Pedja Rogic discrete regulator question

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Next to the SMPS, my favourite power supply for chip amps is the Pedja Rogic discrete regulated. I've just built another pair of these regulators, and they appear to work OK but I have a question about the measured voltages.

I'm using BZX55 C27 zeners (500mW) that are quoted as having a voltage between 25.1 and 28.9. However, in my case I am getting voltages of 29.5, 29.8 29, and 29.4, all above the maximum quoted in the datasheet.

The corresponding output voltages are: 27.1, 27.3, 27.3, 27.4 - so would appear to be what is expected from the diode voltages.

My question is - are these diodes just a little bit out of spec, or is there something wrong with my circuit?

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Seems the zeners are indeed inside their specified parameter range. Using other types won't help you very much, since they have an app. ten percent range as well. Anyway those simple regs tend to sag under load, so they would never be 'DC accurate'.
Luckily the chipamp doesn't care about volt more or less!
 
To reanimate this thread, I also have a few questions about PR regulated PSU.
First of all, what replacements could I use for the J309, they are hard to get here? Is J310 identical as I can read from the datasheet?
Also, what values did you use for resistors R1, R2, and is there a point in trying with a higher voltage - say 33V or 36V to squeeze a bit more from the chips?
 
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