&lpg=PA101&dq=field+coil+in+loudspeaker+-+hum&source=bl&ots=gcw4lZ3ZKr&sig=Q47icOXJYINJP8PYWCk3qlGVBdM&hl=en&ei=l_v0ScPaGpTcMbe-sKkP&sa=X&oi=book_result&ct=result&resnum=2#PPA101,M1>

(TinyUrl)
*http://tinyurl.com/c3lu9x*

  ...shows a representative diagram of the field coil and voice coil 
drive featuring
an antiphase winding intended to cancel the field's hum frequency - the 
"Hum-Bucking winding".

BrianW