🚀 go-pugleaf

In article <9705@june.cs.washington.edu> dylan@cs.washington.edu (Dylan McNamee) writes:
>
>I have this burning desire to modify my CD player ...
>I am going to use the same transformer, and split the supply from
>rectification on down.  Here's my question:  The current power supply
>goes like this:  Transformer -> rectifier -> large cap (150uF) ->
>regulator (78M15CT) -> small cap (47uF) -> 15V out. 
>If I want to improve the quality of this set-up, should I change the
>topology at all?  Can I just make the Caps bigger?  Any other advice?

Before jumping headlong into a project like this, it might help to
enunciate the "theory" behind the idea that power supplies are an area
of weakness in low cost CD players:  (1) their poor regulation does a
disservice to the analog stages, D/A and beyond  (2) time-varying
current demand from the electromechanical parts like the disc motor,
arm servo, and focus servo  manage to modulate the analog stages'
power supplies and muddy the analog waveforms.

The solutions to (1) are to build battleship-like supplies.  (2)
requires you to have _separate_ DC supplies for the electromechanical
stages and the analog stages.  They may perhaps share the transformer
but not the regulator and final +/- NN volt outputs.  ("Enid Lumley"
would of course disagree, stating that they must be separated all
the way back to the pole transformer outside your house).


(1) Power Supply Rectifiers

    It's important to use diodes with a very high current rating, as
they conduct on only a teeny tiny portion of the AC cycle.  (If they
didn't, then you'd have ripple going into your regulator and you
don't want that.)  I'd recommend you use a 4-diode bridge with at least
a 10 ampere rating.


(2) Post-rectifier, pre-regulator  filter caps  ("bulk decoupling")

   The more capacitance you can afford to use here, the easier time your
regulator will have;  it directly decreases ripple and thereby makes
your regulator need to perform less "line regulation".  Here's how to
compute their effect.  Say that your circuitry draws (worst case, peak)
I amperes of current from the regulator.  Presume a full-wave bridge
rectifier that refills the capacitors 120 times/second.  Say that you
have chosen a post-rectifier, pre-regulator capacitor of C farads.  Then
the ripple voltage is given by Vripple = (I / (120 * C)).  For example
if you chose 5,000 microfarads for your supply that draws 300 mA (peak),
then Vripple is 500 millivolts.  Have a look at Senturia and Wedlock's
textbook, section 8.4.2, figure 8.18.

  If you have a lot of "headroom" (i.e. there is a large difference
between the transformer secondary voltage and the desired regulator
output voltage), then you might consider a Capacitance Multiplier
before the regulator.  This is just a series-pass transistor with
a large capacitance at its base; the effective filtering is Beta*C
hence the name.  You can find this circuit in all the cookbooks.


(3)  The regulator circuit itself

   This is a religious topic for many people.  _Audio_Amateur_ for example
maligns the one-chip regulator ICs such as 7815, preferring a discrete-part
implementation.

   One thing to note is that practically _all_ regulators, IC and discrete,
have a lower output impedance at higher current levels.  If you want
to get fancy you can obliquely refer to "the translinear principle" and
amaze your friends.  In practice this means, Put A Resistor Across The
Output Of The Regulator.  It'll increase the current in the regulator
and make it work better, due to a higher Gm in the power semiconductor.
Up to the limit of your transformer, increase the current as much as
you dare (taking care not to overload things and causing a meltdown).
It's usually the case that the circuit can tolerate 250mA more current;
for a 15V supply that's a 60 ohm, 4 Watt resistor.

  If you're determined to use an IC, please choose the LM317/LM337
devices.  They're much newer designs, have tons better performance than
7815/7915, and cost only a bit more.  They're adjustable so consult the
datasheet.  Plus, "Mister POOGE" himself uses them in his Audio Amateur
preamp design.


(4) Decoupling capacitors on the regulator output

   These are examples of "if some is good, more is better" brute-force
application.  Be sure to check the final design with an oscilloscope
to insure there's no instability or parasitic oscillation.


(5) Other

   Single-point "star" grounding will help reduce ground-loops and crosstalk.

   Many people believe that there is a difference in the "sound" of
   different types of capacitors.   However the only type of capacitor
   that comes in 5,000 microfarads is Aluminum Electrolytic.  You may
   choose to parallel that with one of the in-vogue Sidereal Polyprolylene
   of Wonder Cap devices.

   I expect flaming tirades from pals SJC and MH_batcomputer.
-- 
 -- Mark Johnson	
 	MIPS Computer Systems, 930 E. Arques, Sunnyvale, CA 94086
	(408) 991-0208    mark@mips.com  {or ...!decwrl!mips!mark}