Follow-up on noise mitigation

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.

Reply to
Lewin A.R.W. Edwards
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