Re: PSU for precision audio opamps

On Fri, 13 Jun 2008 19:27:52 -0700 (PDT), MooseFET
<kensmith@xxxxxxxxx> wrote:

On Jun 13, 8:34 am, John Larkin
<jjlar...@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
On Fri, 13 Jun 2008 08:24:11 -0700 (PDT), Dirk Bruere at NeoPax

<dirk.bru...@xxxxxxxxx> wrote:
I need +/- 12V from a 5V or 12V supply (PC PSU) to drive precision
audio stuff (LM4562).

Umm, what does "precision audio" mean?

Thinking of using a DC-DC converter eg XP Power or C&D.
The regulation on these isn't brillant at around 10%, but that
shouldn't be a problem.
Ripple is typically 0.5%
Any advice?

PSRR is bad on most all opamps, so the ripple may shoot through. And
the dc/dc guys often make spikes, which can get into the opamp
front-end. If you use a converter brick, it would be prudent to add LC
filtering to the input and the output.


Yes.
Also beware that the PSRR for the minus supply is often worse than for
the positive one. The lower of the two numbers is the one you really
need to worry about.

You should also be very careful about where the DC-DC converter's
input side ripple can flow to and the capacitively driven AC current
that is forced between its input side and output side.

DC-DC converters also radiate AC magnetic fields. The shield over the
top of them never fully stops it. Ideally the DC-DC should be on the
far side of some of the *** metal.


You can get a serious amount of noise magnetically induced from fans,
AC or DC brushless. One has to worry about loop areas, yet another
interesting constraint in PCB layout.

Not to mention the spikey 120 Hz mag field from the ac power wiring,
slamming tens of amps peak into the bridge rectifier and input cap of
the power supply, assisted by some coils in the EMI filter maybe.

And all the RF that's available to rectify.

John

.