Follow-up on noise mitigation

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Hi all,

Just a followup to my question a couple of weeks ago on audio noise
mitigation. To save a little money I decided to start with a handbuilt
prototype rather than etching another PCB. I took a piece of
double-coppered board and dug out a pit on one side to hold the chip.
I soldered it down with small pieces of wire-wrap wire going from its
ground pins to the top copper surface (i.e. lead out of the "pit" and
soldered directly to the "lip").

The results are much better. With a scope, I can see small ripply
noise on the output during certain video output, but I can't hear it.

So, I think my issue can principally be solved with a combination of:

a) Better PCB layout with more solid power planes. My earlier attempts
were based on a PCB to which I had "tacked on" the amplifier -
basically, I just put the amp schematic on the schematic for the other
interface stuff on the same board, and placed the packages in an
unused space. There is nothing high-speed on this board (it just
handles gating of some IR input signals, and it holds a gravity
switch) but it wasn't carefully laid out.

b) Ferrite. We had to add several beads for UL compliance. These have
had some significant effect on the noise.

c) Improved power supply isolation. As suggested, I am running the amp
through a +9V linear regulator [and appropriate bypassing] fed from
the main +12V (switcher) rail. By itself, this change does not make a
HUGE difference, but it is noticeable.

Thanks for everyone who replied with suggestions.

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