less hum: switching power supply?

[Don Tillman]

   From: chris [mailto:espace at e-server.net]
   Sent: Friday, June 02, 2000 11:20 AM

   in a quest to totally eliminate the hum in the audio circuits... 
   i've decided that switching power supply is the way to go to get
   rid of the big 60-cycle EMF.

A switching supply will never have less hum than an equivalent linear
supply (assuming that the other conditions are somewhat similiar and
things are wired correctly).  It just can't happen; a switching supply
has a compromised feedback path because it also has to deal with high
frequency switching and magnetic effects.

   From: Jay Martin <jmar at intface.com>
   Date: Fri, 2 Jun 2000 14:23:54 -0400

   It has been my experience that the biggest problem with linear
   power supplies is that most of them try to make the regulator do
   too much work, and all they use for filtering is capacitance.  If
   you want to make a "clean" linear power supply, all that is needed
   is a true "pi" type filter after the rectification using both
   capacitors for Voltage storage and inductors for Current storage
   (and to keep the current and voltage in phase).  

Are you claiming that an inductor/capacitor power supply will have
less hum than a capacitor only supply?  Of the same weight?  Certainly
not.  Of the same volume?  Same price?  Yeah, correctly adding
inductance will cut down on hum, but so will adding more capacitance. 
Under what circumstances do you think the inductor is better?

   Something like this approach is what I'm trying to convey:
   From full wave bridge >-----+---^^^^^^---+---^^^^^^-----> To regulator
			       |     L1     |     L2
			       |            |
			     C1=          C2=
			       |            |
			       |            |
		  Ground >-----+------------+------->

This is a nice diagram, but it won't work.  The trailing inductor will
give you a very high impedance at high frequencies, which is a bad
thing for a power supply, and regulator chips are not guaranteed to be
stable working from an inductive source.

Perhaps you intended to place the inductor at the front end.

  -- Don

-- 
Don Tillman
Palo Alto, California, USA
don at till.com
http://www.till.com