[sdiy] Low-Dropout regulators... (DIY-style)

[np]

Hi folks. Something semi-synth related, but still about music...


It's difficult to find adjustable low dropout regulators where I live. 
As a quick replacement I've designed one myself out of a opamp, a 
pass-transistor and a stabilized higher supply voltage (low 
current-capability, used to power the opamp and as a voltage-reference).

Schematic is here: http://torus.untergrund.net/tubes/filament_supply.png

That circuit works fine for heating tubes-filaments. I even have a 
soft-start to deal with the high currents of the tube-heaters. However, 
I'm about to use the same circuit to generate 9V out of a 12V supply 
(filtered DC with a good deal of ripple).

Instead of heaters I will power a audio processing circuit that will 
have quite a bit of capacitance across the rails.

Should I worry about oscillation from the opamp in this case? After all 
the opamp will drive a capacitive load. Also how can I avoid oscillation 
for such circuits? I don't need fast resoponse from the opamp. The 
unregulated DC has 100hz ripple, but all fast transients have already 
been filtered out.

Btw - I gave a LM317 a try and found out that the specimen that I have 
here starts to generate squeeky noises on it's output if I operated on 
the edge the minimal drop-out voltage :-)

Cheers,
 Nils