[R-390] OT: Filter for 9VDC PS

[Tim Shoppa]

Barry writes:
>>>I'm thinking of making a 9VDC, 1A supply
>>>Will 35mH be worth the effort or will I be just as well off without it?
>
>> Too small to do any good by a factor of around 100.  There's a reason 
>> chokes have never been used much in low-voltage power supplies.
>
> That's about what I was figuring.  Is there anything better I can do (short 
> of spending bigtime for a really big choke) besides a big ol' capacitor?

Build just about any of the "transistor DC power supplies" in a 60's
or 70's ARRL handbook.

Get up to the 70's and use a LM723 if at all possible. Compared
to a lot of modern regulators it has surprisngly low noise. You'll
need more than 6.3VAC and a bridge to get any headroom over
9VDC. Starting with a 10VAC or 12.6VAC transformer would be a good
start.

And, for god's sake, do NOT JUST PUT HONKIN HUGE CAPACITORS
IN THE PS. Yeah, it's real popular among amateur high-end audiophiles,
which is a good clue you shouldn't do that. The ripple current
through big capacitors will stress the transformer unduly and
the huge current spikes 120 times a second is a great way to induce
buzz into nearby ground loops. Use a reasonable size capacitor
(giving maybe 10 or 15% ripple on the unregulated supply) and
take the ripple out with a nice regulator.

Tim.