[time-nuts] Low noise power supplies?

[Charles P. Steinmetz]

Joe wrote:

>Back when I was in product engineering there was a VCO design that 
>used a "superfilter" circuit. It consisted of a pass transistor and 
>a filter cap from base to ground. The gain of the transistor 
>multiplied the effective capacitance. I have not seen this configuration since.

They are often called "capacitance multipliers" and are popular with 
(among others) audio designers as low-noise supplies for low-level 
circuits (moving coil head amps, RIAA stages, etc.).  They are best 
used following an active regulator.  If the capacitor is 
electrolytic, it needs to be chosen very carefully so that leakage 
current noise doesn't spoil the effort.  Also, it is best to use a 
voltage divider on the base to give the transistor a bit of headroom 
(i.e., base voltage should be a volt or so lower than collector 
voltage, not the same as the collector voltage as happens when there 
is just a pull-up resistor on the base).

Best regards,

Charles