[NJARC] Power Supply Question

Fri, 17 Jan 2003 14:30:50 -0500

Jim: (and the list)

In reality, I always knew the purpose of the "hum bucking coil", however in 
40 years of messing with radios with electro-dynamic speakers in them, I 
can't recall an instance when the field coil got connected in reverse.  Most 
schematics show the extra coil (especially RCA).  What I did not know is 
that the wiring reversal is that dramatic in reduction of hum.

Most radios have speaker plugs which prevent this.  I have rewired plugs to 
the respective points in a chassis many times, but always was careful not to 
change anything.  Usually rotten wiring is just replaced one for one.  One 
has to be particularly careful on sets with PP output and the transformer 
mounted at the speaker to be sure the plates and B+ are in the right places, 
also.

I never thought about this very much and your experience has taught me 
something too.

Lately, have spent more time teaching people who did not grow up on tubed 
equipment that the voltages present on electro-dynamic speaker wiring is 
hazerdous, just something that some of us just know from years of 
experience.

Phil

>From: [email protected]
>Reply-To: [email protected]
>To: [email protected]
>CC: [email protected]
>Subject: Re: [NJARC] Power Supply Question
>Date: Thu, 16 Jan 2003 22:56:25 -0500
>
>Joe and the group
>I did some reading on electrodynamic speakers and learned some
>interesting things about them.  Initially, I couldn't understand how
>reversing the field coil could reduce the power supply hum.  On the face
>of it, what difference would a north pole facing the speaker cone have
>over the south pole?
>
>What I didn't realize was that the field coil, acting as the power supply
>choke, would have a magnetic field that varies in strength at the ripple
>current frequency.  This would affect the voice coil just like the
>amplifier's output.  To counteract this unavoidable coupling, the voice
>coil has two windings on it.  One is for the audio amplifier as expected.
>  The other is a hum-bucker winding which is wired in series opposition,
>with the voice coil, and cancels the induced hum.
>
>So when properly connected, the winding is series subtracting and the hum
>is reduced.  When reversed, just as Joe reported, it is series aiding and
>more hum in produced.
>
>Always something new to learn about old radios!
>Regards,
>Jim
>_______________________________________________
>NJARC mailing list
>[email protected]
>http://mailman.qth.net/mailman/listinfo/njarc

_________________________________________________________________
The new MSN 8 is here: Try it free* for 2 months 
http://join.msn.com/?page=dept/dialup