[quote author="NewYorkDave"]FWIW, a simple unamplified shunt regulator fulfills the "constant current draw" criterion.
This is an interesting topic to me because I have a circuit which requires 150V regulated (at about 25-30mA) and I can't quite decide how I want to go about it. There are so many options, though my personal tendency is to lean toward simple and hard-to-blow-up...[/quote]
I used to use a simple emitter-follower in shunt for apps that weren't too demanding. For example, in a positive supply a PNP fed from a voltage divider off of the regulated voltage supply would take up the "slack" when the load current varied with signal. It was certainly fast, if inefficient.
For more careful work I've used composite current sources feeding composite shunt regulators. Typically the pass and shunt elements are DMOS, and the error amps are discrete bipolar. I like the access to all the parts so that the transient response can be optimized. The noise is way down below typical intergrated parts, and the line and load regulation absurdly good. The low-frequency noise is limited by what is used as a voltage reference. I used to favor concoctions of JFETs selected for operation at a low-tempco point, and more recently have looked at a roll-your-own bandgap reference made from discrete bipolars. But both of these are a lot of work, and for audio, wretched excess. For other instrumentation it may be a different story---for example, the photoarray system mentioned spent up to an hour soaking up photons, and then reading out, and the before-and-after bias was a part of the signal, so needed to be ultra-stable.
Beyond this, I think the explicit use of the signal to drive the regulators is the next frontier, as it were. I veer between thinking it potentially worthwhile and being cynical that it will just be another marketing gimmick (I can already imagine the Stereophoole review). I'm pretty happy with the state of the audio art as it is, and even listening to vintage tube gear with the loosest of specs can be pretty damn enjoyable. I'm bracing myself for one of my highender friends saying I'm taking all the life out of the music by being so precise.
After this, the electronics will have to become precognitive.