less hum: switching power supply?

[Jay Martin]

OK, I have to put my $.02 in here.  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).  The regulator would follow all of
this and would have a very clean input so it would be able to deal with the
rest of the ripple.

Trying to use a switching power supply is probably the wrong approach.  It
is very difficult to get rid of the RF that is introduced, and the circuits
can be downright dangerous to work on (RF has much more transient energy!!)

Something like this approach is what I'm trying to convey:

>From full wave bridge >-----+---^^^^^^---+---^^^^^^-----> To regulator
                            |     L1     |     L2
                            |            |
                          C1=          C2=
                            |            |
                            |            |
               Ground >-----+------------+------->

L1,L2 are inductors, value is probably not critical.
C1,C2 are capacitors, I prefer at least 3300mF ea.

OK, So it's not a "pi" because of the extra L2, but I forget the name for it
with the extra inductor.  Also the values can be calculated so that the LC
of the network produces a proper phase correction..but don't ask me what the
formula is...I'd have to dig out the reference manuals and they are at home.
It usually ends up not being too critical anyway.

Also over design the thing.  It has also been my experience that the noise
ripple will go up dramatically the closer you get to the tolerance of the
parts.  For example, if you are trying to draw 1 amp from a 1 amp power
supply transformer..there will probably be noise no matter how much
filtering you do.  A good rule of thumb is to design 3 times the value that
you plan to draw.  If you are planning on drawing an amp, use a 3 amp
transformer and the appropriate support components to match.

One final note, use the TO-3 based regulators instead of TO-220 based (SK
parts instead of the LM78xx). the regulation is much cleaner on them.
Another idea is to use a zener-to-transistor regulator.  If done right, it
will regulate better than any of the IC based ones and even has some ripple
compensation in the form of a "super capacitor" (the capacitance actually
gets amplified by the bypass transistor).  If anyone is interested in this
type of design, request through the group and I'll dig out the schematics
that I did years ago.

Jay Martin
ISI/CLEO Technical Support


-----Original Message-----
From: chris [mailto:espace at e-server.net]
Sent: Friday, June 02, 2000 11:20 AM
To: synth-diy at node12b53.a2000.nl
Subject: less hum: switching power supply?



hello friends on Synth-Diy !

   i'm usually lurking around here, sorry for the sudden surprise entrance.
   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.
   could anyone point me in the direction of a small, lightweight, 
onboard-style switching power module: input 120VAC, output +/-15V @ 500mA, 
+5V @ 1000mA.

christo