[time-nuts] Anybody want a Thunderbolt power supply?

[Charles Steinmetz]

Nick wrote:>

> 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:

<http://www.ko4bb.com/getsimple/index.php?id=download&file=06_Misc_Test_Equipment/Power_One/Power_One_HTAA-16W-A_mod_for_Thunderbolt_power_supply.pdf>

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.

Best regards,

Charles