Thanks much Mr Tibbs & Martin for your input and help on this!
2) Get hold of a couple of appropriately sized heatsinks and attach them to the smps cases using thermal bonding epoxy.
Already with you on this, and have a couple options in the odd parts box. Will try hopefully this week.
Sounds like low freq oscillation - hard to say what caused it (possibly one of the smps didn't start up properly)
I remember seeing some pretty intense oscillations on SMPS boot up when I was 'scoping them. Took a second or two for them to settle then, so this may be the case. Martin's 10uf suggestion is an easy test/solution.
If you can, split the output 0V on the PS board into two and connect each separately to the preamp 0V point, using as short a link as possible.
Not easy to do. My original board layout had two completely isolated SMPS modules like your setup, but then I thought I'd try to move the preamp star point to the board itself. That seemed to necessitate joining it all into one point. Also, both my SNAPS and DIY HiCap aren't electrically isolated split supplies, just one big supply with multiple outputs hanging off it.
If splitting is not easily done, replace the existing single 0V wire with a heavier gauge wire and keep it short as possible.
Yes, good thought. It has a few turns in it now (mostly for looks I guess) that I could remove/shorten.
Also, I find it improves performance if you arrange the power feeds so that each converter and associated reg's feed the boards of a single channel.
Given that it's single supply feeding both, do you think this is still important? Would involve rerunning 4 lines, not a huge deal, but I'm of the mind to leave well enough alone unless you think it's holding back even more music.
Now for more listening feedback. This the opposite of what I typically expect for hi-fi listening feedback, but it was eye-opening for me.
We have 2 boys, 5 and 3. They are into all sorts of music but have their favorites. Yesterday afternoon I put on They Might Be Giants' "Here Come the ABCs" (great fun for kids and adults) and they started the usual dancing around. About the second song in, both started singing along almost perfectly on pitch with these songs...which I hadn't heard them do before. When the CD was over, the younger said "Dad the radio didn't play any more songs. Make it start again". A little later that evening, the same kid was at the fridge playing with magnetic letters, singing one of the songs from the TMBG collection, again with amazing melodic accuracy and flow. My wife turned and said, "When did he learn to do that?!"
The kicker for me came even later, when I hit "shuffle" on the Squeezebox and let it run through whatever came up. Joe Jackson's "Sunday Papers" came on, which I think was the first time the kids had ever heard it. The 5yr old sang the first line of the chorus the second time it came around, with the right words and melody. Then he paused and asked "Dad why is he singing about a newspaper?" Another first as far as I know. What do these say about the improvement? My kids had never done anything beyond just dancing around, really, when music was playing. Sometimes they'd sing a song later, but I'd never heard them sing along, until today.
In all of this testing and listening, I wonder if my stock preamp wasn't at the same performance level to begin with as others who mod/tweak, and so perhaps what I'm hearing as an improvement may be greater than what others would hear. I don't know if newer designs with better grounding schemes and board layouts than the old 32 (ie 102, 72, 62 or even the 32-5) may be an improvement in and of themselves, without the SMPS/0v work.
Hope this is useful for the rest of you.
M
