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Looking for a transformer

martin clark said:
Easy; use 5-6 , 12v zeners in series. or even 10 x 6.9Vz.

There's a significant noise advantage to doing this.*
Drive them fairly rich, 8-10mA, constant-current, for best effect.
NB that also means the dissipation is divided across teh string.

You'll probably only need 5 at that sort of bias current. Or trim with 5.6-10v zener in last position of 6 to suit.

* a statisical thing: because the zeners noise contributions are random, the addition is RMS... so you'll only have about SQRT(5) x the noise of a single 12v zener at that current; or SQRT(10) = 3.162 x the voltage noise of a 6.9V** zener , for a 70-72VDC voltage reference - and /not/ the full noise of a single c. 70v zener (which will be considerably worse, and need to dissipate up towards ~1W.)

** It's right around this value, maybe a little lower, that treu Zener action happens, not avalanche (higher voltages): and that's both best regulation, and - least noise. 10 zeners at 10mA = 70mW each, buy 20, cost about a buck or two .. about as good as you'll get.

Use a dab of RC following c 470-1K/ 47-100uF, to drive the base/gate of your pass device - and the whole thing is beyond contention. Broadband output noise/ripple at c -100dB, maybe a bit more - easily achieved with good layout.
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Well, that's a wonderful bit of insight! I thought perhaps I could put them in series like that, but this is the first time I've encountered the need, so I wasn't sure whether it was wise to do so. I've got a collection of Zeners on hand, so I may already have everything I need.

I'll sketch the idea using CircuitLab, to confirm that I'm on the right track.
 
They are 30VA so designed to give 45v at the full output current of 0.33A per winding. 10% over at low current output seems about right.
Can you just put a 7v zener in series with the DC into your reg thus dropping 77 to 70? 7v @ say 20mA is 150mW so 1W will suffice as this should handle the inrush current at turn on
 
a.palfreyman said:
They are 30VA so designed to give 45v at the full output current of 0.33A per winding. 10% over at low current output seems about right.
Can you just put a 7v zener in series with the DC into your reg thus dropping 77 to 70? 7v @ say 20mA is 150mW so 1W will suffice as this should handle the inrush current at turn on
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That's an interesting idea, and certainly simpler.

Any dissenting views?

Btw, there is a good possibility that I'm safe without any adjustment to the voltage, but I am finding this discussion to be very educational and interesting. :)
 
What is the o/p voltage of the regs?

The ZTXs will get rather toasty with e.g. 60V rails = 600mW each.
 
S-Man said:
What is the o/p voltage of the regs?

The ZTXs will get rather toasty with e.g. 60V rails = 600mW each.
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The regulators for the input stage will be set to 60V or thereabouts. This is well within the range of the amps I'm using. (It was originally going to be NCC300, but now is SE400.)
 
Sneaky, low cost solution. Get a small (say 10VA) toroid with say 2 x 9V secondaries, and wire them in series, on top of the main transformer secondaries. Obviously, you don't get independence, but the regulation can eat that up, and as the winding only has to supply the front end current, a small device is enough.
 
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