🚀 go-pugleaf

>I have this burning desire to modify my CD player, and maybe learn
>something along the way.  I am in the process of replacing the op-amp
>section with the POOGE-4 mods.  The Audio Amateur's advice for the
>adventurous is to add another power supply to the player (A magnavox.)

I'm not familiar with the player or the mods in question, but the reason for
adding the second supply is likely to avoid the potential problem of the
left and right channels coupling through the supply (i.e., a sudden increase
in the current drawn by the left channel circuits causes a drop in the supply
to the right channel, so affecting its output.

>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. 

Some things to check here: does the transformer have sufficient current
capability to handle the second supply?  Remember, the total current required
will be greater with the second regulator, etc. added, due to losses in that
regulator.

Suggestions:  Increasing the "large cap" slightly might help a little, but
these IC regulators usually do a reasonably good job of getting rid of the
slight ripple left.  Still, a larger filter cap can't hurt, up to the limit
imposed by the ability of the front end of the supply to handle the inrush
current.  In addition, I'd add a small cap - perhaps an 0.1 uF ceramic -
across the 47 uF on the regulator's output.  This doesn't appear to make too
much sense, until you realize that most electrolytic caps - and I'm assuming
that the 47 uF is a small aluminum or tantalum (somewhat better than Al)
electrolytic - don't really appear very capacitive at high frequencies.  Adding
a small cap, keeping the leads as short as possible, can do wonders for keeping
high-frequency noise off the regulator's output.  You'd also be well-advised
to place a diode - practically any part good for a few amps - "backwards" from
the regulator's output to input.  (In other words, the diode's anode - "+" side
- on the regulator's output.)  This will help prevent damage to the regulator
should the output line for any reason ever go higher (voltage-wise) than the
input.

A slightly further-out notion - you didn't mention what the rectifier looks
like.  If it's a cheap half-wave rectifier (a single diode), then using
a full-wave "bridge" rectifer in your "new supply" (as well as the old, if
possible) will reduce the ripple, as the ripple frequency is doubled and
the average voltage at the input to the regulator will be higher.  This mod
may be beyond what's reasonable with your player.

I'd suggest finding a good basic text or article on simple power-supply
design and getting a good understanding of what all these parts *do* before
proceeding with ANY mods.  Good luck!


Bob Myers  KC0EW   HP Graphics Tech. Div.|  Opinions expressed here are not
                   Ft. Collins, Colorado |  those of my employer or any other
myers%hpfcla@hplabs.hp.com               |  sentient life-form on this planet.