wallwarts and regulation..

[KA4HJH]

To John et al:

Sorry for the dalay; I was gone all weekend. A picture is worth a thousand
words. Here's a bipolar PS with two positive regulators and an untapped AC
supply...

   AC in
  |     |
  |     |                    + TTR
  |     |                   -------
  |     |----->|------------|     |---------- V+
  |     |          +|       -------      |
  |     |          _-_         |        ---
  |     |          ___         |        ---
  |     |           |          |         |
  |-----------|<-----------------------------
  |     |                                    |
  |     |                    + TTR           |--- Gnd
  |     |                   -------          |
   ----------->|------------|     |----------
        |          +|       -------      |
        |          _-_         |        ---
        |          ___         |        ---
        |           |          |         |
         -----|<----------------------------- V-

Like I said, the trick is to half-wave rectify both sides of the AC supply.
What do they call this in textbooks, dual half-wave? I can think of some
scenarios where this thing might start to go crazy but that's over my head.

I got this from the article "Hi-fi Noise Filter/Range Expander--Part 2" by
Joseph M. Gorin of Symmetric Sound Systems in the April '81 issue of
Radio-Electronics.

In additon, this device uses a 4049 as six matched N-channel FET's and a
739 Dual Audio Preamp. The latter has the same basic pinout as a 4739, but
it isn't the same IC. I've never seen or heard of it anywhere else. Anyone?


Also, Tony wrote:
>I have tried this with 78L05 and it works OK. Now there are different
>manufacturers of the 7805 chip and it could be that different designs push
>more current down the ground pin than some of the others. This will lead to
>changes in the output pd. Having said that, you might as well get a 317 and
>the two resistors, one cap and two diodes... more elegant somehow. Horses
>for courses as my mother used to say.

You're probably on the right track. All I know is when we looked it over at
work somebody grabbed a Motorola databook which said, no, no, no. It even
showed how to do it "properly" with an op-amp to buffer the current. I just
substituted 15V regulators and it's worked fine ever since.

Terry Bowman, KA4HJH