[time-nuts] Anybody want a Thunderbolt power supply?
Jerry O. Stern
jsternmd at att.net
Wed Aug 31 07:57:38 EDT 2016
Charles, do you have the schematic for the HTAA-16W-A ? The KO4BB mod has limited info
K1jos at arrl.net
From: time-nuts [mailto:time-nuts-bounces at febo.com] On Behalf Of Charles Steinmetz
Sent: Wednesday, August 31, 2016 4:49 AM
To: time-nuts at febo.com
Subject: Re: [time-nuts] Anybody want a Thunderbolt power supply?
> I’m going to shoot for <= 35 mV P-P. If you need better than that,
> then it probably turns into a hybrid switching+linear system
It's been years since I played with Tbolts (as opposed to just using them), but as I recall there was no possibility whatever of getting the best results from any switching supply without at least one stage of linear "polishing." In fact, everything I tried with initial switching regulation benefitted from two levels of linear polishing. It was my conclusion that starting with a switching supply of any sort is a bad idea.
I found that for best performance the -12v supply, which feeds the DAC, needs to be about 1000 times better than what you are shooting for (i.e., low tens of uVp-p). That surprised me, because I assumed there must be some internal regulation of the DAC supplies -- but it was what my testing showed.
Sure, a Tbolt will work with dirtier power than that, but for real time-nuts performance I consider an all-linear supply with an ultra-low-noise -12v rail to be absolutely necessary. Since that is not very hard to provide, I see no reason to try to make do with less.
One low-cost solution is to wait for a cheap Power One HTAA-16W-A triple-output power supply to show up on ebay, and do a very minor modification to raise the +12v current limit (so it will handle the first moments of a cold start without current limiting) as detailed here:
That approach achieves excellent results -- close to the best you can do
-- with very little fiddling. If one wants the very best performance, the -12v supply can be adjusted down to -16v or so to provide the headroom to add a ULN regulator or a capacitance multiplier. (The Tbolt only draws a few mA from the -12v supply, so the ULN regulator can be a suitable op amp with no external pass transistors.)
Last time I bought them, I paid $25 for a lot of three HTAA-16W-A's.
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